Array.prototype.forEach
// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: https://es5.github.io/#x15.4.4.18
if (!Array.prototype['forEach']) {
Array.prototype.forEach = function(callback, thisArg) {
if (this == null) { throw new TypeError('Array.prototype.forEach called on null or undefined'); }
var T, k;
// 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: https://es5.github.io/#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 k be 0
k = 0;
// 7. Repeat, while k < len
while (k < len) {
var kValue;
// 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. Call the Call internal method of callback with T as
// the this value and argument list containing kValue, k, and O.
callback.call(T, kValue, k, O);
}
// d. Increase k by 1.
k++;
}
// 8. return undefined
};
}
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参考:MDN - Array.prototype.forEach - polyfillopen in new window
实现时需要注意的点:
- 第二个参数可以指定回调执行时的 this
- 不能简单地按照从 0 到数组长度去遍历执行回调,因为有些数组下标可能不存在。比如如下代码,
arr.length
为6,但最终只会输出index: 5, value: 1
一组数据。
const arr = [];
arr[5] = 1
arr.forEach((value, index) => {
console.log(`index: ${index}, value: ${value}`)
})
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