js中Array.from的用法

var obj = {0: 'a', 1: 'b', 2:'c', length: 3}

var newObj = Array.from(obj, function(value, index){

  console.log(value, index, this, arguments.length)

  return value.repeat(3)  //必须指定返回值，否则返回 undefined

}, obj)

console.log(newObj)   // ['aaa', 'bbb', 'ccc']