vega-transforms/src/Extent.js

import {Transform} from 'vega-dataflow';
import {accessorName, inherits} from 'vega-util';

/**
 * Computes extents (min/max) for a data field.
 * @constructor
 * @param {object} params - The parameters for this operator.
 * @param {function(object): *} params.field - The field over which to compute extends.
 */
export default function Extent(params) {
  Transform.call(this, [undefined, undefined], params);
}

Extent.Definition = {
  'type': 'Extent',
  'metadata': {},
  'params': [
    { 'name': 'field', 'type': 'field', 'required': true }
  ]
};

var prototype = inherits(Extent, Transform);

prototype.transform = function(_, pulse) {
  var extent = this.value,
      field = _.field,
      min = extent[0],
      max = extent[1],
      mod;

  mod = pulse.changed()
     || pulse.modified(field.fields)
     || _.modified('field');

  if (mod || min == null) {
    min = +Infinity;
    max = -Infinity;
  }

  pulse.visit(mod ? pulse.SOURCE : pulse.ADD, function(t) {
    var v = field(t);
    if (v != null) {
      // coerce to number
      v = +v;
      // NaNs will fail all comparisons!
      if (v < min) min = v;
      if (v > max) max = v;
    }
  });

  if (!Number.isFinite(min) || !Number.isFinite(max)) {
    let name = accessorName(field);
    if (name) name = ` for field "${name}"`;
    pulse.dataflow.warn(`Infinite extent${name}: [${min}, ${max}]`);
    min = max = undefined;
  }
  this.value = [min, max];
};