转载

2019-大龄前端如何准备面试之手写题

今年和以往一个区别就是有一些大厂增加了手写前端逻辑或者收集算法这个环节。前端的一些语言特性，异步控制相对好准备，大多都是Promise的使用外加递归。算法就相对来说难准备一些。需要一个长时间的积累。本文分几类来举例。抛转引玉。

前端语言特性

这部分主要考察对于js语言本身的理解，如this,原型链的理解，instanceOf，new关键字等。

实现function的bind 函数

Function.prototype.mybind = function(context, ...args) {
    let fun = this;
    function bound(...args2) {
        let self = this instanceof bound ? this : context;

        return fun.apply(self, args.concat(args2));
    }
    bound.prototype = Object.create(fun.prototype);
    return bound;
};
复制代码

实现InstanceOf(考察原型链的理解)

function isInstanceOf(child, fun) {
    if (typeof fun !== "function") {
        throw new TypeError("arg2 fun is not a function");
    }
    if (child === null) {
        return false;
    }
    if (child.__proto__ !== fun.prototype) {
        return isInstanceOf(child.__proto__, fun);
    }
    return true;
}
复制代码

实现new 这个关键字函数（考察new的过程）

function myNew(fun, ...arg) {
    if (typeof fun !== "function") {
        throw new TypeError(" fun is not a function");
    }
    let obj = {};
    Object.setPrototypeOf(obj, fun.prototype);
    fun.apply(obj, arg);
    return obj;
}
复制代码

实现JSON.parse函数

function JSONParse(strs) {
    if (strs === "" || typeof strs !== "string") {
        throw new SyntaxError("JSONParse error");
    }
    if (strs[0] === "{") {
        let obj = {};
        if (strs[strs.length - 1] == "}") {
            let fields = strs.substring(1, strs.length - 1).split(",");
            for (let field of fields) {
                let index = field.indexOf(":");
                let temp = [];
                if (index !== -1) {
                    temp[0] = field.substring(0, index);
                    temp[1] = field.substring(index + 1, field.length);
                }
                let key = temp[0].substring(1, temp[0].length - 1);
                let value = parseValue(temp[1]);
                //if (value !== undefined) {
                obj[key] = value;
                //}
            }
        }
        console.log("prase:", obj);
        return obj;
    }
    if (strs[0] === "[") {
        if (strs[strs.length - 1] == "]") {
            let result = [];
            let fields = strs.substring(1, strs.length - 1).split(",");
            for (let field of fields) {
                result.push(parseValue(field));
            }
            return result;
        }
    }
}
复制代码

实现JSON.stringify函数

function JSONStringify(obj) {
    if (
        obj === undefined ||
        obj === null ||
        typeof obj === "string" ||
        typeof obj === "boolean" ||
        typeof obj === "number"
    ) {
        return obj;
    }
    if (typeof obj === "function") {
        return "";
    }
    if (Array.isArray(obj)) {
        let result = [];
        for (let i = 0; i < obj.length; i++) {
            result.push(JSONStringify(obj[i]));
        }
        return "[" + result.join(",") + "]";
    } else {
        let result = [];
        for (let key in obj) {
            result.push(`"${key}":${JSONStringify(obj[key])}`);
        }
        return "{" + result.join(",") + "}";
    }
}
复制代码

实现一个继承（原型链）

function myExtends(parent, child) {
    function nop() {}
    nop.prototype = parent.prototype;
    child.prototype = new nop();
}
复制代码

前端工具类

这部分都是前端的一些高阶函数（闭包）。通常用来解决一些通用的问题。这个思想和J2EE中的（面向切面编程）AOP非常相似。例如debounce,memorize

实现debounce函数

debounce(fun, delay, immediate) {
       let timer = null;
       return (...args) => {
           if (timer) {
               clearTimeout(timer);
           } else {
               timer = setTimeout(() => {
                   fun.apply(this, args);
               }, delay);
           }
       };
   }
复制代码

