Native js

Implement instanceof

function myInstanceOf(left, right) {
  if(typeof left !== 'object' || typeof left === null) return false;
  let protp = Object.getPrototypeOf(left);
  while(true) {
    if(protp === null) return false;
    if(protp === right.prototype) return true;
    prop = Object.getPrototypeOf(prop);
  }
}
console.log(myInstanceOf('aa', String))                 // false
console.log(myInstanceOf(new String('aa'), String))     // true

deepCopy

const isObj = (target) => {(typeof target === 'object' || typeof target === 'function') && target !== null};

const deepCopy = (target, map = new Map()) => {
  if(map.get(target)) {
    return target;
  }
  
  if(isObj(target)) {
    map.set(target, true);
    const cloneTarget = Array.isArray(target) ? [] : {};
    for(let prop of target) {
      if(target.hasOwnProperty(prop)) {
        cloneTarget[prop] = deepCopy(target[prop], map)
      }
    }
  } else {
    return target;
  }
}

const a = {val:2};
a.target = a;
let newA = deepCopy(a);
console.log(newA)//{ val: 2, target: { val: 2, target: [Circular] } }

forEach 中用 await 问题

function handle(x) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			resolve(x)
		}, 1000*x)
	})
}

async function test() {
  var arr = [4,2,1];
  arr.forEach(async item => {
    const res = await handle(item);
    console.log(res)
  })
}
test()

期望结果是:

4
2
1

实际结果是:

1
2
4

**foreach底层实现

// 核心逻辑
for (var i = 0; i < length; i++) {
  if (i in array) {
    var element = array[i];
    callback(element, i, array);
  }
}

可以看到，forEach 拿过来直接执行了，这就导致它无法保证异步任务的执行顺序。比如后面的任务用时短，那么就又可能抢在前面的任务之前执行。
解决方法

function handle(x) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			resolve(x)
		}, 1000*x)
	})
}

async function test() {
  var arr = [4,2,1];
  for(const item of arr) {
    const res = await handle(item);
    console.log(res)
  }
}
test()

解释:

let arr = [4, 2, 1];
// 这就是迭代器
let iterator = arr[Symbol.iterator]();
console.log(iterator.next());
console.log(iterator.next());
console.log(iterator.next());
console.log(iterator.next());


// {value: 4, done: false}
// {value: 2, done: false}
// {value: 1, done: false}
// {value: undefined, done: true}

所以可以这样写:

async function test() {
  let arr = [4, 2, 1]
  let iterator = arr[Symbol.iterator]();
  let res = iterator.next();
  while(!res.done) {
    let value = res.value;
    console.log(value);
    await handle(value);
    res = iterater.next();
  }
	console.log('结束')
}
// 4
// 2
// 1
// 结束

for … of 就是上面代码的语法糖