持续构建个人的源码库(JavaScript 原生、常用库、数据结构、算法)

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*项目地址:awesome-coding-javascript.

出发点

在日常开发中为了提高开发效率,我们一般是直接调用封装好的函数方法。但是作为工程师仅知道这些方法如何调用肯定是不够的,还需要了解这些方法背后的实现原理,以便之后更好地运用这些方法,并可以基于此改进或者写出自己觉得不错的方法。

这方面社区里也有很多关于原理介绍和代码实现的文章,可以从中学到很多。参照着文章里的描述,自己也可以实现这些方法,用 console.log 打印一下输出结果检查看有没有问题,但是这样写起来比较零碎分散不方便代码逻辑的持续改进。

例如:每次要验证代码逻辑的时候,都需要把代码拷到控制台或者一个文件里去运行;测试的数据需要临时构造或者也需要一个一个复制过去;测试用例都需要自己手动执行,不够自动化;正是由于测试不方便,所以写的测试用例也比较粗略;有代码修改时要保证逻辑正确,需要重新手动执行所以测试用例,繁琐重复;没有数据来评估代码质量。

持续构建的思路

如何解决?其实可以从开源的库和插件中找到答案,那就是为这些实现代码加上单元测试,并保证代码覆盖率。

加上单元测试后的优势有:

  1. 单元测试就是一个命令式的运行环境,即可以在开发时调试,也可以在开发完成后验证。
  2. 执行写好的单元测试用例,自动化地帮我们验证逻辑的正确性。
  3. 代码有改动,重新执行所有测试用例,保证改动的代码没有问题。
  4. 单元测试即文档,开发者可以从一个一个的单元测试中看到都实现了哪些功能,实现功能的描述,调用方式和调用的数据,最后调用的结果是什么。所以说单元测试也是一个很直观的用户文档。
  5. 体验 TDD 编程,先写好单元测试用例整理思路,然后实现满足这些测试用例的逻辑。
  6. 提交代码前全量跑一遍单元测试,保证所有逻辑没有问题之后才能提交。

保证代码覆盖率的意义有:

  1. 检测出程序中的无用代码,逆向反推在代码设计中思维未考虑到的点,提升代码质量。
  2. 分析未覆盖部分的代码,反推在前期单元测试设计是否充分,改进测试用例。

所以加上单元测试和保证代码覆盖率之后,能解决掉最开始提到的问题,让我们即方便又有保障地持续构建我们的源码库。接下来就是列出一些我们认为需要实现的源码库。

待构建源码列表

它包括 JavaScript 原生和常用方法的代码实现、常用库和插件的代码实现以及使用 JavaScript 实现的数据结构和常用算法。由于涉及的内容比较多,不可能短时间内全部集齐,这里先把大体的框架搭起来,之后再持续地构建自己的源码库。

  1. JavaScript 原生和常用方法的代码实现
    • bind/apply/call 的实现
    • new 的实现
    • 深拷贝
    • 防抖/节流的实现
    • Promise 原理实现
    • async/await 原理实现
    • 柯里化
    • 模板字符串
    • 订阅发布机制
    • URL 参数解析
  2. 常用的库和插件的代码实现
    • Webpack 的模拟实现
    • Webpack Plugin 的实现
    • Babel 的模拟实现
    • Babel Plugin 的实现
    • React 的模拟实现
    • Redux 的模拟实现
    • Router 的模拟实现
    • Egg.js 的模拟实现
  3. JavaScript 实现的数据结构和常用算法
    • 数据结构
      • 栈、队列、链表、集合
      • 堆、散列表、数组、树
    • 算法
      • 排序、分治、递归
      • 深度优先/广度优先、双指针
      • 动态规划、回溯算法、贪心算法
    • 其他
      • 字符串
      • 数字

源码及单测展示(部分)

为了让大家有个直观的影响,这里贴出一些比较有代表性的代码实现和单元测试,供大家参考。完整的代码展示请见 Repo : awesome-coding-javascript

call 的实现

实现代码:

export default function (...args) {
  // 使用 Symbol 创建一个全局唯一的函数名
  const func = Symbol('func');
  let context = args.shift();

  // 非严格模式下,传 null 或者 undeinfed,context 等于 window 对象
  if (context == null) {
    context = window;
  }
  else {
    context = Object(context);
  }

  // 赋予函数属性
  context[func] = this;
  // 函数执行
  const result = context[func](...args);
  // 删除临时的函数属性
  Reflect.deleteProperty(context, func);

  return result;
}

单元测试用例:

import call from './call';

describe('Function.prototype.call', () => {

  Function.prototype._call = call;

  it('change the direction of this', () => {
    const foo = {
      value: 1,
    };
    function bar() {
      return this.value;
    }

