JavaScript常見的手寫功能
- coldplay.xixi轉載
- 2020-09-18 16:55:402462瀏覽
相關學習推薦：javascript
1. 防抖動
function debounce(func, ms = 500) { let timer; return function (...args) { if (timer) {
clearTimeout(timer);
}
timer = setTimeout(() => {
func.apply(this, args);
}, ms);
};
}复制代码2. 節流
function throttle(func, ms) { let canRun = true; return function (...args) { if (!canRun) return;
canRun = false;
setTimeout(() => {
func.apply(this, args);
canRun = true;
}, ms);
};
}复制代码3. new
function myNew(Func) { const instance = {}; if (Func.prototype) { Object.setPrototypeOf(instance, Func.prototype);
} const res = Func.apply(instance, [].slice.call(arguments, 1)); if (typeof res === "function" || (typeof res === "object" && res !== null)) { return res;
} return instance;
}复制代码4. bind
Function.prototype.myBind = function (context = globalThis) { const fn = this; const args = Array.from(arguments).slice(1); const newFunc = function () { if (this instanceof newFunc) { // 通过 new 调用，绑定 this 为实例对象
fn.apply(this, args);
} else { // 通过普通函数形式调用，绑定 context
fn.apply(context, args);
}
}; // 支持 new 调用方式
newFunc.prototype = fn.prototype; return newFunc;
};复制代码5. call
Function.prototype.myCall = function (context = globalThis) { // 关键步骤，在 context 上调用方法，触发 this 绑定为 context
context.fn = this; let args = [].slice.call(arguments, 1); let res = context.fn(...args); delete context.fn; return res;
};复制代码6 . apply
Function.prototype.myApply = function (context = globalThis) { // 关键步骤，在 context 上调用方法，触发 this 绑定为 context
context.fn = this; let res; if (arguments[1]) {
res = context.fn(...arguments[1]);
} else {
res = context.fn();
} delete context.fn; return res;
};复制代码7. deepCopy
function deepCopy(obj, cache = new WeakMap()) { if (!obj instanceof Object) return obj; // 防止循环引用
if (cache.get(obj)) return cache.get(obj); // 支持函数
if (obj instanceof Function) { return function () {
obj.apply(this, arguments);
};
} // 支持日期
if (obj instanceof Date) return new Date(obj); // 支持正则对象
if (obj instanceof RegExp) return new RegExp(obj.source, obj.flags); // 还可以增加其他对象，比如：Map, Set等，根据情况判断增加即可，面试点到为止就可以了
// 数组是 key 为数字素银的特殊对象
const res = Array.isArray(obj) ? [] : {}; // 缓存 copy 的对象，用于出来循环引用的情况
cache.set(obj, res); Object.keys(obj).forEach((key) => { if (obj[key] instanceof Object) {
res[key] = deepCopy(obj[key], cache);
} else {
res[key] = obj[key];
}
}); return res;
}复制代码8. 事件匯流排| 發布訂閱模式
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) { const 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) { if (this.cache[name]) { for (let fn of this.cache[name]) {
fn();
}
}
}
emit(name, once = false) { if (this.cache[name]) { // 创建事件副本，如果回调函数内继续注册相同事件，触发时，会造成死循环
const tasks = this.cache[name].slice() for (let fn of tasks) {
fn();
} if (once) { delete this.cache[name]
}
}
}
}复制代码9. 柯里化：只傳遞給函數一部分參數來呼叫它，讓它回傳一個函數去處理剩下的參數
function curry(func) { return function curried(...args) { if (args.length >= func.length) {
func.apply(this, args);
} else { return function (...args2) {
curried.apply(this, args.concat(args2));
};
}
};
}复制代码10. es5 實作繼承
function create(proto) { function F() {}
F.prototype = proto; return new F();
}// Parentfunction Parent(name) { this.name = name;
}
Parent.prototype.say = function () { console.log(this.name);
};// Childfunction Child(age, name) {
Parent.call(this, name); this.age = age;
}
Child.prototype = create(Parent.prototype);
Child.prototype.constructor = Child;
Child.prototype.say = function () { console.log(this.age);
};复制代码11. instanceof
function instanceOf(instance, klass) { let proto = instance.__proto__; let prototype = klass.prototype; while (true) { if (proto === null) return false; if (proto === prototype) return true;
proto = proto.__proto__;
}
}复制代码12. 非同步並發數限制
/**
* 关键点说明
* 1. new promise 一经创建，立即执行
* 2. 使用 Promise.resolve().then 可以把任务加到微任务队列，防止立即执行迭代方法
* 3. 微任务处理过程中，产生的新的微任务，会在同一事件循环内，追加到微任务队列里
* 4. 使用 race 在某个任务完成时，继续添加任务，保持任务按照最大并发数进行执行
* 5. 任务完成后，需要从 doingTasks 中移出
*/function limit(count, array, iterateFunc) { const tasks = []; const doingTasks = []; let i = 0; const enqueue = () => { if (i === array.length) { return Promise.resolve();
