[英]What is the use of length property of an Object in Javascript?
This code is polyfill of Array.prototype.map given on Mozilla Developer Network. 此代码是Mozilla Developer Network上给出的Array.prototype.map的polyfill。 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map 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;
};
}
What I do not understand is these two lines - 我不明白的是这两行 -
var O = Object(this); var O = Object(this);
var len = O.length >>> 0; var len = O.length >>> 0;
Can someone please explain this to me? 有人可以向我解释一下吗? What is the use of calling length property on an Object(which has become an array after passing 'this' to it, I suppose) and then zero-fill right shifting it by 0? 在Object上调用length属性有什么用(我猜想,在将'this'传递给它之后它变成了一个数组),然后零填充将它右移0?
Obviously, it needs the length in order to iterate over the elements of the array. 显然,它需要长度才能迭代数组的元素。
You don't really need to worry about the Object(this)
. 你真的不需要担心Object(this)
。 If this is an array, it does nothing. 如果这是一个数组,它什么都不做。 This type of coding is to deal with certain kinds of objects which are not arrays. 这种类型的编码是为了处理某些不是数组的对象。 For instance, I can invoke map
as Array.prototype.map.call(5, function() { })
and it will work (returning []
). 例如,我可以调用map
作为Array.prototype.map.call(5, function() { })
并且它将起作用(返回[]
)。 In this case, the line you are wondering about turns 5
into an object, whose length
property I can examine (although in that case it doesn't exist; the >>> 0
idiom will transform the undefined
into 0
). 在这种情况下,你想知道的行变为5
到一个对象,它的length
属性我可以检查(虽然在这种情况下,它不存在; >>> 0
成语将改造undefined
成0
)。
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