    // 和原生的 call 操作进行比较验证
    expect(bar._call(foo)).toBe(bar.call(foo));
  });

  it('change the direction of this, use in constructor', () => {
    function Product(name, price) {
      this.name = name;
      this.price = price;
    }

    function Food(name, price) {
      Product.call(this, name, price);
      this.category = 'food';
    }
    function Food2(name, price) {
      Product._call(this, name, price);
      this.category = 'food';
    }

    // 和原生的 call 操作进行比较验证
    expect(new Food2('cheese', 5).name).toBe(new Food('cheese', 5).name);
  });

  it('when \'this\' argument is null or undefined', () => {
    window.value = 2;
    function bar() {
      return this.value;
    }
    // 这是非严格模式下的结果,严格模式下会报错
    expect(bar._call(null)).toBe(bar.call(null));
    expect(bar._call(undefined)).toBe(bar.call(undefined));
  });

  it('when \'this\' is other primitive value', () => {
    function bar() {
      return this.length;
    }

    // 和原生的 call 操作进行比较验证
    expect(bar._call('123')).toBe(bar.call('123'));
  });
});

执行单元测试用例:

PASS  src/javascript/call/call.test.js
Function.prototype.call
  ✓ change the direction of this (4ms)
  ✓ change the direction of this, use in constructor (1ms)
  ✓ when 'this' argument is null or undefined (1ms)
  ✓ when 'this' is other primitive value

bind 的实现

实现代码:

export default function (...bindArgs) {
  // 函数自身
  const self = this;
  // 传进来的 this 对象
  let context = bindArgs.shift();

  // 非严格模式下,传 null 或者 undeinfed,context 等于 window 对象
  if (context == null) {
    context = window;
  }
  else {
    context = Object(context);
  }

  // 返回的新函数
  const fBound = function (...args) {
    // 当返回的新函数作为构造函数使用时,之前指定的 this 对象会失效,此时 this 是指向实例对象
    if (this instanceof fBound) {
      context = this;
    }
    // 函数运行并返回结果
    return self.apply(context, [...bindArgs, ...args]);
  };
  // 修改返回函数的原型对象,实例对象就可以从原函数的原型对象上继承属性和方法
  fBound.prototype = Object.create(self.prototype);

  return fBound;
}

单元测试用例:

import bind from './bind';

describe('Function.prototype.bind', () => {

  Function.prototype._bind = bind;

  it('change the direction of this, return a function', () => {
    const foo = {
      value: 1,
    };
    function bar(age1, age2) {
      age1 = age1 || 0;
      age2 = age2 || 0;
      return this.value + age1 + age2;
    }
    const newBar = bar._bind(foo, 3);
    expect(typeof newBar).toBe('function');
    // 和原生 bind 操作进行比较验证
    expect(newBar(2)).toBe((bar.bind(foo, 3))(2));
  });

  it('when return function as a constructor, \'this\' points to the instance object', () => {
    const foo = { value: 1 };
    function bar(name, age) {
      this.name = name;
      this.age = age;
    }
    bar.prototype.friend = 'kevin';

    const bindFoo = bar.bind(foo);  // 原生 bind 操作生成的实例对象
    const bindFoo2 = bar._bind(foo);
    bindFoo2.prototype.address = 1; // 修改返回函数的原型对象

    // 验证返回的函数作为构造函数,实例对象会从原函数的原型对象上继承属性
    expect(new bindFoo2().friend).toBe(new bindFoo().friend);
    // 验证返回的函数作为构造函数,之前绑定的 this 对象会失效,this 会指向实例对象
    expect(new bindFoo2().value).toBe(undefined);
    expect(new bindFoo2().value).toBe(new bindFoo().value);
    // 验证修改返回函数的原型对象,不会引起原始函数 bar 原型对象的修改
    expect(bar.prototype.address).toBe(undefined);
  });

  it('when rebind \'this\', cannot change the direction of this', () => {
    const foo = {
      value: 1,
    };
    function bar(age1, age2) {
      age1 = age1 || 0;
      age2 = age2 || 0;
      return this.value + age1 + age2;
    }
    const bindFoo = bar.bind(foo);  // 原生 bind 操作生成的实例对象
    const bindFoo2 = bar._bind(foo);

    // 对返回的函数用 call 或者 apply 重新绑定 this 对象时,this 对象不会发生改变
    expect(bindFoo2.call({ value: 2 })).toBe(1);
    expect(bindFoo2.call({ value: 2 })).toBe(bindFoo.call({ value: 2 }));
  });

  it('when \'this\' argument is null or undefined', () => {
    window.value = 2;
    function bar(age1, age2) {
      age1 = age1 || 0;
      age2 = age2 || 0;
      return this.value + age1 + age2;
    }

