item-one

1.解构赋值，经常报Cannot read property 'length' of undefined

解决：这种情况利用&&解决,没有值就不赋值

    var obj = {a: 1,b: 2,c: 3}
    let {e} = obj // e = undefined
    e && (this.needList = e)

2.利用forEach实现数组的修改

    var arr = [{a:1},{a:2},{a:3}]
    arr.forEach((e,i)=>{
      e.a = e.a**2
    })
    console.log(arr)
    /**
        0: {a: 1}
        1: {a: 4}
        2: {a: 9}
    */