Array.from()

將類數(shù)組轉(zhuǎn)成真正的組轉(zhuǎn)

let ArrayLike = {
    '0': 'a',
    '1': 'b',
    '2': 'c',
    length: 3
}
console.log(Array.from(ArrayLike,x=>x+'xx'))
            // 等價(jià)于
console.log(Array.from(ArrayLike).map(x=>x+'xx'))

Array.of()

將一維數(shù)轉(zhuǎn)成數(shù)組

console.log(Array.of(1,2,3)) //[1,2,3]
console.log(Array.of(1,2,3).length) //3         

find

找到第一個(gè)符合條件的數(shù)組

console.log([1,2,3,4,5].find((value,index,arr)=>value<1)) //undefined
console.log([1,2,3,4,5].find((value,index,arr)=>value<3)) //1

findIndex

和find類似，他是返回第一個(gè)符合條件的下標(biāo)

console.log([1,2,3,4,5].findIndex((value,index,arr)=>value<1)) //-1
console.log([1,2,3,4,5].findIndex((value,index,arr)=>value<3)) //0

for...of

keys() values() entries()

for(let i of ['a','b'].values()){console.log(i)} //a,b
for(let i of ['a','b'].keys()){console.log(i)} //0,1
for(let [index,value] of ['a','b'].entries()){console.log(index,value)} //0 a,1 b

includes

返回一個(gè)布爾值，判斷數(shù)據(jù)是否有某數(shù)據(jù)，和字符串的includes類似
includes 的第二個(gè)參數(shù)表示搜索的起始位置

console.log('abc'.includes('a')) //true
console.log(['a','b'].includes('a')) //true
console.log(['a','b'].includes('a',0)) //true
console.log(['a','b'].includes('a',1)) //false

數(shù)組空值

• for...of 會(huì)遍歷數(shù)組空位,forEach(),filter(),every(),map() 和some()會(huì)跳過數(shù)據(jù)空位

for(let i of [,,,]){
    console.log(1)
}//1,1,1

• Array.from(),擴(kuò)展運(yùn)算符（...）都會(huì)把數(shù)組空位轉(zhuǎn)成undefined

console.log([...[,,,]]) //[undefined,undefined,undefined]
console.log(Array.from([,,,])) //[undefined,undefined,undefined]

• join，toString,會(huì)把數(shù)據(jù)空位視為undefined，而undefined和null會(huì)處理成空串

console.log([,,,1].join("*"))//***1
console.log([,,,2].toString())//,,,2

• entries()、keys()、values()、find()和findIndex()會(huì)將空位處理成undefined。

console.log([...[,'a'].keys()]) //[0, 1]
console.log([...[,'a'].values()]) //[undefined, "a"]
console.log([...[,'a'].entries()]) //[[0, 1],[undefined, "a"]]
[,'a'].find(x => true) // undefined
[,'a'].findIndex(x => true) // 0