实现throttle函数

throttle(fun, delay, immediate) {
       let flag = false;
       return (...args) => {
           if (!flag) {
               flag = true;
               setTimeout(() => {
                   fun.apply(this, args);
                   flag = false;
               }, delay);
           }
       };
   },
复制代码

实现memeorize函数，可以缓存函数的执行结果。在第二次调用之后会加速。

memeorize(fun) {
      let cache = {};
      return (...args) => {
          const key = args.toString();
          if (cache[key]) {
              return cache[key];
          }
          let value = fun.apply(this, args);
          cache[key] = value;
          return value;
      };
  }
复制代码

实现 promisy 函数。将一个callback的函数转化为promise 链式调用。

promisy(fun) {
      return (...args) => {
          return new Promise((resolve, reject) => {
              try {
                  fun(...args, resolve);
              } catch (e) {
                  reject(e);
              }
          });
      };
  }
  //使用方法
  fun(arg1,callback);
  let promisey = promisy(fun);
  promisey().then((res)=>());
复制代码

实现curry化。这是函数式编程的概念。柯里化。将多个参数的函数调用分布来调用。

currying(fun) {
      function helper(fn, ...arg1) {
          let length = fn.length;
          let self = this;
          return function(...arg2) {
              let arg = arg1.concat(arg2);
              if (arg.length < length) {
                  return helper.call(self, fn, ...arg);
              }
              return fn.apply(this, arg);
          };
      }
      return helper(fun);
  }
  //例子
  function add(a, b) {
      return a + b;
  }
  let curryadd = util.currying(add);
  let add1 = curryadd(1);
  t.is(add1(2), 3);
复制代码

以千分位格式化数字。输入123456，输出123,456

formatNumber(number) {
      if (typeof number !== "number") {
          return null;
      }
      if (isNaN(number)) {
          return null;
      }

      let result = [];
      let tmp = number + "";
      let num = number;
      let suffix = "";
      if (tmp.indexOf(".") !== -1) {
          suffix = tmp.substring(tmp.indexOf(".") + 1);
          num = parseInt(tmp.substring(0, tmp.indexOf(".")));
      }
      while (num > 0) {
          result.unshift(num % 1000);
          num = Math.floor(num / 1000);
      }
      let ret = result.join(",");
      if (suffix !== "") {
          ret += "." + suffix;
      }
      return ret;
  }
复制代码

前端逻辑控制类

这类问题大多是递归外加promise的理解。大家可以着重看看promise的使用。掌握了promise还是很好解决这些问题的。 -实现一个sleep的函数。sleep(3000).then(()=>{})

function sleep(delay){
    return new Promise((resolve,reject)=>{
      setTimeout(()=>{
          resolve()
      },delay);
    })
}
复制代码

-使用XMLHttpRequest 实现一个Promise的ajax

function myRequest(url, method, params) {
  return new Promise((resolve, reject) => {
      let xhr = new XMLHttpRequest();
      xhr.open(method, url);
      xhr.onreadystatechange = () => {
          if (xhr.readyState != 4) {
              return;
          }
          if (xhr.state === 200) {
              resolve(xhr.response);
          }
      };
      xhr.addEventListener("error", e => {
          reject(error);
      });
      xhr.send(params);
  });
}
复制代码

-用promise 实现一个lazyman. LazyManAsync("Hank").sleepFirst(5).eat("supper"); LazyManAsync("Hank").sleep(10).eat("dinner")

export function LazyManAsync(name) {
  return new LazyManFactory(name);
}

function LazyManFactory(name) {

  this.tasks = [];
  this.tasks.push(() => {
      return new Promise((resolve, reject) => {
          console.log("hi", name);
          resolve();
      })
  });
  setTimeout(() => {
      this.run();
  }, 0);

