js总结之数组

let arr1 = [1, 2, 3];
let arr2 = Array(4, 5, 6);

const arr=[]

const arr=Array(3)
[...Array(100).fill(0)]

Array.prototype.slice.call()
[...]
Array.apply(null, { length: 100 }).fill(0);
Array.from({length: 100}, _ => 0)
Array.from({length: 100}).map(()=> 0)

传统方法 for 循环
Object.keys(Array.from({length: 100}));
[...new Array(100).keys()]
Array.from({length:100},(v,i) => i)
Array.from(new Array(100).keys())
[...Array(100).keys()]
[...Array.from({length:100}).keys()]

[,,,]//[empty × 3]
new Array(3)//[empty × 3]

// forEach 忽略空位
[1, , 2].forEach(v => console.log(v)); // 1 2

// for in 忽略空位
for(let key in [1, , 2]){ console.log(key); } // 0 2

// filter 忽略空位
console.log([1, , 2].filter(v => true)); // [1, 2]

// every 忽略空位
console.log([1, , 1].every(v => v === 1)); // true

// some 忽略空位
console.log([1, , 1].some(v => v !== 1)); // false

// map 遍历时忽略空位 新数组保留空位
console.log([1, , 1].map(v => 11)); // (3) [11, empty, 11]

// join 将空位与undefined以及null视为空字符串
console.log([1, , 1, null, undefined].join("|")); // 1||1||

// toString 将空位与undefined以及null视为空字符串
console.log([1, , 1, null, undefined].toString()); // 1,,1,,`

// Array.form 将空位转为undefined
console.log(Array.from([1, , 2])); // (3) [1, undefined, 2]

// ... 将空位转为undefined
console.log([...[1, , 2]]); // (3) [1, undefined, 2]

// copyWithin 将空位一并拷贝
console.log([1, , 2].copyWithin()); // (3) [1, empty, 2]

// for of 遍历空位并将值作为undefined
for(let key of [1, , 2]){ console.log(key); } // 1 undefined 2

// includes 将空位处理成undefined
console.log([, , ,].includes(undefined)); // true

// entries 将空位处理成undefined
console.log([...[1, , 2].entries()]); // [[0, 1], [1, undefined], [2, 2]]

// keys 会取出空位的索引
console.log([...[1, , 2].keys()]); // [0, 1, 2]

// values 将空位处理成undefined
console.log([...[1, , 2].values()]); // [1, undefined, 2]

// find 将空位处理成undefined
console.log([, , 1].find(v => true)); // undefined

// find 将空位处理成undefined
console.log([, , 1].findIndex(v => true)); // 0

const array = [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
console.log([...new Set(array)])//[1]
console.log(Array.from(new Set(array)))//[1]

const arr = [{name: 'zxc'}, {name: 'zxc'}, {name: 'zxc'}, {name: 'zxc'}]
const b=arr.filter((item,idx)=>{
    return arr.findIndex(_=>_.name===item.name)===idx
})
console.log(b)//[{name: "zxc"}]

const arr = [{name: 'zxc'}, {name: 'zxc'}, {name: 'zxc'}, {name: 'zxc'}]
const a=arr.reduce((prev, curr) => {
    if (prev.some(_=>_.name===curr.name)) {
        return prev
    } else {
       return [...prev,curr]
    }
}, [])
console.log(a)//[{name: "zxc"}]

const arr = [{name: 'zxc'}, {name: 'zx1c'}, {name: 'zxc'}, {name: 'zx1c'},]
const newArray = []
arr.forEach((item, idx) => {
    if (arr.findIndex(i => i.name === item.name) === idx) {
        newArray.push(item)
    }
})
console.log(newArray)

const arr = [{name: 'zxc'}, {name: 'zx1c'}, {name: 'zxc'}, {name: 'zx1c'},];
const newArray = []
const set = new Set()
arr.forEach((item) => {
    if (!set.has(item.name)) {
        newArray.push(item)
        set.add(item.name)
    }
})
console.log(newArray)