d3-selection/README.md

# d3-selection

Selections allow powerful data-driven transformation of the document object model (DOM): set [attributes](#selection_attr), [styles](#selection_style), [properties](#selection_property), [HTML](#selection_html) or [text](#selection_text) content, and more. Using the [data join](#joining-data)’s [enter](#selection_enter) and [exit](#selection_enter) selections, you can also [add](#selection_append) or [remove](#selection_remove) elements to correspond to data.

Selection methods typically return the current selection, or a new selection, allowing the concise application of multiple operations on a given selection via method chaining. For example, to set the class and color style of all paragraph elements in the current document:

```js
d3.selectAll("p")
    .attr("class", "graf")
    .style("color", "red");
```

This is equivalent to:

```js
var p = d3.selectAll("p");
p.attr("class", "graf");
p.style("color", "red");
```

By convention, selection methods that return the current selection use *four* spaces of indent, while methods that return a new selection use only *two*. This helps reveal changes of context by making them stick out of the chain:

```js
d3.select("body")
  .append("svg")
    .attr("width", 960)
    .attr("height", 500)
  .append("g")
    .attr("transform", "translate(20,20)")
  .append("rect")
    .attr("width", 920)
    .attr("height", 460);
```

Selections are immutable. All selection methods that affect which elements are selected (or their order) return a new selection rather than modifying the current selection. However, note that elements are necessarily mutable, as selections drive transformations of the document!

## Installing

If you use NPM, `npm install d3-selection`. Otherwise, download the [latest release](https://github.com/d3/d3-selection/releases/latest). You can also load directly from [d3js.org](https://d3js.org), either as a [standalone library](https://d3js.org/d3-selection.v1.min.js) or as part of [D3 4.0](https://github.com/d3/d3). AMD, CommonJS, and vanilla environments are supported. In vanilla, a `d3` global is exported:

```html
<script src="https://d3js.org/d3-selection.v1.min.js"></script>
<script>

var div = d3.selectAll("div");

</script>
```

[Try d3-selection in your browser.](https://tonicdev.com/npm/d3-selection)

## API Reference

* [Selecting Elements](#selecting-elements)
* [Modifying Elements](#modifying-elements)
* [Joining Data](#joining-data)
* [Handling Events](#handling-events)
* [Control Flow](#control-flow)
* [Local Variables](#local-variables)
* [Namespaces](#namespaces)

### Selecting Elements

Selection methods accept [W3C selector strings](http://www.w3.org/TR/selectors-api/) such as `.fancy` to select elements with the class *fancy*, or `div` to select DIV elements. Selection methods come in two forms: select and selectAll: the former selects only the first matching element, while the latter selects all matching elements in document order. The top-level selection methods, [d3.select](#select) and [d3.selectAll](#selectAll), query the entire document; the subselection methods, [*selection*.select](#selection_select) and [*selection*.selectAll](#selection_selectAll), restrict selection to descendants of the selected elements.

<a name="selection" href="#selection">#</a> d3.<b>selection</b>()

[Selects](#select) the root element, `document.documentElement`. This function can also be used to test for selections (`instanceof d3.selection`) or to extend the selection prototype. For example, to add a method to check checkboxes:

```js
d3.selection.prototype.checked = function(value) {
  return arguments.length < 1
      ? this.property("checked")
      : this.property("checked", !!value);
};
```

And then to use:

```js
d3.selectAll("input[type=checkbox]").checked(true);
```

<a name="select" href="#select">#</a> d3.<b>select</b>(<i>selector</i>)

Selects the first element that matches the specified *selector* string. If no elements match the *selector*, returns an empty selection. If multiple elements match the *selector*, only the first matching element(in document order will be selected. For example, to select the first anchor element:

```js
var anchor = d3.select("a");
```

If the *selector* is not a string, instead selects the specified node; this is useful if you already have a reference to a node, such as `this` within an event listener or a global such as `document.body`. For example, to make a clicked paragraph red:

```js
d3.selectAll("p").on("click", function() {
  d3.select(this).style("color", "red");
});
```

<a name="selectAll" href="#selectAll">#</a> d3.<b>selectAll</b>(<i>selector</i>)

Selects all elements that match the specified *selector* string. The elements will be selected in document order (top-to-bottom). If no elements in the document match the *selector*, or if the *selector* is null or undefined, returns an empty selection. For example, to select all paragraphs:

```js
var paragraph = d3.selectAll("p");
```

If the *selector* is not a string, instead selects the specified array of nodes; this is useful if you already have a reference to nodes, such as `this.childNodes` within an event listener or a global such as `document.links`. The nodes may instead be a pseudo-array such as a `NodeList` or `arguments`. For example, to color all links red:

```js
d3.selectAll(document.links).style("color", "red");
```

<a name="selection_select" href="#selection_select">#</a> <i>selection</i>.<b>select</b>(<i>selector</i>)

For each selected element, selects the first descendant element that matches the specified *selector* string. If no element matches the specified selector for the current element, the element at the current index will be null in the returned selection. (If the *selector* is null, every element in the returned selection will be null, resulting in an empty selection.) If the current element has associated data, this data is propagated to the corresponding selected element. If multiple elements match the selector, only the first matching element in document order is selected. For example, to select the first bold element in every paragraph:

```js
var b = d3.selectAll("p").select("b");
```

If the *selector* is a function, it is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. It must return an element, or null if there is no matching element. For example, to select the previous sibling of each paragraph:

```js
var previous = d3.selectAll("p").select(function() {
  return this.previousElementSibling;
});
```

Unlike [*selection*.selectAll](#selection_selectAll), *selection*.select does not affect grouping: it preserves the existing group structure and indexes, and propagates data (if any) to selected children. Grouping plays an important role in the [data join](#joining-data). See [Nested Selections](http://bost.ocks.org/mike/nest/) and [How Selections Work](http://bost.ocks.org/mike/selection/) for more on this topic.