    // 这是非严格模式下的结果,严格模式下会报错
    expect(bar._bind(null, 3)(1)).toBe(6);
    expect(bar._bind(undefined, 3)(1)).toBe(6);
  });
});

执行单元测试用例:

PASS  src/javascript/bind/bind.test.js
  Function.prototype.bind
    ✓ change the direction of this, return a function (3ms)
    ✓ when return function as a constructor, 'this' points to the instance object (1ms)
    ✓ when rebind 'this', cannot change the direction of this (1ms)
    ✓ when 'this' argument is null or undefined

new 的实现

实现代码:

import { isArrowFunction } from './../../shared/is';

export function objectFactory(Factory, ...args) {
  if (typeof Factory !== 'function') {
    throw new Error('need be a function argument');
  }

  if (isArrowFunction(Factory)) {
    throw new Error('arrow function is not allowed');
  }

  const instance = Object.create(Factory.prototype);  // 创建实例对象
  const result = Factory.apply(instance, args);  // 执行构造函数

  return result instanceof Object ? result : instance;
}

单元测试用例:

import { objectFactory } from './new';

describe('new', () => {
  it('take a function as an argument', () => {
    const Factory = 123;
    function excute() {
      objectFactory(Factory);
    }

    expect(excute).toThrowError('need be a function argument');
  });

  it('cannot be an arrow function', () => {
    const Factory = (name, age) => {
      this.name = name;
      this.age = age;

      return 233;
    };

    function excute() {
      objectFactory(Factory);
    }

    expect(excute).toThrowError('arrow function is not allowed');
  });

  it('create a instance', () => {
    function Factory(name, age) {
      this.name = name;
      this.age = age;
    }

    Factory.prototype.getName = function () {
      return this.name;
    };

    const f = objectFactory(Factory, 'jack', 12);
    const nf = new Factory('jack', 12);  // 原生的 new 操作生成的实例对象

    expect(f.name).toBe(nf.name);
    expect(f.age).toBe(nf.age);
    expect(f.getName()).toBe(nf.getName());
  });

  it('if return a primitive value, return the newly instance', () => {
    const Factory = function (name, age) {
      this.name = name;
      this.age = age;

      return 233;
    };

    Factory.prototype.getName = function () {
      return this.name;
    };

    const f = objectFactory(Factory, 'jack', 12);
    const nf = new Factory('jack', 12);  // 原生的 new 操作生成的实例对象

    expect(f.name).toBe(nf.name);
    expect(f.age).toBe(nf.age);
    expect(f.getName()).toBe(nf.getName());
  });

  it('if return a object, return the object', () => {
    const Factory = function (name, age) {
      this.name = name;
      this.age = age;

      return {
        name: 'john',
      };
    };

    Factory.prototype.getName = function () {
      return this.name;
    };

    const f = objectFactory(Factory, 'jack', 12);
    const nf = new Factory('jack', 12);  // 原生的 new 操作生成的实例对象

    expect(f.name).toBe(nf.name);
    expect(f.age).toBe(nf.age);
    expect(f.getName).toBe(nf.getName);
  });
});

执行单元测试用例:

PASS  src/javascript/new/new.test.js
  new
    ✓ take a function as an argument (5ms)
    ✓ cannot be an arrow function (1ms)
    ✓ create a instance (1ms)
    ✓ if return a primitive value, return the newly instance (1ms)
    ✓ if return a object, return the object (1ms)

Throttle 节流实现

实现代码:

export default function throttle(func, interval) {
  let startTime = new Date();

  return function (...args) {
    const curTime = new Date();
    // 大于间隔时才执行函数的逻辑
    if (curTime - startTime > interval) {
      // 重设开始时间
      startTime = curTime;
      // 执行函数
      func.apply(this, args);
    }
  };
}

单元测试用例:

import throttle from './throttle';
import sleep from './../../shared/sleep';

describe('throttle', () => {
  it('when less than interval, will throttle', async () => {
    let count = 0;
    const addCount = () => {
      count += 1;
    };

    const timer = setInterval(throttle(addCount, 400), 200);
    /**
     * 400 -> 1
     * 800 -> 2
     */
    setTimeout(() => {
      clearInterval(timer);
    }, 1000);
    await sleep(1500);
    expect(count).toBe(2);
  });

  it('when greate than interval, normally call', async () => {
    let count = 0;
    const addCount = () => {
      count += 1;
    };

    const timer = setInterval(throttle(addCount, 200), 300);
    /**
     * 300 -> 1
     * 600 -> 2
     * 900 -> 3
     */
    setTimeout(() => {
      clearInterval(timer);
    }, 1000);
    await sleep(1500);
    expect(count).toBe(3);
  });
});

执行单元测试用例:

PASS  src/javascript/throttle/throttle.test.js
  throttle
    ✓ when less than interval, will throttle (1506ms)
    ✓ when greate than interval, normally call (1505ms)

Promise 原理实现

实现代码:

export default class Promise {
  constructor(fn) {
    // 记录 promise 的状态,根据状态来操作 callbacks 队列
    this.state = 'pending';
    // 记录 resolve 或 reject 时的值
    this.value = null;
    // 里面维护了一个队列,是 resolve 时要执行的代码
    this.callbacks = [];

    fn(this.resolve, this.reject);
  }

  static resolve = val => {
    return new Promise(function (resolve) {
      resolve(val);
    });
  }

  static reject = reason => {
    return new Promise(function (resolve, reject) {
      reject(reason);
    });
  }

  static stop = () => {
    return new Promise(function () {});
  }

  // 注册 onFulfilled 和 onRejected 函数
  then = (onFulfilled, onRejected) => {
    return new Promise((resolve, reject) => {
      this.handle({
        onFulfilled: onFulfilled || null,
        onRejected: onRejected || null,
        resolve,
        reject,
      });
    });
  }

  catch = onRejected => {
    return this.then(null, onRejected);
  }

  finally = onFinally => {
    onFinally = typeof onFinally === 'function' ? onFinally : function noop() {};

    return this.then(
      val => Promise.resolve(onFinally()).then(() => val),
      reason => Promise.resolve(onFinally()).then(() => {
        throw reason;
      })
    );
  }

  // 更改状态,调用执行 callbacks 的方法
  resolve = val => {
    // 状态只能更改一次
    if (this.state !== 'pending') {
      return;
    }

    // resolve 的值是 Promise 的情况
    if (val && (typeof val === 'function' || typeof val === 'object')) {
      const then = val.then;
      if (typeof then === 'function') {
        then.call(val, this.resolve, this.reject);
        return;
      }
    }

    this.state = 'fulfilled';
    this.value = val;
    this.execute();
  }

  reject = reason => {
    // 状态只能更改一次
    if (this.state !== 'pending') {
      return;
    }

    this.state = 'rejected';
    this.value = reason;
    this.execute();
  }

  // 执行 callbacks
  execute = () => {
    setTimeout(() => {
      this.callbacks.forEach(callback => {
        this.handle(callback);
      });
    }, 0);
  }

  // 负责处理单个 callback
  handle = callback => {
    if (this.state === 'pending') {
      this.callbacks.push(callback);
      return;
    }

    if (this.state === 'fulfilled') {
      // 如果 then 中没有传递任何东西
      if (!callback.onFulfilled) {
        // 直接执行下一个 promise
        callback.resolve(this.value);
        return;
      }

      try {
        // 执行当前的 promise
        const ret = callback.onFulfilled(this.value);
        // 执行下一个 promise
        callback.resolve(ret);
      }
      catch (err) {
        callback.reject(err);
      }
      return;
    }

    if (this.state === 'rejected') {
      // 如果没有指定 callback.onRejected
      if (!callback.onRejected) {
        callback.reject(this.value);
        return;
      }

      try {
        const ret = callback.onRejected(this.value);
        callback.resolve(ret);
      }
      catch (err) {
        callback.reject(err);
      }
      return;
    }
  }
}

单元测试用例:

import Promise from './promise';
import sleep from './../../shared/sleep';

// ref: https://github.com/promises-aplus/promises-tests
describe('Promise', () => {
  const time = 500;

  it('take a function as an argument', () => {
    expect(() => {
      new Promise(1);
    }).toThrowError('is not a function');
  });

  it('return a promise instace, exposes the public API', () => {
    const promise = new Promise(function (resolve, reject) {
      +new Date() % 2 === 0 ? resolve() : reject();
    });
    expect(promise).toHaveProperty('then');
    expect(promise).toHaveProperty('catch');
    expect(promise).toHaveProperty('finally');
  });

  it('promise.then, onFulfilled', done => {
    const promise = new Promise(function (resolve) {
      setTimeout(function () {
        resolve(time);
      }, time);
    });
    promise.then(ms => {
      expect(ms).toBe(time);
      done();
    });
  });

  it('promise.then, onRejected', done => {
    const promise = new Promise(function (resolve, reject) {
      setTimeout(function () {
        reject(time);
      }, time);
    });
    promise.then(() => {
      // onFulfilled
    }, reason => {
      // onRejected
      expect(reason).toBe(time);
      done();
    });
  });

  it('promise.catch', done => {
    const promise = new Promise(function (resolve, reject) {
      setTimeout(function () {
        reject(time);
      }, time);
    });
    promise.then(() => {
      // onFulfilled
    }).catch(reason => {
      expect(reason).toBe(time);
      done();
    });
  });

