原生js手写

发表于

通过基础的代码，实现原生js的一些方法。更有利于对JavaScript加深理解。

部分内容具体实现可参考冴羽的深入系列。

数据类型判断

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function typeOf(){
  return Object.prototype.toString.call(obj).slice(8, -1).toLowerCase()
}

数组去重

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// 类似的还有indexOf，includes，some等等
function unique(arr) {
    var res = arr.filter(function(item, index, array) {
        return array.indexOf(item) === index
    })
    return res
}

// es6
var unique = arr => [...new Set(arr)]

数组扁平化

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

// es6，参数为处理几层，默认1
Array.prototype.flat(infinity)

浅拷贝

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// 它不会拷贝对象的继承属性；
// 它不会拷贝对象的不可枚举的属性；
// 可以拷贝 Symbol 类型的属性。
Object.assign(target, ...sources)

// 对象扩展符，和上面的类似
let cloneObj = { ...obj };

// concat 拷贝数组
let newArr = arr.concat();

// slice 拷贝数组
let newArr = arr.slice();

// 手动实现
const shallowClone = (target) => {
  if (typeof target === 'object' && target !== null) {
    const cloneTarget = Array.isArray(target) ? []: {};
    for (let prop in target) {
      if (target.hasOwnProperty(prop)) {
          cloneTarget[prop] = target[prop];
      }
    }
    return cloneTarget;
  } else {
    return target;
  }
}

深拷贝

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// 简单版，忽略undefund，symbol，无法转换正则日期等对象，不能循环引用对象
JSON.parse(JSON.stringify(Obj))

// 手写版
const isComplexDataType = obj => (typeof obj === 'object' || typeof obj === 'function') && (obj !== null)

const deepClone = function (obj, hash = new WeakMap()) {
  if (obj.constructor === Date) {
    return new Date(obj)       // 日期对象直接返回一个新的日期对象
  }
  
  if (obj.constructor === RegExp){
    return new RegExp(obj)     //正则对象直接返回一个新的正则对象
  }
  
  //如果循环引用了就用 weakMap 来解决
  if (hash.has(obj)) {
    return hash.get(obj)
  }
  let allDesc = Object.getOwnPropertyDescriptors(obj)

  //遍历传入参数所有键的特性
  let cloneObj = Object.create(Object.getPrototypeOf(obj), allDesc)

  // 把cloneObj原型复制到obj上
  hash.set(obj, cloneObj)

  for (let key of Reflect.ownKeys(obj)) { 
    cloneObj[key] = (isComplexDataType(obj[key]) && typeof obj[key] !== 'function') ? deepClone(obj[key], hash) : obj[key]
  }
  return cloneObj
}

发布订阅模式

同观察者模式区别在于，没有中间消息处理中心，可参考观察者模式 vs 发布订阅模式

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class EventEmitter {
    constructor() {
        this.cache = {}
    }
    on(name, fn) {
        if (this.cache[name]) {
            this.cache[name].push(fn)
        } else {
            this.cache[name] = [fn]
        }
    }
    off(name, fn) {
        let tasks = this.cache[name]
        if (tasks) {
            const index = tasks.findIndex(f => f === fn || f.callback === fn)
            if (index >= 0) {
                tasks.splice(index, 1)
            }
        }
    }
    emit(name, once = false, ...args) {
        if (this.cache[name]) {
            // 创建副本，如果回调函数内继续注册相同事件，会造成死循环
            let tasks = this.cache[name].slice()
            for (let fn of tasks) {
                fn(...args)
            }
            if (once) {
                delete this.cache[name]
            }
        }
    }
}

字符串模版

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function render(template, data) {
    const reg = /\{\{(\w+)\}\}/; // 模板字符串正则
    if (reg.test(template)) { // 判断模板里是否有模板字符串
        const name = reg.exec(template)[1]; // 查找当前模板里第一个模板字符串的字段
        template = template.replace(reg, data[name]); // 将第一个模板字符串渲染
        return render(template, data); // 递归的渲染并返回渲染后的结构
    }
    return template; // 如果模板没有模板字符串直接返回
}

防抖

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function debounce(func, wait) {
    var timeout;
    return function () {
        var context = this;
        var args = arguments;
        clearTimeout(timeout)
        timeout = setTimeout(function(){
            func.apply(context, args)
        }, wait);
    }
}

// 最终版
function debounce(func, wait, immediate) {
    var timeout, result;
    
    var debounced = function () {
        var context = this;
        var args = arguments;
        
        if (timeout) clearTimeout(timeout);
        if (immediate) {
            var callNow = !timeout;
            timeout = setTimeout(function(){
                timeout = null;
            }, wait)
            if (callNow) result = func.apply(context, args)
        } else {
            timeout = setTimeout(function(){
                func.apply(context, args)
            }, wait);
        }
        return result;
    };

    debounced.cancel = function() {
        clearTimeout(timeout);
        timeout = null;
    };

    return debounced;
}

节流

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function throttle(func, wait) {
    var context, args;
    var previous = 0;

    return function() {
        var now = +new Date();
        context = this;
        args = arguments;
        if (now - previous > wait) {
            func.apply(context, args);
            previous = now;
        }
    }
}