<a name="selection_selectAll" href="#selection_selectAll">#</a> <i>selection</i>.<b>selectAll</b>(<i>selector</i>)

For each selected element, selects the descendant elements that match the specified *selector* string. The elements in the returned selection are grouped by their corresponding parent node in this selection. If no element matches the specified selector for the current element, or if the *selector* is null, the group at the current index will be empty. The selected elements do not inherit data from this selection; use [*selection*.data](#selection_data) to propagate data to children. For example, to select the bold elements in every paragraph:

```js
var b = d3.selectAll("p").selectAll("b");
```

If the *selector* is a function, it is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. It must return an array of elements (or a pseudo-array, such as a NodeList), or the empty array if there are no matching elements. For example, to select the previous and next siblings of each paragraph:

```js
var sibling = d3.selectAll("p").selectAll(function() {
  return [
    this.previousElementSibling,
    this.nextElementSibling
  ];
});
```

Unlike [*selection*.select](#selection_select), *selection*.selectAll does affect grouping: each selected descendant is grouped by the parent element in the originating selection. Grouping plays an important role in the [data join](#joining-data). See [Nested Selections](http://bost.ocks.org/mike/nest/) and [How Selections Work](http://bost.ocks.org/mike/selection/) for more on this topic.

<a name="selection_filter" href="#selection_filter">#</a> <i>selection</i>.<b>filter</b>(<i>filter</i>)

Filters the selection, returning a new selection that contains only the elements for which the specified *filter* is true. The *filter* may be specified either as a selector string or a function. If a function, it is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element.

For example, to filter a selection of table rows to contain only even rows:

```js
var even = d3.selectAll("tr").filter(":nth-child(even)");
```

This is approximately equivalent to using [d3.selectAll](#selectAll) directly, although the indexes may be different:

```js
var even = d3.selectAll("tr:nth-child(even)");
```

Similarly, using a function:

```js
var even = d3.selectAll("tr").filter(function(d, i) { return i & 1; });
```

Or using [*selection*.select](#selection_select):

```js
var even = d3.selectAll("tr").select(function(d, i) { return i & 1 ? this : null; });
```

Note that the `:nth-child` pseudo-class is a one-based index rather than a zero-based index. Also, the above filter functions do not have precisely the same meaning as `:nth-child`; they rely on the selection index rather than the number of preceeding sibling elements in the DOM.

The returned filtered selection preserves the parents of this selection, but like [*array*.filter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter), it does not preserve indexes as some elements may be removed; use [*selection*.select](#selection_select) to preserve the index, if needed.

<a name="selection_merge" href="#selection_merge">#</a> <i>selection</i>.<b>merge</b>(<i>other</i>)

Returns a new selection merging this selection with the specified *other* selection. The returned selection has the same number of groups and the same parents as this selection. Any missing (null) elements in this selection are filled with the corresponding element, if present (not null), from the specified *selection*. (If the *other* selection has additional groups or parents, they are ignored.)

This method is commonly used to merge the [enter](#selection_enter) and [update](#selection_data) selections after a [data-join](#joining-data). After modifying the entering and updating elements separately, you can merge the two selections and perform operations on both without duplicate code. For example:

```js
var circle = svg.selectAll("circle").data(data) // UPDATE
    .style("fill", "blue");

circle.exit().remove(); // EXIT

circle.enter().append("circle") // ENTER
    .style("fill", "green")
  .merge(circle) // ENTER + UPDATE
    .style("stroke", "black");
```

This method is not intended for concatenating arbitrary selections, however: if both this selection and the specified *other* selection have (non-null) elements at the same index, this selection’s element is returned in the merge and the *other* selection’s element is ignored.

<a name="matcher" href="#matcher">#</a> d3.<b>matcher</b>(<i>selector</i>)

Given the specified *selector*, returns a function which returns true if `this` element [matches](https://developer.mozilla.org/en-US/docs/Web/API/Element/matches) the specified selector. This method is used internally by [*selection*.filter](#selection_filter). For example, this:

```js
var div = selection.filter("div");
```

Is equivalent to:

```js
var div = selection.filter(d3.matcher("div"));
```

(Although D3 is not a compatibility layer, this implementation does support vendor-prefixed implementations due to the recent standardization of *element*.matches.)

