Array.prototype.copyWithin()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

copyWithin() 方法會對陣列的一部分進行淺拷貝至同一陣列的另一位置並回傳此陣列，而不修改其大小。

嘗試一下

語法

arr.copyWithin(target)
arr.copyWithin(target, start)
arr.copyWithin(target, start, end)

參數

target

要複製序列（sequence）至該位置的索引（起始為 0）。若為負數，target 將會自陣列末項開始計算。假如 target 大於等於 arr.length，則沒有項目會被複製。如果 target 的索引在 start 之後，則拷貝的序列將會被修剪以符合 arr.length。

start 選擇性

開始拷貝的起始元素索引（起始為 0）。若為負數，start 將會自陣列末項開始計算。如果省略 start，copyWithin 將會自陣列首項開始複製（預設為 0）。

end 選擇性

結束拷貝的結尾元素索引（起始為 0）。copyWithin 會拷貝至此索引，但不包含 end。若為負數，end 將會自陣列末項開始計算。如果省略 end，copyWithin 將會一路拷貝至陣列末項（預設至 arr.length）。

回傳值

被修改後的陣列。

描述

The copyWithin works like C and C++'s memmove, and is a high-performance method to shift the data of an Array. This especially applies to the TypedArray method of the same name. The sequence is copied and pasted as one operation; pasted sequence will have the copied values even when the copy and paste region overlap.

The copyWithin function is intentionally generic, it does not require that its this value be an Array object.

The copyWithin method is a mutable method. It does not alter the length of this, but will change its content and create new properties if necessary.

範例

js
[1, 2, 3, 4, 5].copyWithin(-2);
// [1, 2, 3, 1, 2]

[1, 2, 3, 4, 5].copyWithin(0, 3);
// [4, 5, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(0, 3, 4);
// [4, 2, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(-2, -3, -1);
// [1, 2, 3, 3, 4]

[].copyWithin.call({ length: 5, 3: 1 }, 0, 3);
// {0: 1, 3: 1, length: 5}

// ES2015 Typed Arrays are subclasses of Array
var i32a = new Int32Array([1, 2, 3, 4, 5]);

i32a.copyWithin(0, 2);
// Int32Array [3, 4, 5, 4, 5]

// On platforms that are not yet ES2015 compliant:
[].copyWithin.call(new Int32Array([1, 2, 3, 4, 5]), 0, 3, 4);
// Int32Array [4, 2, 3, 4, 5]

Polyfill

js
if (!Array.prototype.copyWithin) {
  Array.prototype.copyWithin =
    // Array: Number[, Number[, Number]]
    function copyWithin(target, start, stop) {
      var positiveT = target >= 0,
        positiveS = (start = start | 0) >= 0,
        length = this.length,
        zero = 0,
        r = function () {
          return (+new Date() * Math.random()).toString(36);
        },
        delimiter = "\b" + r() + "-" + r() + "-" + r() + "\b",
        hold;

      stop = stop || this.length;
      hold = this.slice
        .apply(
          this,
          positiveT ? [start, stop] : positiveS ? [start, -target] : [start],
        )
        .join(delimiter);

      return (
        this.splice.apply(
          this,
          positiveT
            ? [target, stop - start, hold]
            : positiveS
              ? [target, stop, hold]
              : [target, start, hold],
        ),
        this.join(delimiter).split(delimiter).slice(zero, length)
      );
    };
}

規範

Specification
ECMAScript Language Specification
# sec-array.prototype.copywithin

瀏覽器相容性

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參見