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    Array.prototype.copyWithin()

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    该特性处于 ECMAScript 6 规范草案中,目前的实现在未来可能会发生微调,请谨慎使用。

    Summary

    The copyWithin() method copies the sequence of array elements within the array to the position starting at target. The copy is taken from the index positions of the second and third arguments start and end. The end argument is optional and defaults to the length of the array.

    Syntax

    arr.copyWithin(target, start[, end = this.length])

    Parameters

    target
    Target start index position where to copy the elements to.
    start
    Source start index position where to start copying elements from.
    end
    Optional. Source end index position where to end copying elements from.

    Description

    The arguments target, start, and end are coerced to Number and truncated to integer values.

    If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end.

    The copyWithin function is intentionally generic, it does not require that its this value be an Array object and in addition, copyWithin is a mutable method, it will change this object itself, and return it, not just return a copy of it.

    Examples

    [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(0, -2, -1);
    // [4, 2, 3, 4, 5]
    
    [].copyWithin.call({length: 5, 3: 1}, 0, 3);
    // {0: 1, 3: 1, length: 5}
    
    // ES6 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 ES6 compliant: 
    [].copyWithin.call(new Int32Array([1, 2, 3, 4, 5]), 0, 3, 4);
    // Int32Array [4, 2, 3, 4, 5]
    

    Polyfill

    if (!Array.prototype.copyWithin) {
      Array.prototype.copyWithin = function(target, start/*, end*/) {
        // Steps 1-2.
        if (this == null) {
          throw new TypeError('this is null or not defined');
        }
    
        var O = Object(this);
    
        // Steps 3-5.
        var len = O.length >>> 0;
    
        // Steps 6-8.
        var relativeTarget = target >> 0;
    
        var to = relativeTarget < 0 ?
          Math.max(len + relativeTarget, 0) :
          Math.min(relativeTarget, len);
    
        // Steps 9-11.
        var relativeStart = start >> 0;
    
        var from = relativeStart < 0 ?
          Math.max(len + relativeStart, 0) :
          Math.min(relativeStart, len);
    
        // Steps 12-14.
        var end = arguments[2];
        var relativeEnd = end === undefined ? len : end >> 0;
    
        var final = relativeEnd < 0 ?
          Math.max(len + relativeEnd, 0) :
          Math.min(relativeEnd, len);
    
        // Step 15.
        var count = Math.min(final - from, len - to);
    
        // Steps 16-17.
        var direction = 1;
    
        if (from < to && to < (from + count)) {
          direction = -1;
          from += count - 1;
          to += count - 1;
        }
    
        // Step 18.
        while (count > 0) {
          if (from in O) {
            O[to] = O[from];
          } else {
            delete O[to];
          }
    
          from += direction;
          to += direction;
          count--;
        }
    
        // Step 19.
        return O;
      };
    }
    

    Specifications

    Specification Status Comment
    ECMAScript 6 (ECMA-262)
    Array.prototype.copyWithin
    Release Candidate Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 未实现 32 (32) 未实现 未实现 未实现
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support 未实现 未实现 32.0 (32) 未实现 未实现 未实现

    See also

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