<a name="selector" href="#selector">#</a> d3.<b>selector</b>(<i>selector</i>)

Given the specified *selector*, returns a function which returns the first descendant of `this` element that matches the specified selector. This method is used internally by [*selection*.select](#selection_select). For example, this:

```js
var div = selection.select("div");
```

Is equivalent to:

```js
var div = selection.select(d3.selector("div"));
```

<a name="selectorAll" href="#selectorAll">#</a> d3.<b>selectorAll</b>(<i>selector</i>)

Given the specified *selector*, returns a function which returns all descendants of `this` element that match the specified selector. This method is used internally by [*selection*.selectAll](#selection_selectAll). For example, this:

```js
var div = selection.selectAll("div");
```

Is equivalent to:

```js
var div = selection.selectAll(d3.selectorAll("div"));
```

<a name="window" href="#window">#</a> d3.<b>window</b>(<i>node</i>)

Returns the owner window for the specified *node*. If *node* is a node, returns the owner document’s default view; if *node* is a document, returns its default view; otherwise returns the *node*.

### Modifying Elements

After selecting elements, use the selection’s transformation methods to affect document content. For example, to set the name attribute and color style of an anchor element:

```js
d3.select("a")
    .attr("name", "fred")
    .style("color", "red");
```

To experiment with selections, visit [d3js.org](https://d3js.org) and open your browser’s developer console! (In Chrome, open the console with ⌥⌘J.) Select elements and then inspect the returned selection to see which elements are selected and how they are grouped. Call selection methods and see how the page content changes.

<a name="selection_attr" href="#selection_attr">#</a> <i>selection</i>.<b>attr</b>(<i>name</i>[, <i>value</i>])

If a *value* is specified, sets the attribute with the specified *name* to the specified value on the selected elements and returns this selection. If the *value* is a constant, all elements are given the same attribute value; otherwise, if the *value* is a function, the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to set each element’s attribute. A null value will remove the specified attribute.

If a *value* is not specified, returns the current value of the specified attribute for the first (non-null) element in the selection. This is generally useful only if you know that the selection contains exactly one element.

The specified *name* may have a namespace prefix, such as `xlink:href` to specify the `href` attribute in the XLink namespace. See [namespaces](#namespaces) for the map of supported namespaces; additional namespaces can be registered by adding to the map.

<a name="selection_classed" href="#selection_classed">#</a> <i>selection</i>.<b>classed</b>(<i>names</i>[, <i>value</i>])

If a *value* is specified, assigns or unassigns the specified CSS class *names* on the selected elements by setting the `class` attribute or modifying the `classList` property and returns this selection. The specified *names* is a string of space-separated class names. For example, to assign the classes `foo` and `bar` to the selected elements:

```js
selection.classed("foo bar", true);
```

If the *value* is truthy, then all elements are assigned the specified classes; otherwise, the classes are unassigned. If the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to assign or unassign classes on each element. For example, to randomly associate the class *foo* with on average half the selected elements:

```js
selection.classed("foo", function() { return Math.random(); });
```

If a *value* is not specified, returns true if and only if the first (non-null) selected element has the specified *classes*. This is generally useful only if you know the selection contains exactly one element.

<a name="selection_style" href="#selection_style">#</a> <i>selection</i>.<b>style</b>(<i>name</i>[, <i>value</i>[, <i>priority</i>]])

If a *value* is specified, sets the style property with the specified *name* to the specified value on the selected elements and returns this selection. If the *value* is a constant, then all elements are given the same style property value; otherwise, if the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to set each element’s style property. A null value will remove the style property. An optional *priority* may also be specified, either as null or the string `important` (without the exclamation point).

If a *value* is not specified, returns the current computed value of the specified style property for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element. The computed value **may be different than the previously-set value**, particularly if it was set using a shorthand property (such as the `font` style, which is shorthand for `font-size`, `font-face`, etc.).

Caution: unlike many SVG attributes, CSS styles typically have associated units. For example, `3px` is a valid stroke-width property value, while `3` is not. Some browsers implicitly assign the `px` (pixel) unit to numeric values, but not all browsers do: IE, for example, throws an “invalid arguments” error!

<a name="selection_property" href="#selection_property">#</a> <i>selection</i>.<b>property</b>(<i>name</i>[, <i>value</i>])

Some HTML elements have special properties that are not addressable using attributes or styles, such as a form field’s text `value` and a checkbox’s `checked` boolean. Use this method to get or set these properties.

If a *value* is specified, sets the property with the specified *name* to the specified value on selected elements. If the *value* is a constant, then all elements are given the same property value; otherwise, if the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to set each element’s property. A null value will delete the specified property.

If a *value* is not specified, returns the value of the specified property for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element.

<a name="selection_text" href="#selection_text">#</a> <i>selection</i>.<b>text</b>([<i>value</i>])

If a *value* is specified, sets the [text content](http://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-textContent) to the specified value on all selected elements, replacing any existing child elements. If the *value* is a constant, then all elements are given the same text content; otherwise, if the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to set each element’s text content. A null value will clear the content.

If a *value* is not specified, returns the text content for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element.

