PR #6

This document is a preview of merging PR #6 as commit 64a5844fd24e4580fc77643a04e9db67fd74fde2.

Do not reference it as authoritative in any way. Instead, see the living specification at https://github.com/tc39/proposal-typedarray-concat.

Stage 1 Draft / February 4, 2026

TypedArray.concat

1 TypedArray Objects

1.1 The %TypedArray% Intrinsic Object

1.1.1 Properties of the %TypedArray% Intrinsic Object

1.1.1.1 %TypedArray%.concat ( items [ , length ] )

This method concatenates the elements of multiple TypedArrays of the same type into a new TypedArray. It performs the following steps when called:

  1. Let C be the this value.
  2. If IsConstructor(C) is false, throw a TypeError exception.
  3. If C does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
  4. Let arrayList be ? IteratorToList(? GetIteratorFromMethod(items, ? GetMethod(items, %Symbol.iterator%))).
  5. Let totalLength be 0.
  6. Let elementType be TypedArrayElementType(C).
  7. For each element item of arrayList, do
    1. Let taRecord be ? ValidateTypedArray(item, seq-cst).
    2. If TypedArrayElementType(item) is not elementType, throw a TypeError exception.
    3. Let itemLength be TypedArrayLength(taRecord).
    4. Set totalLength to totalLength + itemLength.
  8. If length is present and length is not undefined, then
    1. Let newLength be ? ToIndex(length).
    2. If newLength > totalLength, throw a RangeError exception.
  9. Else,
    1. Let newLength be totalLength.
  10. Let A be a TypedArray that is an instance of C with a length of newLength, containing the concatenated elements from arrayList in order, truncated to newLength elements.
  11. Return A.
 Note

This method requires that the this value be a TypedArray constructor and that all elements of items be TypedArrays of the same type. For example, Uint8Array.concat([a, b]) requires that both a and b are Uint8Array instances.

If the optional length parameter is provided and is less than the sum of the lengths of all input TypedArrays, the result is truncated to the specified length. If length is greater than the total length of input arrays, a RangeError is thrown.

Implementations may use any technique to allocate and populate the result TypedArray, such as direct memory copy, deferred concatenation, or element-by-element copying, provided the observable result is a TypedArray containing the concatenated elements in order.

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