  it('promise.finally', done => {
    const promise = new Promise(function (resolve, reject) {
      setTimeout(function () {
        reject(time);
      }, time);
    });
    promise.then(() => {
      // onFulfilled
    }).catch(() => {
      // onRejected
    }).finally(() => {
      // Finally
      expect(true).toBe(true);
      done();
    });
  });

  it('promise chain call, onFulfilled', done => {
    new Promise(function (resolve) {
      setTimeout(function () {
        resolve(time);
      }, time);
    }).then(ms => {
      return new Promise(function (resolve) {
        setTimeout(function () {
          resolve(ms + time);
        }, time);
      });
    }).then(total => {
      expect(total).toBe(time * 2);
      done();
    });
  });

  it('promise chain call, onRejected', done => {
    new Promise(function (resolve, reject) {
      setTimeout(function () {
        reject(time);
      }, time);
    }).then(ms => {
      return new Promise(function (resolve) {
        setTimeout(function () {
          resolve(ms + time);
        }, time);
      });
    }).then(total => {
      return new Promise(function (resolve) {
        setTimeout(function () {
          resolve(total + time);
        }, time);
      });
    }).catch(reason => {
      expect(reason).toBe(time);
      done();
    });
  });

  it('can only change status once, cannot from fulfilled to rejected', async () => {
    let result = '';
    const promise = new Promise(function (resolve, reject) {
      setTimeout(function () {
        resolve(time);
        setTimeout(function () {  // 设定之后再 reject 一次
          reject(time);
        }, 0);
      }, time);
    });
    promise.then(() => {
      result += '=fulfilled=';
    }).catch(() => {
      result += '=rejected=';
    });

    await sleep(2000);
    // 不等于 'fulfilled rejected'
    expect(result).not.toBe('=fulfilled==rejected=');
    // 等于 'fulfilled'
    expect(result).toBe('=fulfilled=');
  });

  it('can only change status once, cannot from rejected to fulfilled', async () => {
    let result = '';
    const promise = new Promise(function (resolve, reject) {
      setTimeout(function () {
        reject(time);
        setTimeout(function () {  // 设定之后再 resolve 一次
          resolve(time);
        }, 0);
      }, time);
    });
    promise.then(() => {
      result += '=fulfilled=';
    }).catch(() => {
      result += '=rejected=';
    });

    await sleep(2000);
    // 不等于 'fulfilled rejected'
    expect(result).not.toBe('=rejected==fulfilled=');
    // 等于 'rejected'
    expect(result).toBe('=rejected=');
  });

  it('Promise.resolve', done => {
    Promise.resolve(1).then(num => {
      expect(num).toBe(1);
      done();
    });
  });

  it('Promise.reject', done => {
    Promise.reject(1).catch(num => {
      expect(num).toBe(1);
      done();
    });
  });
});

执行单元测试用例:

PASS  src/javascript/promise/promise.test.js (8.328s)
  Promise
    ✓ take a function as an argument (13ms)
    ✓ return a promise instace, exposes the public API (2ms)
    ✓ promise.then, onFulfilled (507ms)
    ✓ promise.then, onRejected (505ms)
    ✓ promise.catch (506ms)
    ✓ promise.finally (509ms)
    ✓ chain call, onFulfilled (1007ms)
    ✓ chain call, onRejected (505ms)
    ✓ can only change status once, cannot from fulfilled to rejected (2002ms)
    ✓ can only change status once, cannot from rejected to fulfilled (2002ms)
    ✓ Promise.resolve
    ✓ Promise.reject (1ms)

async/await 原理实现

实现代码:

export function asyncGenerator(fn) {
  return function () {
    const gen = fn.apply(this, arguments);

    return new Promise(function(resolve, reject) {
      function step(result) {
        try {
          // 执行结束
          if (result.done) {
            return resolve(result.value);
          }
          // 继续往下执行
          Promise.resolve(result.value)
            .then(val => step(gen.next(val)))
            .catch(err => reject(err));
        }
        catch(e) {
          return reject(e);
        }
      }
      // 递归地一步一步执行 gen.next()
      step(gen.next());
    });
  };
}

单元测试用例:

import { asyncGenerator } from './async';

describe('async generator', () => {
  it('auto excute generator', done => {
    const asyncFunc = asyncGenerator(function*() {
      const a = yield new Promise(resolve => {
        setTimeout(() => {
          resolve('a');
        }, 1000);
      });
      const b = yield Promise.resolve('b');
      const c = yield 'c';
      const d = yield Promise.resolve('d');
      return [a, b, c, d];
    });

    asyncFunc().then(res => {
      expect(res).toEqual(['a', 'b', 'c', 'd']);
      done();
    });
  });

  it('auto excute generator, when get reject', done => {
    const errorMsg = 'error';
    const asyncFunc = asyncGenerator(function*() {
      const s = yield 's';
      yield Promise.reject(errorMsg);
      return s;
    });

    asyncFunc()
      .catch(res => {
        expect(res).toBe(errorMsg);
        done();
      });
  });
});