<a name="selection_html" href="#selection_html">#</a> <i>selection</i>.<b>html</b>([<i>value</i>])

If a *value* is specified, sets the [inner HTML](http://dev.w3.org/html5/spec-LC/apis-in-html-documents.html#innerhtml) to the specified value on all selected elements, replacing any existing child elements. If the *value* is a constant, then all elements are given the same inner HTML; otherwise, if the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function’s return value is then used to set each element’s inner HTML. A null value will clear the content.

If a *value* is not specified, returns the inner HTML for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element.

Use [*selection*.append](#selection_append) or [*selection*.insert](#selection_insert) instead to create data-driven content; this method is intended for when you want a little bit of HTML, say for rich formatting. Also, *selection*.html is only supported on HTML elements. SVG elements and other non-HTML elements do not support the innerHTML property, and thus are incompatible with *selection*.html. Consider using [XMLSerializer](https://developer.mozilla.org/en-US/docs/XMLSerializer) to convert a DOM subtree to text. See also the [innersvg polyfill](https://code.google.com/p/innersvg/), which provides a shim to support the innerHTML property on SVG elements.

<a name="selection_append" href="#selection_append">#</a> <i>selection</i>.<b>append</b>(<i>type</i>)

If the specified *type* is a string, appends a new element of this type (tag name) as the last child of each selected element, or the next following sibling in the update selection if this is an [enter selection](#selection_enter). (The enter behavior allows you to insert elements into the DOM in an order consistent with bound data; however, the slower [*selection*.order](#selection_order) may still be required if updating elements change order.) Otherwise, the *type* may be a function which is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. This function should return an element to be appended. (The function typically creates a new element, but it may instead return an existing element.) For example, to append a DIV element to each paragraph:

```js
d3.selectAll("p").append("div");
```

This is equivalent to:

```js
d3.selectAll("p").append(function() {
  return document.createElement("div");
});
```

Which is equivalent to:

```js
d3.selectAll("p").select(function() {
  return this.appendChild(document.createElement("div"));
});
```

In both cases, this method returns a new selection containing the appended elements. Each new element inherits the data of the current elements, if any, in the same manner as [*selection*.select](#selection_select).

The specified *name* may have a namespace prefix, such as `svg:text` to specify a `text` attribute in the SVG namespace. See [namespaces](#namespaces) for the map of supported namespaces; additional namespaces can be registered by adding to the map. If no namespace is specified, the namespace will be inherited from the parent element; or, if the name is one of the known prefixes, the corresponding namespace will be used (for example, `svg` implies `svg:svg`).

<a name="selection_insert" href="#selection_insert">#</a> <i>selection</i>.<b>insert</b>(<i>type</i>, <i>before</i>)

If the specified *type* is a string, inserts a new element of this type (tag name) before the element matching the specified *before* selector for each selected element. For example, a *before* selector `:first-child` will prepend nodes before the first child. Both *type* and *before* may instead be specified as functions which are evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The *type* function should return an element to be inserted; the *before* function should return the child element before which the element should be inserted. For example, to insert a DIV element to each paragraph:

```js
d3.selectAll("p").insert("div");
```

This is equivalent to:

```js
d3.selectAll("p").insert(function() {
  return document.createElement("div");
});
```

Which is equivalent to:

```js
d3.selectAll("p").select(function() {
  return this.insertBefore(document.createElement("div"), null);
});
```

In both cases, this method returns a new selection containing the appended elements. Each new element inherits the data of the current elements, if any, in the same manner as [*selection*.select](#selection_select).

The specified *name* may have a namespace prefix, such as `svg:text` to specify a `text` attribute in the SVG namespace. See [namespaces](#namespaces) for the map of supported namespaces; additional namespaces can be registered by adding to the map. If no namespace is specified, the namespace will be inherited from the parent element; or, if the name is one of the known prefixes, the corresponding namespace will be used (for example, `svg` implies `svg:svg`).

<a name="selection_remove" href="#selection_remove">#</a> <i>selection</i>.<b>remove</b>()

Removes the selected elements from the document. Returns this selection (the removed elements) which are now detached from the DOM. There is not currently a dedicated API to add removed elements back to the document; however, you can pass a function to [*selection*.append](#selection_append) or [*selection*.insert](#selection_insert) to re-add elements.

<a name="selection_sort" href="#selection_sort">#</a> <i>selection</i>.<b>sort</b>(<i>compare</i>)

Returns a new selection that contains a copy of each group in this selection sorted according to the *compare* function. After sorting, re-inserts elements to match the resulting order (per [*selection*.order](#selection_order)).

The compare function, which defaults to [ascending](https://github.com/d3/d3-array#ascending), is passed two elements’ data *a* and *b* to compare. It should return either a negative, positive, or zero value. If negative, then *a* should be before *b*; if positive, then *a* should be after *b*; otherwise, *a* and *b* are considered equal and the order is arbitrary.

Note that sorting is not guaranteed to be stable; however, it is guaranteed to have the same behavior as your browser’s built-in [sort](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/sort) method on arrays.

<a name="selection_order" href="#selection_order">#</a> <i>selection</i>.<b>order</b>()

Re-inserts elements into the document such that the document order of each group matches the selection order. This is equivalent to calling [*selection*.sort](#selection_sort) if the data is already sorted, but much faster.