// 最终版
function throttle(func, wait, options) {
    var timeout, context, args, result;
    var previous = 0;
    if (!options) options = {};

    var later = function() {
        previous = options.leading === false ? 0 : new Date().getTime();
        timeout = null;
        func.apply(context, args);
        if (!timeout) context = args = null;
    };

    var throttled = function() {
        var now = new Date().getTime();
        if (!previous && options.leading === false) previous = now;
        var remaining = wait - (now - previous);
        context = this;
        args = arguments;
        if (remaining <= 0 || remaining > wait) {
            if (timeout) {
                clearTimeout(timeout);
                timeout = null;
            }
            previous = now;
            func.apply(context, args);
            if (!timeout) context = args = null;
        } else if (!timeout && options.trailing !== false) {
            timeout = setTimeout(later, remaining);
        }
    };
    
    throttled.cancel = function() {
        clearTimeout(timeout);
        previous = 0;
        timeout = null;
    }
    return throttled;
}

柯里化

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function curry(fn) {
    let judge = (...args) => {
        if (args.length == fn.length) return fn(...args)
        return (...arg) => judge(...args, ...arg)
    }
    return judge
}

偏函数

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function partial(fn, ...args) {
    return (...arg) => {
        args[index] = 
        return fn(...args, ...arg)
    }
}

JSONP

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const jsonp = ({ url, params, callbackName }) => {
    const generateUrl = () => {
        let dataSrc = ''
        for (let key in params) {
            if (params.hasOwnProperty(key)) {
                dataSrc += `${key}=${params[key]}&`
            }
        }
        dataSrc += `callback=${callbackName}`
        return `${url}?${dataSrc}`
    }
    return new Promise((resolve, reject) => {
        const scriptEle = document.createElement('script')
        scriptEle.src = generateUrl()
        document.body.appendChild(scriptEle)
        window[callbackName] = data => {
            resolve(data)
            document.removeChild(scriptEle)
        }
    })
}

Ajax

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const getJSON = function(url) {
    return new Promise((resolve, reject) => {
        const xhr = XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Mscrosoft.XMLHttp');
        xhr.open('GET', url, false);
        xhr.setRequestHeader('Accept', 'application/json');
        xhr.onreadystatechange = function() {
            if (xhr.readyState !== 4) return;
            if (xhr.status === 200 || xhr.status === 304) {
                resolve(xhr.responseText);
            } else {
                reject(new Error(xhr.responseText));
            }
        }
        xhr.send();
    })
}

call、apply、bind

利用obj.fn的this指向obj。可参考JavaScript深入之call和apply的模拟实现相关内容。

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Function.prototype.call2 = function (context) {
    var context = context || window;
    context.fn = this;

    var args = [];
    for(var i = 1, len = arguments.length; i < len; i++) {
        args.push('arguments[' + i + ']');
    }

    var result = eval('context.fn(' + args +')');

    delete context.fn
    return result;
}
Function.prototype.apply2 = function (context, arr) {
    var context = context || window;
    context.fn = this;

    var result;
    if (!arr) {
        result = context.fn();
    } else {
        var args = [];
        for (var i = 0, len = arr.length; i < len; i++) {
            args.push('arr[' + i + ']');
        }
        result = eval('context.fn(' + args + ')')
    }

    delete context.fn
    return result;
}
Function.prototype.bind2 = function (context) {
    var self = this;
    var args = Array.prototype.slice.call(arguments, 1);

    var fNOP = function () {};

    var fBound = function () {
        var bindArgs = Array.prototype.slice.call(arguments);
        // 处理new时
        return self.apply(this instanceof fNOP ? this : context, args.concat(bindArgs));
    }

    fNOP.prototype = this.prototype;
    fBound.prototype = new fNOP();
    return fBound;
}

new

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function objectFactory() {
    var obj = new Object()
    Constructor = [].shift.call(arguments);
    obj.__proto__ = Constructor.prototype;
    var ret = Constructor.apply(obj, arguments);
    
    // 存在值为对象的情况应返回对象，否则返回生成的对象
    return typeof ret === 'object' ? ret || obj : obj;
};

instanceof

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function instanceOf(left, right) {
    let proto = left.__proto__
    while (true) {
        if (proto === null) return false
        if (proto === right.prototype) {
            return true
        }
        proto = proto.__proto__
    }
}

Object.create

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Object.create2 = function(proto, propertyObject = undefined) {
    if (typeof proto !== 'object' && typeof proto !== 'function') {
        throw new TypeError('Object prototype may only be an Object or null.')
    if (propertyObject == null) {
        new TypeError('Cannot convert undefined or null to object')
    }
    function F() {}
    F.prototype = proto
    const obj = new F()
    if (propertyObject != undefined) {
        Object.defineProperties(obj, propertyObject)
    }
    if (proto === null) {
        // 创建一个没有原型对象的对象，Object.create(null)
        obj.__proto__ = null
    }
    return obj
}

Object.assign

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Object.assign2 = function(target, ...source) {
    if (target == null) {
        throw new TypeError('Cannot convert undefined or null to object')
    }
    let ret = Object(target) 
    source.forEach(function(obj) {
        if (obj != null) {
            for (let key in obj) {
                if (obj.hasOwnProperty(key)) {
                    ret[key] = obj[key]
                }
            }
        }
    })
    return ret
}

Promise

从一道让我失眠的 Promise 面试题开始，深入分析 Promise 实现细节
要就来45道Promise面试题一次爽到底

------本文结束 感谢阅读------