map的用法和实现原理
用法
"map"即"映射"，也就是原数组被"映射"成对应新数组。
let arr = [1,2,3];
arr1 = arr.map(item => item * 2)
实现原理
Array.prototype._map = function (callback) {
let newArr = [];
for (let i = 0; i < this.length; i++) {
newArr.push(callback && callback(this[i]))
}
return newArr
}
let result = [1,2,3]._map(item => item + 1)
console.log(result); //[2, 3, 4]
filter的用法和实现原理
用法
过滤
let arr=[2,4,6,8];
let arr1=arr.filter(function(item){
return item>5
})
console.log(arr1) //[6,8]
实现原理
Array.prototype._filter = function (callback) {
let newArr = [];
for (let i = 0; i < this.length; i++) {
callback && callback(this[i], i) && newArr.push(this[i])
}
return newArr
}
let filter = [1,2,3]._filter(item => item > 1)
console.log(filter); // [2,3]
find的用法和实现原理
用法
该方法返回通过测试的数组的第一个元素的值
let arr=[2,4,6,8];
let arr1=arr.find(function(item){
return item>5
})
console.log(arr1) // 6
实现原理
Array.prototype._find = function (callback) {
let currentVal
for (let i = 0; i <this.length; i++) {
if (callback(this[i])) {
currentVal = this[i]
break
}
}
return currentVal
}
let result = [1,2,3,4]._find(item => item > 1)
console.log(result); // 2
reduce的用法和实现原理
用法
累加
let arr=[2,4,6,8];
let arr1=arr.reduce((prev, item) => prev + item, 0)
console.log(arr1) // 20
实现原理
Array.prototype._reduce = function (callback, initVal) {
for (let i = 0; i < this.length; i++) {
initVal = callback(initVal, this[i], i, this)
}
return initVal
}
let result = [1,2,3,4]._reduce((prev, item) => prev + item, 0)
console.log(result); // 10
some的用法和实现原理
用法
只要有一个元素满足条件就返回true
let arr = [1,2,3,4];
let res = arr.some(item => item > 4)
console.log(res); // false
实现原理
Array.prototype._some = function (callback) {
// callback &&
let result = false
for (let i = 0; i < this.length; i++) {
result = callback && callback(this[i])
}
return result
}
let result = [1,2,3]._some(item => item > 1)
console.log(result); // true
every的用法和实现原理
用法
数组中每一个元素满足条件就返回true
let arr = [1,2,3,4];
let res = arr.every(item => item > 0)
console.log(res); // true
实现原理
Array.prototype._every = function (callback) {
let result = true
for (let i = 0; i < this.length; i++) {
if(!callback(this[i])) {
result = false
break
}
}
return result
}
let result = [1,2,3]._every(item => item > 0)
console.log(result); //true
let result = [1,2,3]._every(item => item > 1)
console.log(result); //false
reduce实现filter,map 数组扁平化等 https://juejin.im/post/5cc80c6e6fb9a0324936c0d9
