Stage 1 Draft / July 15, 2021

Array Filtering

1 Scope

This is the spec text of the Array Filtering proposal in ECMAScript.

2 Properties of the Array Prototype Object (22.1.3)

2.1 Array.prototype.filterReject ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filterReject calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filterReject does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filterReject is set before the first call to callbackfn. Elements which are appended to the array after the call to filterReject begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filterReject visits them; elements that are deleted after the call to filterReject begins and before being visited are not visited.

When the filterReject method is called with one or two arguments, the following steps are taken:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let A be ? ArraySpeciesCreate(O, 0).
  5. Let k be 0.
  6. Let to be 0.
  7. Repeat, while k < len
    1. Let Pk be ! ToString(k).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Let rejected be ! ToBoolean(? Call(callbackfn, thisArg, « kValue, k, O »)).
      3. If rejected is false, then
        1. Perform ? CreateDataPropertyOrThrow(A, ! ToString(to), kValue).
        2. Set to to to + 1.
    4. Set k to k + 1.
  8. Return A.
 Note 2

The filterReject function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

A Copyright & Software License

Copyright Notice

© 2021 Justin Ridgewell

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