Stage 2 Draft / August 2, 2021

Proposal-array-find-from-last

1 Array.prototype.findLast ( predicate [ , thisArg ] )

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. findLast calls predicate once for each element of the array, in descending order, until it finds one where predicate returns true. If such an element is found, findLast immediately returns that element value. Otherwise, findLast returns undefined.

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

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

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

The range of elements processed by findLast is set before the first call to predicate. Elements that are appended to the array after the call to findLast begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findLast visits them.

When the findLast method is called, the following steps are taken:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(predicate) is false, throw a TypeError exception.
  4. Let k be len - 1.
  5. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(O, Pk).
    3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is true, return kValue.
    5. Set k to k - 1.
  6. Return undefined.
Note 2

The findLast 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.

2 Array.prototype.findLastIndex ( predicate [ , thisArg ] )

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. findLastIndex calls predicate once for each element of the array, in descending order, until it finds one where predicate returns true. If such an element is found, findLastIndex immediately returns the index of that element value. Otherwise, findLastIndex returns -1.

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

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

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

The range of elements processed by findLastIndex is set before the first call to predicate. Elements that are appended to the array after the call to findLastIndex begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findLastIndex visits them.

When the findLastIndex method is called, the following steps are taken:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(predicate) is false, throw a TypeError exception.
  4. Let k be len - 1.
  5. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(O, Pk).
    3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is true, return 𝔽(k).
    5. Set k to k - 1.
  6. Return -1𝔽.
Note 2

The findLastIndex 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.

3 Array.prototype [ @@unscopables ]

The initial value of the @@unscopables data property is an object created by the following steps:

  1. Let unscopableList be OrdinaryObjectCreate(null).
  2. Perform ! CreateDataPropertyOrThrow(unscopableList, "copyWithin", true).
  3. Perform ! CreateDataPropertyOrThrow(unscopableList, "entries", true).
  4. Perform ! CreateDataPropertyOrThrow(unscopableList, "fill", true).
  5. Perform ! CreateDataPropertyOrThrow(unscopableList, "find", true).
  6. Perform ! CreateDataPropertyOrThrow(unscopableList, "findIndex", true).
  7. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLast", true).
  8. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLastIndex", true).
  9. Perform ! CreateDataPropertyOrThrow(unscopableList, "flat", true).
  10. Perform ! CreateDataPropertyOrThrow(unscopableList, "flatMap", true).
  11. Perform ! CreateDataPropertyOrThrow(unscopableList, "includes", true).
  12. Perform ! CreateDataPropertyOrThrow(unscopableList, "keys", true).
  13. Perform ! CreateDataPropertyOrThrow(unscopableList, "values", true).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

4 %TypedArray%.prototype.findLast ( predicate [ , thisArg ] )

The interpretation and use of the arguments of %TypedArray%.prototype.findLast are the same as for Array.prototype.findLast as defined in 1.

When the findLast method is called, the following steps are taken:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(predicate) is false, throw a TypeError exception.
  5. Let k be len - 1.
  6. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is true, return kValue.
    5. Set k to k - 1.
  7. Return undefined.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

5 %TypedArray%.prototype.findLastIndex ( predicate [ , thisArg ] )

The interpretation and use of the arguments of %TypedArray%.prototype.findLastIndex are the same as for Array.prototype.findLastIndex as defined in 2.

When the findLastIndex method is called, the following steps are taken:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(predicate) is false, throw a TypeError exception.
  5. Let k be len - 1.
  6. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is true, return 𝔽(k).
    5. Set k to k - 1.
  7. Return -1𝔽.

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

A Copyright & Software License

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© 2021 Wenlu Wang, Daniel Rosenwasser

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