}

LazyManFactory.prototype.run = function () {

  if (this.tasks.length === 0) {
      return;
  }
  let task = this.tasks.shift();

  task().then(() => {
      this.run();
  }).catch(() => {
      this.run();
  })
}

LazyManFactory.prototype.sleep = function (time) {
  this.tasks.push(() => {
      return new Promise((resolve, reject) => {
          setTimeout(() => {
              resolve();
          }, time * 1000)
      })
  })
  return this;
}


LazyManFactory.prototype.eat = function (name) {
  this.tasks.push(() => {
      return new Promise((resolve, reject) => {
          console.log("eat:", name);
          resolve();
      })
  })
  return this;
}


LazyManFactory.prototype.sleepFirst = function (time) {
  this.tasks.unshift(() => {
      return new Promise((resolve, reject) => {
          setTimeout(() => {
              resolve();
          }, time * 1000)
      })
  })
  return this;
}
复制代码

算法类

这类一般涉及到算法。这部分短时间不好准备，建议可以作为长期战略来复习。这里暂时只列出非常高频的简单题。实际面试中有可能难度大大超过下面。

两数之和

const twoSum = function(arys, target) {
  if (!Array.isArray(arys)) {
      throw new TypeError("arg1 is not a array");
  }
  const map = new Map();
  for (let i = 0; i < arys.length; i++) {
      let num = target - arys[i];
      if (map.get(num) !== undefined) {
          return [map.get(num), i];
      }
      map.set(arys[i], i);
  }
  return [];
};
复制代码

快速排序

const quickSort = function(ary = [], start = 0, end = ary.length - 1) {
  if (!Array.isArray(ary)) {
      throw new TypeError("arg1 is not a array");
  }
  if (start >= end || isNaN(start) || isNaN(end)) {
      return;
  }
  let index = partition(start, end);
  quickSort(ary, start, index - 1);
  quickSort(ary, index + 1, end);
  function partition(left, right) {
      let priviot = ary[right];
      let k = left - 1;
      for (let i = left; i <= right - 1; i++) {
          if (ary[i] <= priviot) {
              swap(++k, i);
          }
      }
      swap(++k, right);
      return k;
  }
  function swap(i, j) {
      let temp = ary[i];
      ary[i] = ary[j];
      ary[j] = temp;
  }
};
复制代码

二分查找.给定一个排序好的数组，用二分查找的办法找出目标元素。

const binarySearch = function(ary, target) {
  if (!Array.isArray(ary)) {
      throw new TypeError("arg1 is not a array");
  }
  let start = 0,
      end = ary.length - 1;
  while (start <= end) {
      let mid = Math.floor(start + (end - start) / 2);
      if (ary[mid] === target) {
          return mid;
      } else if (ary[mid] < target) {
          start = mid + 1;
      } else {
          end = mid - 1;
      }
  }
  return -1;
};
复制代码

数组洗牌。将数组中的数字，打乱顺序，保证每个位置的概率相等。

const flush = function(num = []) {
  for (let i = 0; i < num.length; i++) {
      let index = Math.floor(Math.random() * (num.length - 1));
      let temp = num[i];
      num[i] = num[index];
      num[index] = temp;
  }
};
复制代码

斐波那契数列实现函数 f(n) = f(n-1)+f(n-2)

const f = (n)=>{
    if(n<0){
      return 0;
    }
    if(n === 0 ){
        return 1;
    }
    return f(n-1)+f(n-2);
    
}
复制代码

集合的子集.求一个函数所有的子集。比如集合[A,B]。输出[],[A],[B],[A,B]

var subsets = function(nums) {
  
  let result = [];
  function dfs(index,ans){
      let ans2 = ans.concat();
      ans2.push(nums[index]);
      if(index === 0){
          result.push(ans);
          result.push(ans2);
          return;
      }else{
          dfs(index-1,ans);
          dfs(index-1,ans2);
      }
  }
  dfs(nums.length-1,[]);
  return result;
};
复制代码