用js拍平一个多维数组

拍平数组：

Array.prototype.concat.apply([], [1, [2,3], [4]])    //[1,2,3,4]

深度拍平：

function flattenDeep(arrs) {
  let result = Array.prototype.concat.apply([], arrs);
  while (result.some(item => item instanceof Array)) {
    result = Array.prototype.concat.apply([], result);
  }
  return result;
}

 console.log(flattenDeep([1,2,[5,[22,11],7,90]]));   //[1,2,5,22，11，7，90]

交换变量：

[a,b] = [b,a]

some和every

有等于3的吗？true
  console.log( arr.some( function( item, index, array ){
        return item == 3;
    }));

所有都等于3吗？false
    console.log( arr.every( function( item, index, array ){
        return item == 3;
    }));

length

传统Length它并不是用于统计数组中元素的数量，而是代表数组中最高索引的值

const arrs = [1, 2, 3];
delete arrs[2]; // 长度依然为3

const arrs = [];
arrs[5] = 1;
console.log(arrs.length); // 6

const arrs = [1, 2, 3, 4];
arrs.length = 2; // arrs = [1, 2]
arrs.length = 0; // arrs = []

const arrs = [1, 2];
arrs.length = 5; // arrs = [1, 2,,,,]

const arrs = [1, 2, 3, 4];
arrs.length = 0; // arrs = []

let a = [1,2,3];
let b = [1,2,3];
let a1 = a;
let b1 = b;
a = [];
b.length = 0;
console.log(a, b, a1, b1); // [], [], [1, 2, 3], []

Object.assign()

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);