Javascript 中对象的长度属性有什么用？

Question

此代码是 Mozilla Developer Network 上提供的 Array.prototype.map 的 polyfill。 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
if (!Array.prototype.map) {

  Array.prototype.map = function(callback, thisArg) {

    var T, A, k;

    if (this == null) {
      throw new TypeError(' this is null or not defined');
    }

    // 1. Let O be the result of calling ToObject passing the |this| 
    //    value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get internal 
    //    method of O with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If IsCallable(callback) is false, throw a TypeError exception.
    // See: http://es5.github.com/#x9.11
    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function');
    }

    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
    if (arguments.length > 1) {
      T = thisArg;
    }

    // 6. Let A be a new array created as if by the expression new Array(len) 
    //    where Array is the standard built-in constructor with that name and 
    //    len is the value of len.
    A = new Array(len);

    // 7. Let k be 0
    k = 0;

    // 8. Repeat, while k < len
    while (k < len) {

      var kValue, mappedValue;

      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the HasProperty internal 
      //    method of O with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      if (k in O) {

        // i. Let kValue be the result of calling the Get internal 
        //    method of O with argument Pk.
        kValue = O[k];

        // ii. Let mappedValue be the result of calling the Call internal 
        //     method of callback with T as the this value and argument 
        //     list containing kValue, k, and O.
        mappedValue = callback.call(T, kValue, k, O);

        // iii. Call the DefineOwnProperty internal method of A with arguments
        // Pk, Property Descriptor
        // { Value: mappedValue,
        //   Writable: true,
        //   Enumerable: true,
        //   Configurable: true },
        // and false.

        // In browsers that support Object.defineProperty, use the following:
        // Object.defineProperty(A, k, {
        //   value: mappedValue,
        //   writable: true,
        //   enumerable: true,
        //   configurable: true
        // });

        // For best browser support, use the following:
        A[k] = mappedValue;
      }
      // d. Increase k by 1.
      k++;
    }

    // 9. return A
    return A;
  };
}

我不明白的是这两行 -

var O = Object(this);

var len = O.length >>> 0;

有人可以给我解释一下吗？在一个对象上调用 length 属性有什么用（我想在将 'this' 传递给它之后它已经变成了一个数组）然后零填充右移它 0?

Answer 1

显然，它需要长度才能遍历数组的元素。

您真的不需要担心 Object(this)。如果这是一个数组，它什么都不做。这种类型的编码是为了处理某些类型的不是数组的对象。例如，我可以将 map 调用为 Array.prototype.map.call(5, function() { })，它会起作用（返回 []）。在这种情况下，您想知道的行将 5 变成一个对象，我可以检查其 length 属性（尽管在那种情况下它不存在；>>> 0 成语会将 undefined 转换为 0)。

Javascript 中对象的长度属性有什么用？

What is the use of length property of an Object in Javascript?

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Javascript 中对象的长度 属性 有什么用？

What is the use of length property of an Object in Javascript?

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Javascript 中对象的长度属性有什么用？