Stage 3 Draft / January 4, 2019

Array.prototype.flatMap & Array.prototype.flat

Introduction

Array.prototype.flat returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Array.prototype.flatMap first maps each element using a mapping function, then flattens the result into a new array. It is identical to a map followed by a flatten of depth 1, but flatMap is quite often useful and merging both into one method is slightly more efficient.

1Array.prototype.flatMap ( mapperFunction [ , thisArg ] )

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

  1. Let O be ? ToObject(this value).
  2. Let sourceLen be ? ToLength(? Get(O, "length")).
  3. If IsCallable(mapperFunction) is false, throw a TypeError exception.
  4. If thisArg is present, let T be thisArg; else let T be undefined.
  5. Let A be ? ArraySpeciesCreate(O, 0).
  6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction, T).
  7. Return A.

2Array.prototype.flat( [ depth ] )

When the flat method is called with zero or one arguments, the following steps are taken:

  1. Let O be ? ToObject(this value).
  2. Let sourceLen be ? ToLength(? Get(O, "length")).
  3. Let depthNum be 1.
  4. If depth is not undefined, then
    1. Set depthNum to ? ToInteger(depth).
  5. Let A be ? ArraySpeciesCreate(O, 0).
  6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
  7. Return A.

3FlattenIntoArray(target, source, sourceLen, start, depth [ , mapperFunction, thisArg ])

  1. Let targetIndex be start.
  2. Let sourceIndex be 0.
  3. Repeat, while sourceIndex < sourceLen
    1. Let P be ! ToString(sourceIndex).
    2. Let exists be ? HasProperty(source, P).
    3. If exists is true, then
      1. Let element be ? Get(source, P).
      2. If mapperFunction is present, then
        1. Assert: thisArg is present.
        2. Set element to ? Call(mapperFunction, thisArg , « element, sourceIndex, source »).
      3. Let shouldFlatten be false.
      4. If depth > 0, then
        1. Set shouldFlatten to ? IsArray(element).
      5. If shouldFlatten is true, then
        1. Let elementLen be ? ToLength(? Get(element, "length")).
        2. Set targetIndex to ? FlattenIntoArray(target, element, elementLen, targetIndex, depth - 1).
      6. Else,
        1. If targetIndex ≥ 253-1, throw a TypeError exception.
        2. Perform ? CreateDataPropertyOrThrow(target, ! ToString(targetIndex), element).
        3. Increase targetIndex by 1.
    4. Increase sourceIndex by 1.
  4. Return targetIndex.

4Array.prototype [ @@unscopables ]

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

  1. Let unscopableList be ObjectCreate(null).
  2. Perform CreateDataProperty(unscopableList, "copyWithin", true).
  3. Perform CreateDataProperty(unscopableList, "entries", true).
  4. Perform CreateDataProperty(unscopableList, "fill", true).
  5. Perform CreateDataProperty(unscopableList, "find", true).
  6. Perform CreateDataProperty(unscopableList, "findIndex", true).
  7. Perform CreateDataProperty(unscopableList, "flat", true).
  8. Perform CreateDataProperty(unscopableList, "flatMap", true).
  9. Perform CreateDataProperty(unscopableList, "includes", true).
  10. Perform CreateDataProperty(unscopableList, "keys", true).
  11. Perform CreateDataProperty(unscopableList, "values", true).
  12. Assert: Each of the above calls will returns true.
  13. Return unscopableList.

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

Note

The own property names of this object are property names that were not included as standard properties of Array.prototype prior to in earlier editions of the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.

ACopyright & Software License

Copyright Notice

© 2019 Michael Ficarra, Brian Terlson, and Mathias Bynens

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