方法一

function it(){
    let keys = Object.keys(this)
    let obj = this
    let idx = 0
    let length = keys.length
    return {
        next: function() {
            if (idx < length) {
                let ret = {
                    value: {
                        key: keys[idx],
                        val: obj[keys[idx]]
                    },
                    done: false
                }
                idx ++
                return ret
            } else {
                return {
                    value: undefined,
                    done: true
                }
            }
        }
    }
}

var a = {x:1, y:2, z:3}

a[Symbol.iterator] = it

for (let v of a) 
{
    console.log('------------')
    console.log(v)
}
/*
------------
{ key: 'x', val: 1 }
------------
{ key: 'y', val: 2 }
------------
{ key: 'z', val: 3 }
*/

for，of循环会在迭代器返回对象的done属性为true时，停止循环，且不执行当前循环体。

方法二

function* it(obj) {
    let propKeys = Object.keys(obj);
    // console.log(propKeys)  // [ 'x', 'y', 'z' ]
    for (let propKey of propKeys) {
        yield {
            key: propKey, 
            val: obj[propKey]
        }
    }
}

var a = {x:1, y:2, z:3}

for (let v of it(a)) 
{
    console.log('------------')
    console.log(v)
}

方法三

function* it(obj) {
    let propKeys = Object.keys(this);
    for (let propKey of propKeys) {
        yield {
            key: propKey, 
            val: this[propKey]
        }
    }
}

var a = {x:1, y:2, z:3}

a[Symbol.iterator] = it

for (let v of a) 
{
    console.log('------------')
    console.log(v)
}