<a name="selection_raise" href="#selection_raise">#</a> <i>selection</i>.<b>raise</b>()

Re-inserts each selected element, in order, as the last child of its parent. Equivalent to:

```js
selection.each(function() {
  this.parentNode.appendChild(this);
});
```

<a name="selection_lower" href="#selection_lower">#</a> <i>selection</i>.<b>lower</b>()

Re-inserts each selected element, in order, as the first child of its parent. Equivalent to:

```js
selection.each(function() {
  this.parentNode.insertBefore(this, this.parentNode.firstChild);
});
```

<a name="creator" href="#creator">#</a> d3.<b>creator</b>(<i>name</i>)

Given the specified element *name*, returns a function which creates an element of the given name, assuming that `this` is the parent element. This method is used internally by [*selection*.append](#selection_append) and [*selection*.insert](#selection_insert) to create new elements. For example, this:

```js
selection.append("div");
```

Is equivalent to:

```js
selection.append(d3.creator("div"));
```

See [namespace](#namespace) for details on supported namespace prefixes, such as for SVG elements.

### Joining Data

For an introduction to D3’s data joins, see [Thinking With Joins](http://bost.ocks.org/mike/join/). Also see the [General Update Pattern](http://bl.ocks.org/mbostock/3808218) examples.

<a name="selection_data" href="#selection_data">#</a> <i>selection</i>.<b>data</b>([<i>data</i>[, <i>key</i>]])

Joins the specified array of *data* with the selected elements, returning a new selection that it represents the *update* selection: the elements successfully bound to data. Also defines the [enter](#selection_enter) and [exit](#selection_exit) selections on the returned selection, which can be used to add or remove elements to correspond to the new data. The specified *data* is an array of arbitrary values (*e.g.*, numbers or objects), or a function that returns an array of values for each group. When data is assigned to an element, it is stored in the property `__data__`, thus making the data “sticky” and available on re-selection.

The *data* is specified **for each group** in the selection. If the selection has multiple groups (such as [d3.selectAll](#selectAll) followed by [*selection*.selectAll](#selection_selectAll)), then *data* should typically be specified as a function. This function will be evaluated for each group in order, being passed the group’s parent datum (*d*, which may be undefined), the group index (*i*), and the selection’s parent nodes (*nodes*), with *this* as the group’s parent element. For example, to create an HTML table from a matrix of numbers:

```js
var matrix = [
  [11975,  5871, 8916, 2868],
  [ 1951, 10048, 2060, 6171],
  [ 8010, 16145, 8090, 8045],
  [ 1013,   990,  940, 6907]
];

var tr = d3.select("body")
  .append("table")
  .selectAll("tr")
  .data(matrix)
  .enter().append("tr");

var td = tr.selectAll("td")
  .data(function(d) { return d; })
  .enter().append("td")
    .text(function(d) { return d; });
```

In this example the *data* function is the identity function: for each table row, it returns the corresponding row from the data matrix.

If a *key* function is not specified, then the first datum in *data* is assigned to the first selected element, the second datum to the second selected element, and so on. A *key* function may be specified to control which datum is assigned to which element, replacing the default join-by-index. This key function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The key function is then also evaluated for each new datum in *data*, being passed the current datum (*d*), the current index (*i*), and the group’s new *data*, with *this* as the group’s parent DOM element. The datum for a given key is assigned to the element with the matching key. If multiple elements have the same key, the duplicate elements are put into the exit selection; if multiple data have the same key, the duplicate data are put into the enter selection.

For example, given this document:

```html
<div id="Ford"></div>
<div id="Jarrah"></div>
<div id="Kwon"></div>
<div id="Locke"></div>
<div id="Reyes"></div>
<div id="Shephard"></div>
```

You could join data by key as follows:


```js
var data = [
  {name: "Locke", number: 4},
  {name: "Reyes", number: 8},
  {name: "Ford", number: 15},
  {name: "Jarrah", number: 16},
  {name: "Shephard", number: 31},
  {name: "Kwon", number: 34}
];

d3.selectAll("div")
  .data(data, function(d) { return d ? d.name : this.id; })
    .text(function(d) { return d.number; });
```

This example key function uses the datum *d* if present, and otherwise falls back to the element’s id property. Since these elements were not previously bound to data, the datum *d* is null when the key function is evaluated on selected elements, and non-null when the key function is evaluated on the new data.

The *update* and *enter* selections are returned in data order, while the *exit* selection preserves the selection order prior to the join. If a key function is specified, the order of elements in the selection may not match their order in the document; use [*selection*.order](#order) or [*selection*.sort](#sort) as needed. For more on how the key function affects the join, see [A Bar Chart, Part 2](http://bost.ocks.org/mike/bar/2/) and [Object Constancy](http://bost.ocks.org/mike/constancy/).

If *data* is not specified, this method returns the array of data for the selected elements.

This method cannot be used to clear bound data; use [*selection*.datum](#selection_datum) instead.

<a name="selection_enter" href="#selection_enter">#</a> <i>selection</i>.<b>enter</b>()

Returns the enter selection: placeholder nodes for each datum that had no corresponding DOM element in the selection. The enter selection is determined by [*selection*.data](#selection_data), and is empty on a selection that is not joined to data.