执行单元测试用例:

PASS  src/javascript/async/async.test.js
  async generator
    ✓ auto excute generator (1012ms)

订阅发布机制

实现代码:

export default class EventEmitter {
  constructor() {
    this.listeners = {};
  }

  on = (event, listener) => {
    this.bindEvent(event, listener, false);
  }

  once = (event, listener) => {
    this.bindEvent(event, listener, true);
  }

  emit = (event, ...args) => {
    if (!this.hasBind(event)) {
      console.warn(`this event: ${event} don't has bind listener.`);
      return;
    }

    const { listeners, isOnce } = this.listeners[event];
    listeners.forEach(listener => listener.call(this, ...args));
    if (isOnce) {
      this.off(event);
    }
  }

  off = (event, listener) => {
    if (!this.hasBind(event)) {
      console.warn(`this event: ${event} don't exist.`);
      return;
    }

    // remove all listener
    if (!listener) {
      delete this.listeners[event];
      return;
    }

    // remove specific listener
    const listeners = this.listeners[event].listeners;
    listeners.forEach(listener => {
      const index = listeners.indexOf(listener);
      if (index !== -1) {
        listeners.splice(index, 1);
      }
    });
  }

  hasBind = event => {
    return this.listeners[event]
      && this.listeners[event].listeners
      && this.listeners[event].listeners.length;
  }

  bindEvent = (event, listener, isOnce = false) => {
    if (!event || !listener) {
      return;
    }

    this.listeners[event] = this.listeners[event] || {
      isOnce: false,
      listeners: []
    };
    this.listeners[event].isOnce = isOnce;

    if (isOnce) {
      this.listeners[event].listeners = [listener];
    }
    else {
      this.listeners[event].listeners.push(listener);
    }
  }
}

单元测试用例:

import EventEmitter from './event-emitter';

describe('EventEmitter', () => {
  let emitter;

  beforeEach(() => {
    emitter = new EventEmitter();
  });

  it('exposes the public API', () => {
    expect(emitter).toHaveProperty('on');
    expect(emitter).toHaveProperty('emit');
    expect(emitter).toHaveProperty('once');
    expect(emitter).toHaveProperty('off');
  });

  it('emitter.on', () => {
    const foo = jest.fn();
    const bar = jest.fn();
    emitter.on('foo', foo);
    expect(emitter.listeners['foo'].listeners).toEqual([foo]);
    emitter.on('foo', bar);
    expect(emitter.listeners['foo'].listeners).toEqual([foo, bar]);
  });

  it('emitter.once', () => {
    const foo = jest.fn();
    const bar = jest.fn();
    emitter.once('foo', foo);
    expect(emitter.listeners['foo'].listeners).toEqual([foo]);
    emitter.once('foo', bar);
    expect(emitter.listeners['foo'].listeners).toEqual([bar]);
  });

  it('emitter.emit', () => {
    // emitter.on
    const foo = jest.fn();
    emitter.on('foo', foo);
    emitter.emit('foo', 'x');
    expect(foo).toHaveBeenNthCalledWith(1, 'x');
    emitter.emit('foo', 'x');
    expect(foo).toHaveBeenCalledTimes(2);

    // emitter.once
    const bar = jest.fn();
    emitter.once('bar', bar);
    emitter.emit('bar', 'x');
    expect(bar).toHaveBeenNthCalledWith(1, 'x');
    emitter.emit('bar', 'x');
    expect(bar).toHaveBeenCalledTimes(1);
  });

  it('emitter.off, remove all listener', () => {
    const foo = jest.fn();
    emitter.on('foo', foo);
    emitter.emit('foo', 'x');
    emitter.off('foo');
    emitter.emit('foo', 'x');
    expect(foo).toHaveBeenCalledTimes(1);
  });

  it('emitter.off, remove specific listener', () => {
    const foo = jest.fn();
    const bar = jest.fn();
    emitter.on('foo', foo);
    emitter.on('foo', bar);
    emitter.emit('foo', 'x');
    expect(foo).toHaveBeenCalledTimes(1);
    expect(bar).toHaveBeenCalledTimes(1);
    emitter.off('foo', foo);
    emitter.emit('foo', 'x');
    expect(foo).toHaveBeenCalledTimes(1);
    expect(bar).toHaveBeenCalledTimes(2);
  });
});

执行单元测试用例:

