SOURCE

const myKeys = (obj)=>{
//定义返回值
const res = [];
//遍历所有可枚举属性，推入res
for (prop in obj){
    //过滤掉原型上的属性用hasOwnProperty
    if(obj.hasOwnPoperty(prop)){
        res.push(prop)
    }
}
return res
}
var arr = ['a', 'b', 'c'];
console.log(Object.keys(arr)); // console: ['0', '1', '2']

// array like object with random key ordering
var anObj = { 100: 'a', 2: 'b', 7: 'c' };
console.log(Object.keys(anObj)); // console: ['2', '7', '100']