The enter selection is typically used to create “missing” elements corresponding to new data. For example, to create DIV elements from an array of numbers:

```js
var div = d3.select("body")
  .selectAll("div")
  .data([4, 8, 15, 16, 23, 42])
  .enter().append("div")
    .text(function(d) { return d; });
```

If the body is initially empty, the above code will create six new DIV elements, append them to the body in-order, and assign their text content as the associated (string-coerced) number:

```html
<div>4</div>
<div>8</div>
<div>15</div>
<div>16</div>
<div>23</div>
<div>42</div>
```

Conceptually, the enter selection’s placeholders are pointers to the parent element (in this example, the document body). The enter selection is typically only used transiently to append elements, and is often [merged](#selection_merge) with the update selection after appending, such that modifications can be applied to both entering and updating elements.

<a name="selection_exit" href="#selection_exit">#</a> <i>selection</i>.<b>exit</b>()

Returns the exit selection: existing DOM elements in the selection for which no new datum was found. The exit selection is determined by the previous [*selection*.data](#selection_data), and is thus empty until the selection is joined to data. If the exit selection is retrieved more than once after a data join, subsequent calls return the empty selection.

The exit selection is typically used to remove “superfluous” elements corresponding to old data. For example, to update the DIV elements created previously with a new array of numbers:

```js
div = div.data([1, 2, 4, 8, 16, 32], function(d) { return d; });
```

Since a key function was specified (as the identity function), and the new data contains the numbers [4, 8, 16] which match existing elements in the document, the update selection contains three DIV elements. Leaving those elements as-is, we can append new elements for [1, 2, 32] using the enter selection:

```js
div.enter().append("div").text(function(d) { return d; });
```

Likewise, to remove the exiting elements [15, 23, 42]:

```js
div.exit().remove();
```

Now the document body looks like this:

```html
<div>1</div>
<div>2</div>
<div>4</div>
<div>8</div>
<div>16</div>
<div>32</div>
```

The order of the DOM elements matches the order of the data because the old data’s order and the new data’s order were consistent. If the new data’s order is different, use [*selection*.order](#selection_order) to reorder the elements in the DOM. See the [General Update Pattern](http://bl.ocks.org/mbostock/3808218) example thread for more on data joins.

<a name="selection_datum" href="#selection_datum">#</a> <i>selection</i>.<b>datum</b>([<i>value</i>])

Gets or sets the bound data for each selected element. Unlike [*selection*.data](#selection_data), this method does not compute a join and does not affect indexes or the enter and exit selections.

If a *value* is specified, sets the element’s bound data to the specified value on all selected elements. If the *value* is a constant, all elements are given the same datum; otherwise, if the *value* is a function, then the function is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. The function is then used to set each element’s new data. A null value will delete the bound data.

If a *value* is not specified, returns the bound datum for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element.

This method is useful for accessing HTML5 [custom data attributes](http://www.w3.org/TR/html5/dom.html#custom-data-attribute). For example, given the following elements:

```html
<ul id="list">
  <li data-username="shawnbot">Shawn Allen</li>
  <li data-username="mbostock">Mike Bostock</li>
</ul>
```

You can expose the custom data attributes by setting each element’s data as the built-in [dataset](http://www.w3.org/TR/html5/dom.html#dom-dataset) property:

```js
selection.datum(function() { return this.dataset; })
```

### Handling Events

For interaction, selections allow listening for and dispatching of events.

<a name="selection_on" href="#selection_on">#</a> <i>selection</i>.<b>on</b>(<i>typenames</i>[, <i>listener</i>[, <i>capture</i>]])

Adds or removes a *listener* to each selected element for the specified event *typenames*. The *typenames* is a string event type, such as `click`, `mouseover`, or `submit`; any [DOM event type](https://developer.mozilla.org/en-US/docs/Web/Events#Standard_events) supported by your browser may be used. The type may be optionally followed by a period (`.`) and a name; the optional name allows multiple callbacks to be registered to receive events of the same type, such as `click.foo` and `click.bar`. To specify multiple typenames, separate typenames with spaces, such as `input change` or `click.foo click.bar`.

When a specified event is dispatched on a selected node, the specified *listener* will be evaluated for each selected element, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. Listeners always see the latest datum for their element, but the index is a property of the selection and is fixed when the listener is assigned; to update the index, re-assign the listener. To access the current event within a listener, use [d3.event](#event).

If an event listener was previously registered for the same *typename* on a selected element, the old listener is removed before the new listener is added. To remove a listener, pass null as the *listener*. To remove all listeners for a given name, pass null as the *listener* and `.foo` as the *typename*, where `foo` is the name; to remove all listeners with no name, specify `.` as the *typename*.

An optional *capture* flag may be specified which corresponds to the W3C [useCapture flag](http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-registration): “After initiating capture, all events of the specified type will be dispatched to the registered EventListener before being dispatched to any EventTargets beneath them in the tree. Events which are bubbling upward through the tree will not trigger an EventListener designated to use capture.”

If a *listener* is not specified, returns the currently-assigned listener for the specified event *typename* on the first (non-null) selected element, if any. If multiple typenames are specified, the first matching listener is returned.

