阿里面试题为例

完成 EventEmitter 模块，它是一个类，它的实例具有以下几个方法：on、emit、off：

on(eventName, func)：监听 eventName 事件，事件触发的时候调用 func 函数。
emit(eventName, arg1, arg2, arg3...)：触发 eventName 事件，并且把参数 arg1, arg2, arg3... 传给事件处理函数。
off(eventName, func)：停止监听某个事件。

    class EventEmitter {
        constructor() {
            this.handlers = {}
        }

        on(eventName, func) {
            let callbacks = eventName in this.handlers ? this.handlers[eventName] : []
            callbacks.push(func)
            this.handlers[eventName] = callbacks
        }

        emit(eventName, ...args) {
            if (!eventName in this.handlers) return
            const callbacks = this.handlers[eventName]
            callbacks.map(cb => {
                cb(...args)
            })
        }

        off(eventName, func) {
            if (!eventName in this.handlers) return
            let callbacks = this.handlers[eventName]
            let index = callbacks.indexOf(func)
            callbacks.splice(index, 1)
        }
    }

    const emitter = new EventEmitter()
    const sayHi = (name) => console.log(`Hello ${name}`)
    const sayHi2 = (name) => console.log(`Good night, ${name}`)

    emitter.on('hi', sayHi)
    emitter.on('hi', sayHi2)
    emitter.emit('hi', 'ScriptOJ')
    // => Hello ScriptOJ
    // => Good night, ScriptOJ

    emitter.off('hi', sayHi)
    emitter.emit('hi', 'ScriptOJ')
    // => Good night, ScriptOJ

    const emitter2 = new EventEmitter()
    emitter2.on('hi', (name, age) => {
        console.log(`I am ${name}, and I am ${age} years old`)
    })
    emitter2.emit('hi', 'Jerry', 12)
    // -------------------------------------------------------------------------------
    class Person extends EventEmitter {
        constructor(name) {
        super()
        this.name = name;
        this.age = 1;
        // this.on('growup', () => {
        //     console.log('ceshi' + this.name);
        // })
        // this.growup()
    }

        growup() {
            setInterval(() => {
                this.age++;
                this.emit('growup');
            }, 1000);
        }
    }

    const p1 = new Person('xiao_liu');
    const p2 = new Person('xiao_liu1');
    p1.on('growup', () => {
        console.log('长大一岁');
    })
    p2.on('growup', () => {
        console.log('长大一岁');
    })

    p1.growup()
    p2.growup()