Stage 1 Draft / October 13, 2021

ES Array.fromAsync (2021)

Introduction

This is the formal specification for a proposed Array.fromAsync factory method in JavaScript. It modifies the original ECMAScript specification with several new or revised clauses. See the proposal's explainer for the proposal's background, motivation, and usage examples.

1 Indexed Collections

1.1 Array Objects

1.1.1 Properties of the Array Constructor

1.1.1.1 Array.fromAsync ( asyncItems [ , mapfn [ , thisArg ] ] )

Editor's Note

This section is a wholly new subsection of the original Properties of the Array Constructor clause, to be inserted before the Array.from clause.

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

  1. Let C be the this value.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let fromAsyncClosure be a new Abstract Closure with no parameters that captures C and mapfn and performs the following steps when called:
    1. If mapfn is undefined, let mapping be false.
    2. Else,
      1. If IsCallable(mapfn) is false, throw a TypeError exception.
      2. Let mapping be true.
    3. Let usingAsyncIterator be ? GetMethod(asyncItems, @@asyncIterator).
    4. If usingAsyncIterator is undefined,
      1. Let usingSyncIterator be ? GetMethod(asyncItems, @@iterator).
      2. If usingSyncIterator is undefined, set usingSyncIterator to %Array.prototype.values%.
    5. If IsConstructor(C) is true, then
      1. Let A be ? Construct(C).
    6. Else,
      1. Let A be ! ArrayCreate(0).
    7. If usingAsyncIterator is not undefined, then
      1. Let iteratorRecord be ? GetIterator(asyncItems, async, usingAsyncIterator).
    8. Else,
      1. Let iteratorRecord be ? CreateAsyncFromSyncIterator(GetIterator(syncItems, sync, usingSyncIterator)).
    9. Let k be 0.
    10. Repeat,
      1. If k ≥ 253 - 1, then
        1. Let error be ThrowCompletion(a newly created TypeError object).
        2. Return ? AsyncIteratorClose(iteratorRecord, error).
      2. Let Pk be ! ToString(𝔽(k)).
      3. Let next be ? Await(IteratorStep(iteratorRecord)).
      4. If next is false, then
        1. Perform ? Set(A, "length", 𝔽(k), true).
        2. Return A.
      5. Let nextValue be ? IteratorValue(next).
      6. If mapping is true, then
        1. Let mappedValue be Call(mapfn, thisArg, « nextValue, 𝔽(k) »).
        2. IfAbruptCloseAsyncIterator(iteratorRecord, mappedValue).
        3. Let mappedValue to Await(mappedValue).
        4. IfAbruptCloseAsyncIterator(iteratorRecord, mappedValue).
        5. Set mappedValue to mappedValue.[[Value]].
      7. Else, let mappedValue be nextValue.
      8. Let defineStatus be CreateDataPropertyOrThrow(A, Pk, mappedValue).
      9. If defineStatus is an abrupt completion, return ? AsyncIteratorClose(iteratorRecord, defineStatus).
      10. Set k to k + 1.
  4. Perform AsyncFunctionStart(promiseCapability, fromAsyncClosure).
  5. Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.
 Note

The fromAsync function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.

1.2 TypedArray Objects

1.2.1 Properties of the %TypedArray% Intrinsic Object

1.2.1.1 %TypedArray%.fromAsync ( source [ , mapfn [ , thisArg ] ] )

Editor's Note

This section is a wholly new subsection of the original Properties of the %TypedArray% Intrinsic Object clause, to be inserted before the %TypedArray%.from clause.

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

  1. Let C be the this value.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let fromAsyncClosure be a new Abstract Closure with no parameters that captures C and mapfn and performs the following steps when called:
    1. If IsConstructor(C) is false, throw a TypeError exception.
    2. If mapfn is undefined, let mapping be false.
    3. Else,
      1. If IsCallable(mapfn) is false, throw a TypeError exception.
      2. Let mapping be true.
    4. Let usingIterator be ? GetMethod(source, @@iterator).
    5. If usingIterator is not undefined, then
      1. Let values be ? IterableToList(source, usingIterator).
      2. Let len be the number of elements in values.
      3. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
      4. Let k be 0.
      5. Repeat, while k < len,
        1. Let Pk be ! ToString(𝔽(k)).
        2. Let kValue be the first element of values and remove that element from values.
        3. If mapping is true, then
          1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
        4. Else, let mappedValue be kValue.
        5. Perform ? Set(targetObj, Pk, mappedValue, true).
        6. Set k to k + 1.
      6. Assert: values is now an empty List.
      7. Return targetObj.
    6. NOTE: source is not an Iterable so assume it is already an array-like object.
    7. Let arrayLike be ! ToObject(source).
    8. Let len be ? LengthOfArrayLike(arrayLike).
    9. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
    10. Let k be 0.
    11. Repeat, while k < len,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Let kValue be ? Get(arrayLike, Pk).
      3. If mapping is true, then
        1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
      4. Else, let mappedValue be kValue.
      5. Perform ? Set(targetObj, Pk, mappedValue, true).
      6. Set k to k + 1.
    12. Return targetObj.
  4. Perform AsyncFunctionStart(promiseCapability, fromAsyncClosure).
  5. Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

A Copyright & Software License

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© 2021 J. S. Choi, Ecma International

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