} const task = Promise.resolve().then(() => iterateFunc(array[i++]));
tasks.push(task); const doing = task.then(() => doingTasks.splice(doingTasks.indexOf(doing), 1));
doingTasks.push(doing); const res = doingTasks.length >= count ? Promise.race(doingTasks) : Promise.resolve(); return res.then(enqueue);
}; return enqueue().then(() => Promise.all(tasks));
}// testconst timeout = i => new Promise(resolve => setTimeout(() => resolve(i), i));
limit(4, [1000, 1000, 1000, 1000], timeout).then((res) => { console.log(res);
})复制代码 13. 非同步串列| 非同步並行
// 大厂面试题，实现一个异步加法function asyncAdd(a, b, callback) {
setTimeout(function () {
callback(null, a + b);
}, 1000);
}// 0. promisifyconst promiseAdd = (a, b) => new Promise((resolve, reject) => {
asyncAdd(a, b, (err, res) => { if (err) {
reject(err)
} else {
resolve(res)
}
})
})// 1. 串行处理async function serialSum(...args) { return args.reduce((task, now) => task.then(res => promiseAdd(res, now)), Promise.resolve(0))
}// 2. 并行处理async function parallelSum(...args) { if (args.length === 1) return args[0] const tasks = [] for (let i = 0; i < args.length; i += 2) {
tasks.push(promiseAdd(args[i], args[i + 1] || 0))
} const results = await Promise.all(tasks) return parallelSum(...results)
}// 测试(async () => { const res1 = await serialSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12) console.log(res1) const res2 = await parallelSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12) console.log(res2)
})()复制代码14. vue reactive
// Dep moduleclass Dep { static stack = [] static target = null
deps = null
constructor() { this.deps = new Set()
}
depend() { if (Dep.target) { this.deps.add(Dep.target)
}
}
notify() { this.deps.forEach(w => w.update())
} static pushTarget(t) { if (this.target) { this.stack.push(this.target)
} this.target = t
} static popTarget() { this.target = this.stack.pop()
}
}// reactivefunction reactive(o) { if (o && typeof o === 'object') { Object.keys(o).forEach(k => {
defineReactive(o, k, o[k])
})
} return o
}function defineReactive(obj, k, val) { let dep = new Dep() Object.defineProperty(obj, k, {
get() {
dep.depend() return val
},
set(newVal) {
val = newVal
dep.notify()
}
}) if (isObj(val)) {
reactive(val)
}
}// watcherclass Watcher { constructor(effect) { this.effect = effect this.update()
}
update() {
Dep.pushTarget(this) this.value = this.effect()
Dep.popTarget() return this.value
}
}// 测试代码const data = reactive({ msg: 'aaa'})new Watcher(() => { console.log('===> effect', data.msg);
})
setTimeout(() => {
data.msg = 'hello'}, 1000)复制代码15. promise
// 建议阅读 [Promises/A+ 标准](https://promisesaplus.com/)class MyPromise { constructor(func) { this.status = 'pending'
this.value = null
this.resolvedTasks = [] this.rejectedTasks = [] this._resolve = this._resolve.bind(this) this._reject = this._reject.bind(this) try {
func(this._resolve, this._reject)
} catch (error) { this._reject(error)
}
}
_resolve(value) {
setTimeout(() => { this.status = 'fulfilled'
this.value = value this.resolvedTasks.forEach(t => t(value))
})
}
_reject(reason) {
setTimeout(() => { this.status = 'reject'
this.value = reason this.rejectedTasks.forEach(t => t(reason))
})
}
then(onFulfilled, onRejected) { return new MyPromise((resolve, reject) => { this.resolvedTasks.push((value) => { try { const res = onFulfilled(value) if (res instanceof MyPromise) {
res.then(resolve, reject)
} else {
resolve(res)
}
} catch (error) {
reject(error)
}
}) this.rejectedTasks.push((value) => { try { const res = onRejected(value) if (res instanceof MyPromise) {
res.then(resolve, reject)
} else {
reject(res)
}
} catch (error) {
reject(error)
}
})
})
} catch(onRejected) { return this.then(null, onRejected);
}
}// 测试new MyPromise((resolve) => {
setTimeout(() => {
resolve(1);
}, 500);
})
.then((res) => { console.log(res); return new MyPromise((resolve) => {
setTimeout(() => {
resolve(2);
}, 500);
});
})
.then((res) => { console.log(res);
}, err => { console.log('==>', err);
});复制代码歡迎大家一起補充~
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