@function can-stache-bindings.twoWay key:bind
@parent can-stache-bindings.syntaxes

@description Two-way bind a value in the [can-stache-element StacheElement], [can-component.prototype.ViewModel can-component ViewModel], or element to the parent scope.

@signature `childProp:bind="key"`

  Two-way binds `childProp` in the  [can-stache-element StacheElement] or [can-component.prototype.ViewModel can-component ViewModel] to
  [can-stache.key] in the parent [can-view-scope scope].  If `childProp` is updated `key` will be updated
  and vice-versa.

  ```html
  <my-element someProp:bind="value" />
  ```

  When setting up the binding:

  - If `childProp` is `undefined`, `key` will be set to `childProp`.
  - If `key` is `undefined`, `childProp` will be set to `key`.
  - If both `childProp` and `key` are defined, `key` will be set to `childProp`.



  @param {String} childProp The name of the property of the [can-stache-element StacheElement] or [can-component.prototype.ViewModel can-component ViewModel] to two-way bind.

  @param {can-stache/expressions/literal|can-stache/expressions/key-lookup|can-stache/expressions/call|can-stache/expressions/helper} key A call expression whose value will be used to two-way bind in the parent scope.

@signature `child-prop:bind="key"`

  Two-way binds the element’s `child-prop` property or attribute to
  [can-stache.key] in the parent [can-view-scope scope].  If `child-prop` is updated, `key` will be updated
  and vice-versa.

  ```html
  <input value:bind="name" />
  ```

  @param {String} child-prop The name of the element’s property or attribute to two-way bind.

  @param {can-stache/expressions/literal|can-stache/expressions/key-lookup|can-stache/expressions/call|can-stache/expressions/helper} key A call expression whose value will be used to two-way bind in the parent scope.

@signature `vm:childProp:bind="key"`

  Two-way binds `childProp` in the  [can-stache-element StacheElement] or [can-component.prototype.ViewModel can-component ViewModel] to
  [can-stache.key] in the parent [can-view-scope scope].  If `childProp` is updated, `key` will be updated
  and vice-versa.

  ```html
  <my-element vm:someProp:bind="value" />
  ```

  When setting up the binding:

  - If `childProp` is `undefined`, `key` will be set to `childProp`.
  - If `key` is `undefined`, `childProp` will be set to `key`.
  - If both `childProp` and `key` are defined, `key` will be set to `childProp`.



Parameters are the same as [can-stache-bindings.twoWay#childProp_bind__key_ childProp:bind="key"]

@signature `el:child-prop:bind="key"`

  Two-way binds the element’s `child-prop` property or attribute to
  [can-stache.key] in the parent [can-view-scope scope].  If `child-prop` is updated, `key` will be updated
  and vice-versa.

  ```html
  <input el:value:bind="name" />
  ```

Parameters are the same as [can-stache-bindings.twoWay#child_prop_bind__key_ child-prop:bind="key"]

@body

## Initialization

When a binding is being initialized, the behavior of what the component and scope properties
are set to depends on their initial values:

- If the component’s property value is **not** `undefined` and the scope’s property is `undefined`, scope will be set to the component’s property value.
- If the component’s property value is `undefined` and the scope’s property is **not** `undefined`, component’s property will be set to the scope value.
- If both the component and scope properties are **not** `undefined`, component’s property will be set to the scope value.

## Use

See [can-stache-bindings#Keepaparentandchildinsync Keep a parent and child in sync].