<a name="selection_dispatch" href="#selection_dispatch">#</a> <i>selection</i>.<b>dispatch</b>(<i>type</i>[, <i>parameters</i>])

Dispatches a [custom event](http://www.w3.org/TR/dom/#interface-customevent) of the specified *type* to each selected element, in order. An optional *parameters* map may be specified to set additional properties of the event. It may contain the following fields:

* [`bubbles`](https://www.w3.org/TR/dom/#dom-event-bubbles) - if true, the event is dispatched to ancestors in reverse tree order.
* [`cancelable`](https://www.w3.org/TR/dom/#dom-event-cancelable) - if true, *event*.preventDefault is allowed.
* [`detail`](https://www.w3.org/TR/dom/#dom-customevent-detail) - any custom data associated with the event.

If *parameters* is a function, it is evaluated for each selected element, in order, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. It must return the parameters map for the current element.

<a name="event" href="#event">#</a> d3.<b>event</b>

The current [event](https://developer.mozilla.org/en-US/docs/DOM/event), if any. This is set during the invocation of an event listener, and is reset after the listener terminates. Use this to access standard event fields such as [*event*.timeStamp](https://www.w3.org/TR/dom/#dom-event-timestamp) and methods such as [*event*.preventDefault](https://www.w3.org/TR/dom/#dom-event-preventdefault). While you can use the native [*event*.pageX](https://developer.mozilla.org/en/DOM/event.pageX) and [*event*.pageY](https://developer.mozilla.org/en/DOM/event.pageY), it is often more convenient to transform the event position to the local coordinate system of the container that received the event using [d3.mouse](#mouse), [d3.touch](#touch) or [d3.touches](#touches).

If you use Babel, Webpack, or another ES6-to-ES5 bundler, be aware that the value of d3.event changes during an event! An import of d3.event must be a [live binding](http://www.2ality.com/2015/07/es6-module-exports.html), so you may need to configure the bundler to import from D3’s ES6 modules rather than from the generated UMD bundle; not all bundlers observe [jsnext:main](https://github.com/rollup/rollup/wiki/jsnext:main). Also beware of conflicts with the [*window*.event](https://developer.mozilla.org/en-US/docs/Web/API/Window/event) global.

<a name="customEvent" href="#customEvent">#</a> d3.<b>customEvent</b>(<i>event</i>, <i>listener</i>[, <i>that</i>[, <i>arguments</i>]])

Invokes the specified *listener*, using the specified *that* `this` context and passing the specified *arguments*, if any. During the invocation, [d3.event](#event) is set to the specified *event*; after the listener returns (or throws an error), d3.event is restored to its previous value. In addition, sets *event*.sourceEvent to the prior value of d3.event, allowing custom events to retain a reference to the originating native event. Returns the value returned by the *listener*.

<a name="mouse" href="#mouse">#</a> d3.<b>mouse</b>(<i>container</i>)

Returns the *x* and *y* coordinates of the [current event](#event) relative to the specified *container*. The container may be an HTML or SVG container element, such as a [G element](http://www.w3.org/TR/SVG/struct.html#Groups) or an [SVG element](http://www.w3.org/TR/SVG/struct.html#SVGElement). The coordinates are returned as a two-element array of numbers [*x*, *y*].

<a name="touch" href="#touch">#</a> d3.<b>touch</b>(<i>container</i>[, <i>touches</i>], <i>identifier</i>)

Returns the *x* and *y* coordinates of the touch with the specified *identifier* associated with the [current event](#event) relative to the specified *container*. The container may be an HTML or SVG container element, such as a [G element](http://www.w3.org/TR/SVG/struct.html#Groups) or an [SVG element](http://www.w3.org/TR/SVG/struct.html#SVGElement). The coordinates are returned as an array of two-element arrays of numbers \[\[*x1*, *y1*], [*x2*, *y2*], …\]. If there is no touch with the specified identifier in *touches*, returns null; this can be useful for ignoring touchmove events where the only some touches have moved. If *touches* is not specified, it defaults to the current event’s [changedTouches](http://developer.apple.com/library/safari/documentation/UserExperience/Reference/TouchEventClassReference/TouchEvent/TouchEvent.html#//apple_ref/javascript/instp/TouchEvent/changedTouches) property.

<a name="touches" href="#touches">#</a> d3.<b>touches</b>(<i>container</i>[, <i>touches</i>])

Returns the *x* and *y* coordinates of the touches associated with the [current event](#event) relative to the specified *container*. The container may be an HTML or SVG container element, such as a [G element](http://www.w3.org/TR/SVG/struct.html#Groups) or an [SVG element](http://www.w3.org/TR/SVG/struct.html#SVGElement). The coordinates are returned as an array of two-element arrays of numbers \[\[*x1*, *y1*], [*x2*, *y2*], …\]. If *touches* is not specified, it defaults to the current event’s [touches](http://developer.apple.com/library/safari/documentation/UserExperience/Reference/TouchEventClassReference/TouchEvent/TouchEvent.html#//apple_ref/javascript/instp/TouchEvent/touches) property.

### Control Flow

For advanced usage, selections provide methods for custom control flow.

