filter
Array.prototype.filter( callback(current, index), thisArg)
- current: 当前元素*
- index: 当前元素的索引
-
thisArg: 作为回调函数的值
作用：返回经过符合操作的元素组成的数组
function myFilter(callback) {
if (this === undefined) throw new TypeError("this is null or undefined")
if (! callback instanceof Function)
throw new TypeError(callback + " is not a function")
let result = [] // 定义返回数组
const that = Object(this)
for(let key in that) {
if(that.hasOwnProperty(key) && callback.call(that, that[key], key))
result.push(that[key])
}
return result
}
const arr = [1, 1, '1', 17, true, true, false, false, 'true', 'a', {}, {}];
Array.prototype.filter = myFilter
const res4 = arr.filter( (item, index) => {
return arr.indexOf(item) == index
})
console.log(res4);
forEach
forEach 是遍历数组，对每一项执行对应的操作，没有返回值
forEach 会改变原数组
function myForEach(callback) {
if (this == undefined) throw new TypeError("this is null or undefined")
if (! callback instanceof Function) throw new TypeError(callback + " is not a function")
const that = Object(this)
for(let key in that) {
if(that.hasOwnProperty(key)) {
callback.call(that, that[key], Number(key))
}
}
}
map
map: 一个由原数组中每个元素执行回调的结果组成的新数组
function myMap(callback) {
if (this == undefined) throw new TypeError("this is null or undefined")
if (! callback instanceof Function) throw new TypeError(callback + " is not function")
const that = Object(this)
const result = []
for(let key in that) {
if (that.hasOwnProperty(key)) {
result[key] = callback.call(that, that[key], key)
}
}
return result
}
Array.prototype.map = myMap
const arr = [1, 1, '1', 17, true, true, false, false, 'true', 'a', {}, {}];
const newArr = arr.map((item, key) => {
return arr.indexOf(item) % 3 !== 0
})
console.log(newArr);
reduce
reduce 返回函数累计处理的结果
initialValue是调用时第一个参数额值，没写的话默认是数组的第一个有效值
空数组调用报错
function myReduce(callback, initialValue) {
if (this == undefined) {
throw new TypeError("this is null or not defined");
}
if (typeof callback !== "function") {
throw new TypeError(callbackfn + " is not a function");
}
const that = this;
let preValue = initialValue;
let key = 0
// 当没有传第一个值时，使用数组的第一个有效值
if (preValue === undefined) {
for (; key < that.length; key ++) {
if (!preValue && !that.includes(preValue)) {
preValue = that[key] === undefined ? undefined : that[key];
key = key === 0 ? 1 : key
break
}
}
}
if (preValue === undefined)
throw new TypeError("Reduce of empty array with no initial value");
for (; key < that.length; key ++) {
preValue = callback.call(undefined, preValue, that[key], key);
}
return preValue;
}
const arr = [1, 2, 3, 4];
Array.prototype.reduce = myReduce;
const res = arr.reduce((pre, cur) => {
return pre + cur;
});
console.log(res);
