js中循环对比(for循环，foreach，for in，for of ,map)

对空位的处理

for循环(不会忽略空位，标记undefined)

var arr =[1,2,undefined,3,null,,7]
for (let i=0;i<arr.length;i++) {
    console.log('for循环',arr[i])
}

结果：
for循环 1
for循环 2
for循环 undefined
for循环 3
for循环 null
for循环 undefined
for循环 7

for of(不会忽略空位，标记undefined)

for(let i of arr) {
    console.log('for of',i)
}

结果：
for of 1
for of 2
for of undefined
for of 3
for of null
for of undefined
for of 7

for in(会忽略空位）

for(let i in arr) {
    console.log('for in',arr[i])
}

结果：
for in 1
for in 2
for in undefined
for in 3
for in null
for in 7

forEach(会忽略空位)

arr.forEach(item => console.log('foreach',item))

结果：
foreach 1
foreach 2
foreach undefined
foreach 3
foreach null
foreach 7

map(会忽略空位)，filter，every，some，find，findIndex都会忽略空位

arr.map(item => console.log('map',item))

结果：
map 1
map 2
map undefined
map 3
map null
map 7

性能对比

var arr =new Array(100000).fill(1)
console.time('for循环')
for (let i=0;i<arr.length;i++) {
}
console.timeEnd('for循环')

console.time('缓存优化的for循环')
for (let i=0,len=arr.length;i<len;i++) {
}
console.timeEnd('缓存优化的for循环')

console.time('foreach循环')
arr.forEach(item => {} )
console.timeEnd('foreach循环')

console.time('for of 循环')
for(let i of arr) {
}
console.timeEnd('for of 循环')

console.time('for in 循环')
for(let i in arr) {
}
console.timeEnd('for in 循环')

console.time('map循环')
arr.map(item => {})
console.timeEnd('map循环')

结果：

for循环: 3.226ms
缓存优化的for循环: 1.965ms
foreach循环: 2.144ms
for of 循环: 5.128ms
for in 循环: 11.501ms
map循环: 13.134ms