<a name="selection_each" href="#selection_each">#</a> <i>selection</i>.<b>each</b>(<i>function</i>)

Invokes the specified *function* for each selected element, passing in passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* of the current DOM element. This method can be used to invoke arbitrary code for each selected element, and is useful for creating a context to access parent and child data simultaneously, such as:

```js
parent.each(function(p, j) {
  d3.select(this)
    .selectAll(".child")
      .text(function(d, i) { return "child " + d.name + " of " + p.name; });
});
```

See [Sized Donut Multiples](http://bl.ocks.org/mbostock/4c5fad723c87d2fd8273) for an example.

<a name="selection_call" href="#selection_call">#</a> <i>selection</i>.<b>call</b>(<i>function</i>[, <i>arguments…</i>])

Invokes the specified *function* exactly once, passing in this selection along with any optional *arguments*. Returns this selection. This is equivalent to invoking the function by hand but facilitates method chaining. For example, to set several styles in a reusable function:

```javascript
function name(selection, first, last) {
  selection
      .attr("first-name", first)
      .attr("last-name", last);
}
```

Now say:

```javascript
d3.selectAll("div").call(name, "John", "Snow");
```

This is equivalent to:

```javascript
name(d3.selectAll("div"), "John", "Snow");
```

<a name="selection_empty" href="#selection_empty">#</a> <i>selection</i>.<b>empty</b>()

Returns true if this selection contains no (non-null) elements.

<a name="selection_nodes" href="#selection_nodes">#</a> <i>selection</i>.<b>nodes</b>()

Returns an array of all (non-null) elements in this selection.

<a name="selection_node" href="#selection_node">#</a> <i>selection</i>.<b>node</b>()

Returns the first (non-null) element in this selection. If the selection is empty, returns null.

<a name="selection_size" href="#selection_size">#</a> <i>selection</i>.<b>size</b>()

Returns the total number of elements in this selection.

### Local Variables

D3 locals allow you to define local state independent of data. For instance, when rendering [small multiples](http://bl.ocks.org/mbostock/e1192fe405703d8321a5187350910e08) of time-series data, you might want the same *x*-scale for all charts but distinct *y*-scales to compare the relative performance of each metric. D3 locals are scoped by DOM elements: on set, the value is stored on the given element; on get, the value is retrieved from given element or the nearest ancestor that defines it.

<a name="local" href="#local">#</a> d3.<b>local</b>()

Declares a new local variable. For example:

```js
var foo = d3.local();
```

Like `var`, each local is a distinct symbolic reference; unlike `var`, the value of each local is also scoped by the DOM.

<a name="local_set" href="#local_set">#</a> <i>local</i>.<b>set</b>(<i>node</i>, <i>value</i>)

Sets the value of this local on the specified *node* to the *value*, and returns the specified *value*. This is often performed using [*selection*.each](#selection_each):

```js
selection.each(function(d) { foo.set(this, d.value); });
```

If you are just setting a single variable, consider using [*selection*.property](#selection_property):

```js
selection.property(foo, function(d) { return d.value; });
```

<a name="local_get" href="#local_get">#</a> <i>local</i>.<b>get</b>(<i>node</i>)

Returns the value of this local on the specified *node*. If the *node* does not define this local, returns the value from the nearest ancestor that defines it. Returns undefined if no ancestor defines this local.

<a name="local_remove" href="#local_remove">#</a> <i>local</i>.<b>remove</b>(<i>node</i>)

Deletes this local’s value from the specified *node* and returns its previous value. Returns true if the *node* defined this local prior to removal, and false otherwise. If ancestors also define this local, those definitions are unaffected, and thus [*local*.get](#local_get) will still return the inherited value.

<a name="local_toString" href="#local_toString">#</a> <i>local</i>.<b>toString</b>()

Returns the automatically-generated identifier for this local. This is the name of the property that is used to store the local’s value on elements, and thus you can also set or get the local’s value using *element*[*local*] or by using [*selection*.property](#selection_property).

### Namespaces

XML namespaces are fun! Right? Fortunately you can mostly ignore them.

<a name="namespace" href="#namespace">#</a> d3.<b>namespace</b>(<i>name</i>)

Qualifies the specified *name*, which may or may not have a namespace prefix. If the name contains a colon (`:`), the substring before the colon is interpreted as the namespace prefix, which must be registered in [d3.namespaces](#namespaces). Returns an object `space` and `local` attributes describing the full namespace URL and the local name. For example:

```js
d3.namespace("svg:text"); // {space: "http://www.w3.org/2000/svg", local: "text"}
```

If the name does not contain a colon, this function merely returns the input name.

<a name="namespaces" href="#namespaces">#</a> d3.<b>namespaces</b>

The map of registered namespace prefixes. The initial value is:

```js
{
  svg: "http://www.w3.org/2000/svg",
  xhtml: "http://www.w3.org/1999/xhtml",
  xlink: "http://www.w3.org/1999/xlink",
  xml: "http://www.w3.org/XML/1998/namespace",
  xmlns: "http://www.w3.org/2000/xmlns/"
}
```

Additional prefixes may be assigned as needed to create elements or attributes in other namespaces.