非同步IO與同步IO在時間表現上，非同步IO預期於最慢一個IO，而同步IO則是所有IO的總和。但是非同步IO的結果也是需要統一處理的。因此就涉及到多非同步之間的協作。

在node中，我們在全域性上設定一個哨兵變數來協作。

下面程式碼使用process.nextTick來模擬一次非同步的IO：

"use strict";
process.nextTick(function(){
    console.log('nextTick的執行');
    done();//每次呼叫count自增1

});
process.nextTick(function(){
    console.log('nextTick的執行');
    done();//每次呼叫count自增1
 


});
process.nextTick(function(){
    console.log('nextTick的執行');
    done();//每次呼叫count自增1
});

var count=0;//哨兵變數

var done=function(){
    count++;
    if(count===3){//當count為3的時候，說明三個非同步都完成了，執行非同步完成的處理
        console.log('全部非同步IO完成');
    }
}

"use strict";
process.nextTick(function(){
    console.log('nextTick的執行');
    done();
    done2();
});
process.nextTick(function(){
    console.log('nextTick的執行');
    done();
    done2();
});
process.nextTick(function(){
    console.log('nextTick的執行');
    done();
    done2();
});

var count=0;

var done=function(){
    count++;
    if(count===3){
        console.log('done1:全部非同步IO完成');
    }
}
var done2=function(){
    if(count===3){
        console.log('done2:全部非同步IO完成');
    }
}
 

"use strict";

const events = require('events');
var emitter=new events.EventEmitter();

process.nextTick(function(){
    console.log('nextTick的執行');
    emitter.emit("done");
});
process.nextTick(function(){
    console.log('nextTick的執行');
    emitter.emit("done");
});
process.nextTick(function(){
    console.log('nextTick的執行');
    emitter.emit("done");
});

var count=0;

var done=function(){
    count++;
    if(count===3){
        console.log('done1:全部非同步IO完成');
    }
}
var done2=function(){
    if(count===3){
        console.log('done2:全部非同步IO完成');
    }
}
emitter.on('done',done);
emitter.on('done',done2);