我整理了一下前端面试常见的手写题，面试出现概率极高，建议每个都过自己过一遍，加深印象。

所有代码都是自己手敲的，并附带了测试的方法。

你也可以 watch 一下这个 GitHub 仓库，会持续收集、整理和维护。

实现 bind()

Function.prototype.MyBind = function (context, ...args) {
  let self = this;
  return function() {
    return self.apply(context, args);
  }
}

// test
let a = {name: 'jack'} 
let test = function() {
  console.log(this.name); // jack
}
let rs = test.MyBind(a);
rs();

实现 apply()

Function.prototype.myApply = function (context, args) {
  context.fn = this;
  let res;
  if (!args){
    res = context.fn();
  } else  {
    res = context.fn(...args)
  }
  return res;
}

// test
let obj = {
  name: 'jack'
}
function test(arg1, arg2, arg3) {
  console.log(this.name)   // jack
  console.log(arg1, arg2, arg3);  // 1 2 3
}
test.myApply(obj, [1,2,3]);

实现 call()

Function.prototype.myCall = function (context, ...rest) {
  context.fn = this;
  var result = context.fn(...rest);
  delete context.fn;
  return result;
}

// test
let obj = {
  name: 'jack'
}
function test(arg1, arg2, arg3) {
  console.log(this.name)   // jack
  console.log(arg1, arg2, arg3);  // 1 2 3
}
test.myCall(obj, 1,2,3);

实现 instanceof

function myInstanceOf(left, right) {
  let prototype = right.prototype;
  left = left.__proto__;
  while(true) {
    if (!left) return false;
    if (left == prototype) return true;
    left = left.__proto__;
  }
}

console.log(myInstanceOf([], Array));  // true

实现 new

function myNew (fun, ...args) {
  let obj = {};
  obj.__proto__ = fun.prototype;
  let res = fun.apply(obj, args);
  return res instanceof Object ? res : obj;
}

function Animal(name) {
  this.name = name;
}
let animal = myNew(Animal, 'dog');
console.log(animal.name)  // dog

实现 jsonp

var newscript = document.createElement('script');
newscript.src = '[https://www.adb.com?callback=fn](https://www.adb.com?callback=fn)'
document.body.appendChild(newscript);
function fn(data) {
  console.log(data);
}

实现 Promise

const PENDING = Symbol();
const REJECTED = Symbol();
const FULLFILLED = Symbol();

const MyPromise = function(fn) {
  this.state = PENDING;
  this.value = '';

  const resolve = (value) => {
    this.state = FULLFILLED;
    this.value = value;
  }

  const reject = (error) => {
    this.state = REJECTED;
    this.value = error;
  }

  this.then = (onFullFill, onReject) => {
    if (this.state == FULLFILLED) {
      onFullFill(this.value);
    } else {
      onReject(this.value);
    }
  }

  try {
    fn(resolve, reject);
  } catch(error) {
    reject(error);
  }
}

// test
let p = new MyPromise((resolve, reject) => {
  resolve('hello');
})
p.then(res => {
  console.log(res);  // hello
})

实现 Promise.all()

function isPromise(obj) {
  return !!obj && (typeof obj === 'function' || typeof obj === 'object') && typeof obj.then == 'function';
}

function myPromiseAll(arr) {
  let res = []
  return new Promise((resolve, reject) => {
    for (let i = 0; i < arr.length; i++) {
      if (isPromise(arr[i])) {
        arr[i].then(data => {
          res[i] = data;
          if (res.length === arr.length) {
            resolve(res)
          }
        }).catch(error => {
          reject(error)
        })
      } else {
        res[i] = arr[i];
      }
    }
  })
}

实现 Promise.race()

function myPromiseRace(arr) {
  return new Promise((resolve, reject) => {
    for (let i = 0; i < arr.length; i++) {
      return arr[i].then(resolve, reject)
    }
  })
}

实现 EventEmitter 订阅监听模式

class EventEmitter {
  constructor() {
    this.events = {};
  }
  on (eventName, callback) {
    if(!this.events[eventName]) {
      this.events[eventName] = [callback];
    } else {
      this.events[eventName].push(callback);
    }
  }

  emit(eventName, ...args) {
    this.events[eventName].forEach(fn => fn.apply(this, args));
  }

  once(eventName, callback) {
    const fn = () => {
      callback();
      this.remove(eventName, fn);
    }
    this.on(eventName, fn)
  }

  remove(eventName, callback) {
    this.events[eventName] = this.events[eventName].filter(fn => fn != callback);
  }
}

setTimout 实现 setInterval

function myInterval(fn, time) {
  let context = this;
  setTimeout(() => {
    fn.call(context);
    myInterval(fn, time);
  }, time);
}

深拷贝

function deepClone(obj) {
  // 先判断是对象还是数组
  let copy = obj instanceof Array ? [] : {};
  for (let key in obj) {
    // 判断是否是对象上的属性，而不是原型上的属性
    if (obj.hasOwnProperty(key)) {
      // obj[key] 是否是对象，如果是对象，递归遍历
      copy[key] = typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key];
    }
  }
  return copy;
}

// test
console.log(deepClone({name: 'jack', birth: {year: '1997', month: '10'}})) // {name: 'jack', birth: {…}}

数组拍平

var flatten = function(arr) {
  let res = [];
  for (let i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      res = res.concat(flatten(arr[i]))
    } else {
      res.push(arr[i])
    }
  }
  return res;
}

console.log(flatten([1,[1,2,[2,4]],3,5]));  // [1, 1, 2, 2, 4, 3, 5]

函数防抖

function debounce(fn, wait) {
  let timeout = null;
  return function() {
    let context = this;
    let args = arguments;
    if (timeout) clearTimeout(timeout);
    timeout = setTimeout(() => {
      fn.apply(context, args);
    }, wait);
  }
}

函数节流

function throttle(fn, wait) {
  let  pre = new Date();
  return function() {
    let context = this;
    let args = arguments;
    let now = new  Date();
    if (now - pre >= wait) {
      fn.apply(context, args);
      pre = now;
    }
  }
}

函数柯里化（实现 sum(1,2)(3)()）

function sum(...args1) {
  let x = args1.reduce((prev, next) => {return prev+next;})
  return function(...args2) {
    if (args2.length == 0) return x;
    let y = args2.reduce((prev, next) => {return prev+next;})
    return sum(x+y)
  }
}

console.log(sum(1,2,2,5)(7)()) // 17

快速排序

function quicksort(arr) {
  if (arr.length <= 1) return arr;
  let pivotIndex = arr.length >> 1;
  let pivot = arr.splice(pivotIndex, 1)[0];
  let left = [];
  let right = [];
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] <= pivot)  {
      left.push(arr[i]);
    } else {
      right.push(arr[i]);
    }
  }
  return quicksort(left).concat(pivot, quicksort(right));

}

console.log(quicksort([4,3,5,2,1,6]));   //  [1, 2, 3, 4, 5, 6]

归并排序

function merge(left, right) {
  let res = [];
  while(left.length > 0 && right.length > 0) {
    if (left[0] < right[0]) {
      res.push(left.shift());
    } else {
      res.push(right.shift());
    }
  }
  return res.concat(left).concat(right);
}

function mergeSort(arr) {
  if (arr.length == 1) return arr;
  var middle = Math.floor(arr.length / 2);
  var left = arr.slice(0, middle);
  var right = arr.slice(middle);
  return merge(mergeSort(left), mergeSort(right));
}

console.log(mergeSort([3,2,4,5,1,6]));  // [1, 2, 3, 4, 5, 6]

长期内推，欢迎私信👏