PASS  src/javascript/event-emitter/event-emitter.test.js
  EventEmitter
    ✓ exposes the public API (4ms)
    ✓ emitter.on (2ms)
    ✓ emitter.once (1ms)
    ✓ emitter.emit (14ms)
    ✓ emitter.off, remove all listener (2ms)
    ✓ emitter.off, remove specific listener (1ms)

数据结构:堆

实现代码:

/**
 * 堆(Heap)是计算机科学中的一种特别的树状数据结构,若是满足一下特性,即可成为堆:给定堆中任意节点 P 和 C,
 * 若 P 是 C 的母节点,那么 P 的值会小于等于(或大于等于)C 的值。若母节点的值恒小于等于子节点的值,此堆成为最小堆。
 * 若母节点恒小于等于子节点的值,此堆成为最小堆(min heap)
 * 若母节点恒大于等于子节点的值,此堆成为堆大堆(max heap)
 * 在堆中最顶端的那一个节点,称作根节点(root node),根节点本身没有母节点(parent node)
 *
 * 在队列中,调度程序反复提取队列中第一个作业并运行,因为实际情况中某些事件较短的任务将等待很长事件才能结束。
 * 或者某些不短小,但具有重要性的作业,同样应当具有优先权。堆即为解决此类问题设计的一种数据结构。
 * 也就是优先队列:一种特殊的队列,队列中元素出栈的顺序是按照元素的优先权大小,而不是元素入队的先后顺序。
 *
 * 堆的特性:
 * - 堆是一棵完全二叉树。即除了最底层,其他层的节点都被元素填满,且最底层尽可能地从左到右填入。
 * - 任意节点小于(或大于)它的所有子节点。
 * - 可以用数组来存储二叉堆。
 */

/**
 * 实现堆(最大堆、最小堆)
 */
export default class Heap {
  constructor(type, nums) {
    this.type = type || 'max';  // 默认为最大堆
    this.items = [];

    if (Array.isArray(nums) && nums.length) {
      nums.forEach(n => this.add(n));
    }
  }

  isMaxHeap = () => {
    return this.type === 'max';
  }

  isMinHeap = () => {
    return this.type === 'min';
  }

  size = () => {
    return this.items.length;
  }

  getParentIndex = i => {
    return Math.floor((i - 1) / 2);
  }

  getLeftChildIndex = i => {
    return i * 2 + 1;
  }

  getRightChildIndex = i => {
    return i * 2 + 2;
  }

  swap = (i, j) => {
    const temp = this.items[i];
    this.items[i] = this.items[j];
    this.items[j] = temp;
  }

  // 向堆底插入元素
  add = el => {
    this.items.push(el);
    this.siftUP(this.size() - 1);
  }

  siftUP = index => {
    // 递归终止条件
    if (index <= 0) {
      return;
    }

    // 找到父元素的索引和值
    const item = this.items[index];
    const parentIndex = this.getParentIndex(index);
    const parent = this.items[parentIndex];

    // 如果是最大堆
    if (this.isMaxHeap()) {
      // 如果母节点的值小于子节点,则该节点需要上浮,即需要交换位置
      if (item > parent) {
        this.swap(index, parentIndex);
        // 再递归对 parent 做上浮操作
        this.siftUP(parentIndex);
      }
    }
    else if (this.isMinHeap()) {
      // 如果母节点的值大于子节点,则该节点需要上浮,即需要交换位置
      if (item < parent) {
        this.swap(index, parentIndex);
        // 再递归对 parent 做上浮操作
        this.siftUP(parentIndex);
      }
    }
  }

  // 取出栈顶元素,并重新构建完全二叉树
  extract = () => {
    // 获取堆顶元素
    const item = this.items.shift();

    // 如果当前堆的元素个数小于 2 时,可以直接返回,不需要重新构建完全二叉树
    if (this.size() < 2) {
      return item;
    }

    // 现在分离成了两个完全二叉树,需要重新构建成一颗完全二叉树
    // 获取最后一个元素,并把它放到堆顶
    this.items.unshift(this.items.pop());
    // 进行 siftDown 操作,重新构建一颗完全二叉树
    this.siftDown(0);

    // 返回堆顶元素
    return item;
  }

  siftDown = index => {
    const leftChildIndex = this.getLeftChildIndex(index);

