新冠疫情慢慢消退，各行各业裁员跳槽大洗牌，为了收割满意的offer，每天不得不逼着自己天天刷题。下面是来自阿里的面试题，希望可以帮到你。

例1

实现一个 normalize 函数，能将输入的特定的字符串转化为特定的结构化数据。

符串仅由小写字母和[,]组成，且字符串不会包含多余的空格。
示例一: 'abc' --> {value: 'abc'}
示例二：'[abc[bcd[def]]]' -> {value: 'abc', children: {value: 'bcd', children: {value: 'def'}}}

通过栈可以用来判断一个表达式中括号是否匹配。栈是一种LIFO（Last-In-First-Out，后进先出）的数据结构，也就是最新添加的项最早被移除。而栈中项的插入（叫做推入）和移除（叫做弹出），只发生在一个位置——栈的顶部。数组专门提供了 push() 和 pop() 方法，以便实现类似栈的行为。

function todo(str) {let s = [];let list = [];let obj = {}for (let i = 0; i < str.length; i++) {let value = str[i]switch (value) {case '[':s.push({ index: i, value: '[' })break;case ']':let last = s.pop();list.unshift([last.index, i])default:break;}}let [start, end] = list[0]let parent = objfor (let i = 1; i < list.length; i++) {let [a, b] = list[i];let result = str.slice(start + 1, a) + str.slice(b + 1, end);start = a;end = b;parent.value = result;parent.children = {};parent = parent.children;}let [x, y] = list[list.length - 1]parent.value = str.slice(x + 1, y)return obj
}

例2

实现一个事件收发器 Event 类，继承自此类的对象拥有 on,off,once 和 trigger 方法。

// 例如
const event = new Event();
function log(val) {console.log(val);};
event.on('foo_event', log);
event.trigger('foo_event', 'abc'); // 打印出 abc
event.off('foo_event', log);
event.trigger('foo_event', 'abc'); // 打印出 undefined

这个是Node中的events模块里面的知识，直接上源码。（发布订阅模式大家一定要掌握，这是非常常见的设计模式！）

function checkListener(listener) {if (typeof listener !== 'function') {throw new Error('listener', 'Function', listener);}
}function spliceOne(list, index) {for (; index + 1 < list.length; index++)list[index] = list[index + 1];list.pop();
}function EventEmitter() {EventEmitter.init.call(this)
}EventEmitter.prototype._events = undefined
EventEmitter.prototype._eventsCount = 0;EventEmitter.init = function () {if (this._events === undefined || this._events === Object.getPrototypeOf(this)._events) {this._events = Object.create(null)this._eventsCount = 0;}
}function _addListener(target, type, listener) {let events;let exiting;checkListener(listener)events = target._eventsif (events === undefined) {events = target._events = Object.create(null)target._eventsCount = 0;} else {exiting = events[type]}if (exiting === undefined) {events[type] = listener;++target._eventsCount;} else {if (typeof exiting === 'function') {exiting = events[type] = [exiting, listener]} else {exiting.push(listener)}}return target
}EventEmitter.prototype.addListener = function addListener(type, listener) {return _addListener(this, type, listener)
}EventEmitter.prototype.on = EventEmitter.prototype.addListener;EventEmitter.prototype.removeListener = function removeListener (type, listener) {let list;let events;checkListener(listener)events = this._events;if (events === undefined) {return this;}list = events[type]if (list === undefined) {return this;}if (list === listener || list.listener === listener) {if (--this._eventsCount === 0) {this._events = Object.create(null)} else {delete events[type];}}
}EventEmitter.prototype.off = EventEmitter.prototype.removeListenerEventEmitter.prototype.trigger = function trigger(type, ...args) {const events = this._events;const handler = events[type]if (handler === undefined) {console.log(undefined)return false;}if (typeof handler === 'function') {Reflect.apply(handler, this, args)} else {const len = handler.length;const listeners = arrayClone(handler, len);for (let i = 0; i < len; i++) {Reflect.apply(listeners[i], this, args)}}return true
}function arrayClone(arr, n) {var copy = new Array(n);for (var i = 0; i < n; ++i)copy[i] = arr[i];return copy;
}function _onceWrap(target, type, listener) {let state = { fired: false, wrapFn: undefined, target, type, listener }var wrapped = onceWrapper.bind(state);wrapped.listener = listener;state.wrapFn = wrapped;return wrapped
}function onceWrapper(...args) {if (!this.fired) {this.target.removeListener(this.type, this.wrapFn);this.fired = true;Reflect.apply(this.listener, this.target, args)}
}EventEmitter.prototype.once = function once(type, listener) {checkListener(listener)this.on(type, _onceWrap(this, type, listener))return this
}

例3

实现一个 DOM 事件代理方法 delegate，接收一个 dom 元素，有两个方法，分别是添加 on 和 off。

// 例如
delegate(parentElement).on('.childClassname', 'click', callback);
delegate(parentElement).off('.childClassname', 'click', callback);

这里考察事件委托，如果你了解React合成事件原理，把事件都代理到document中，那么你很容易写出来。

function delegate(parentElement) {function on(childClassname, eventType, listener) {let list = parentElement.querySelectorAll(childClassname)Array.from(list, (dom) => {let eventStore = dom.eventStore || (dom.eventStore = {})eventStore[eventType] = listener;})document.addEventListener(eventType, dispatchEvent, false)}function dispatchEvent(event) {let { type, target } = eventlet eventType = type;while (target) {let { eventStore } = target;let listener = eventStore && eventStore[eventType]if (listener) {listener.call(target, event)}target = target.parentNode;}}function off(childClassname, eventType, listener) {let list = parentElement.querySelectorAll(childClassname)Array.from(list, (dom) => {let eventStore = dom.eventStorelet listener = eventStore && eventStore[eventType]if (listener) {eventStore[eventType] = null}})}return { on, off }
}

解决问题方式多种多样，答案仅供参考，欢迎在评论区留言，说说你对题目的看法。

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