js中push、pop、slice、map 和 reduce实现 javascript前端es6

push方法

Array.prototype.push = function (...items) { let O = Object(this); // ecma 中提到的先转换为对象 // >>> 无符号右移 let len = this.length >>> 0 let argCount = items.length >>> 0 // 2 ^ 53 - 1 为JS能表示的最大正整数 if (len + argCount > 2 ** 53 - 1) { throw new TypeError("The number of array is over the max value") } for (let i = 0; i < argCount; i++) { O[len + i] = items[i]; } let newLength = len + argCount; O.length = newLength; return newLength; }

pop 方法

Array.prototype.pop = function() { let O = Object(this); let len = this.length >>> 0; if (len === 0) { O.length = 0; return undefined; } len --; let value = https://www.it610.com/article/O[len]; delete O[len]; O.length = len; return value; }

map方法

Array.prototype.map = function(callbackFn, thisArg) { if (this === null || this === undefined) { throw new TypeError("Cannot read property 'map' of null"); } if (Object.prototype.toString.call(callbackfn) != "[object Function]") { throw new TypeError(callbackfn + ' is not a function') } let O = Object(this); let T = thisArg; let len = O.length >>> 0; let A = new Array(len); // 赋值给新数组，并不改变原数组的值 for(let k = 0; k < len; k++) { if (k in O) { let kValue = https://www.it610.com/article/O[k]; // 依次传入this, 当前项，当前索引，整个数组 let mappedValue = callbackfn.call(T, KValue, k, O); A[k] = mappedValue; } } return A; }

reduce方法
两个注意点

初始值默认值不传的特殊处理；
【js中push、pop、slice、map 和 reduce实现】累加器以及 callbackfn 的处理逻辑。

Array.prototype.reduce= function(callbackfn, initialValue) { // 异常处理，和 map 类似 if (this === null || this === undefined) { throw new TypeError("Cannot read property 'reduce' of null"); } // 处理回调类型异常 if (Object.prototype.toString.call(callbackfn) != "[object Function]") { throw new TypeError(callbackfn + ' is not a function') } let O = Object(this); let len = O.length >>> 0; let k = 0; let accumulator = initialValue; // reduce方法第二个参数作为累加器的初始值 if (accumulator === undefined) { throw new Error('Each element of the array is empty'); // 初始值不传的处理 for(; k < len ; k++) { if (k in O) { accumulator = O[k]; k++; break; } } } for(; k < len; k++) { if (k in O) { // 注意 reduce 的核心累加器 accumulator = callbackfn.call(undefined, accumulator, O[k], O); } } return accumulator; }