    // 当没有左子树(也就没有右子树)时,递归终止
    if (leftChildIndex >= this.size()) {
      return;
    }

    const leftChild = this.items[leftChildIndex];
    const rightChildIndex = this.getRightChildIndex(index);
    const rightChild = this.items[rightChildIndex];
    let nextIndex = leftChildIndex;

    if (this.isMaxHeap()) {
      // 找到左右子树的最大值
      if (typeof rightChild !== undefined && rightChild > leftChild) {
        nextIndex = rightChildIndex;
      }
    }
    else if (this.isMinHeap()) {
      // 找到左右子树的最小值
      if (typeof rightChild !== undefined && rightChild < leftChild) {
        nextIndex = rightChildIndex;
      }
    }

    const parent = this.items[index];
    const next = this.items[nextIndex];

    if (this.isMaxHeap()) {
      // 如果左右子树的最大值大于母节点的值,则母节点需要下沉,即需要交换位置
      if (next > parent) {
        this.swap(index, nextIndex);
        // 再递归对母节点进行下沉
        this.siftDown(nextIndex);
      }
    }
    else if (this.isMinHeap()) {
      // 如果左右子树的最小值小于母节点的值,则母节点需要下沉,即需要交换位置
      if (next < parent) {
        this.swap(index, nextIndex);
        // 再递归对母节点进行下沉
        this.siftDown(nextIndex);
      }
    }
  }

  toString = connector => {
    return this.items.join(connector);
  }
}

单元测试用例:

import Heap from './heap';

describe('Heap', () => {
  let heap;

  beforeEach(() => {
    heap = new Heap();
  });

  it('exposes the public API', () => {
    expect(heap).toHaveProperty('add');
    expect(heap).toHaveProperty('extract');
    expect(heap).toHaveProperty('toString');
  });
});

describe('Max Heap', () => {
  let heap;

  beforeEach(() => {
    heap = new Heap();
  });

  it('maxHeap.add', () => {
    // 乱序加入
    heap.add(3);
    heap.add(1);
    heap.add(5);
    heap.add(2);
    heap.add(4);
    heap.add(6);
    expect(heap.toString()).toBe('6,4,5,1,2,3');
  });

  it('maxHeap.extract', () => {
    // 乱序加入
    heap.add(1);
    heap.add(4);
    heap.add(2);
    heap.add(5);
    heap.add(3);
    heap.add(6);
    expect(heap.extract()).toBe(6);
    expect(heap.toString()).toBe('5,4,2,1,3');
  });
});

describe('Min Heap', () => {
  let heap;

  beforeEach(() => {
    heap = new Heap('min');
  });

  it('minHeap.add', () => {
    // 乱序加入
    heap.add(3);
    heap.add(1);
    heap.add(5);
    heap.add(2);
    heap.add(4);
    heap.add(6);
    expect(heap.toString()).toBe('1,2,5,3,4,6');
  });

  it('minHeap.extract', () => {
    // 乱序加入
    heap.add(1);
    heap.add(4);
    heap.add(2);
    heap.add(5);
    heap.add(3);
    heap.add(6);
    expect(heap.extract()).toBe(1);
    expect(heap.toString()).toBe('2,3,6,5,4');
  });
});

执行单元测试用例:

PASS  src/dsa/heap/heap.test.js
  Heap
    ✓ exposes the public API (4ms)
  Max Heap
    ✓ maxHeap.add (1ms)
    ✓ maxHeap.extract (1ms)
  Min Heap
    ✓ minHeap.add
    ✓ minHeap.extract (1ms)

算法:双指针

实现代码:

/**
 * 输入一个链表,输出该链表中倒数第 k 个结点。
 */
export default function findKthToTail(head, k) {
  if (!head || !k) {
    return null;
  }

  // 双指针
  let first = head;
  let second = head;
  let index = 1;

  while (first.next) {
    first = first.next;
    // 当 first 和 second 相距 k 时,second 开始逐渐前进
    if (index >= k) {
      second = second.next;
    }
    index++;
  }

  // 循环结束后, k 如果大于 index 则说明倒数第 k 个节点不存在
  if (k > index) {
    return null;
  }
  return second;
}

单元测试用例:

import { LinkedList } from './../linked-list/linkedList';
import findKthToTail from './findKthToTail';

describe('find Kth to tail', () => {
  it('when k is less or equal than linked list size', () => {
    const sourceArr = [1, 2, 3, 4, 5];
    const linkedList = LinkedList.from(sourceArr);
    expect(findKthToTail(linkedList.head, 3).val).toEqual(3);
    expect(findKthToTail(linkedList.head, 5).val).toEqual(1);
  });

  it('when k is greate than linked list size', () => {
    const sourceArr = [1, 2, 3, 4, 5];
    const linkedList = LinkedList.from(sourceArr);
    expect(findKthToTail(linkedList.head, 6)).toEqual(null);
  });
});

执行单元测试用例:

PASS  src/dsa/doublePointer/findKthToTail.test.js
  find Kth to tail
    ✓ when k is less or equal than linked list size (4ms)
    ✓ when k is greate than linked list size (9ms)

可以看出来,写单测并不复杂,测试框架的接口简单易用,写单测用例也比较容易。目前的代码覆盖率在 90% 以上。

结语

希望这篇文章能对你积累自己的源码库有所帮助,如果有其他想要加入的内容,欢迎提 IssuePR