2020.03.17
arr.forEach(callback(currentValue [, index [, array]])[, thisArg])
callback 为数组中每个元素执行的函数currentValue 当前元素index 当前元素位于数组中的索引array 当前正在处理的数组thisArg 当执行 callback 回调函数时作为 this 的值，注意针对于箭头函数的回调函数无效Array.prototype.myForEach = function (fn, ctx = this) {
for (let i = 0; i < this.length; i++) {
fn.call(ctx, this[i], i, this);
}
};
let obj1 = {
count: 2,
print: function (arr = [], ctx = this) {
arr.myForEach(function (v) {
console.log(v * this.count)
}, ctx);
}
}
let obj2 = {
count: 3,
}
let arr = [1,1,1];
obj1.print(arr);
// 2 2 2
obj1.print.call(obj2, arr);
// 3 3 3
obj1.print(arr, obj2);
// 3 3 3
摘自 MDN
// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
if (!Array.prototype.forEach) {
Array.prototype.forEach = function(callback, thisArg) {
var T, 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 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
};
}