javascript - 为什么以及何时使用 process.nextTick？

Question

下面是“practise01.js”文件中的代码，

function fn(name){
   return f;

   function f(){
       var n = name;
       console.log("Next TICK "+n+", ");        
   }
}

function myTimeout(time,msg){
   setTimeout(function(){
       console.log("TIMEOUT "+msg);
   },time); 
}

process.nextTick(fn("ONE"));
myTimeout(500,"AFTER-ONE");
process.nextTick(fn("TWO"));
myTimeout(500,"AFTER-TWO");
process.nextTick(fn("THREE"));
myTimeout(500,"AFTER-THREE");
process.nextTick(fn("FOUR"));

运行上面代码的输出是

rahul@rahul:~/myPractise/PlainNodeJSPractise01/Process$ node practise01.js                  

Next TICK ONE, 
Next TICK TWO, 
Next TICK THREE, 
Next TICK FOUR, 
TIMEOUT AFTER-ONE
TIMEOUT AFTER-TWO
TIMEOUT AFTER-THREE

现在我在“practise02.js”中不使用process.nextTick写了代码，如下，

function myTimeout(time,msg){
  setTimeout(function(){
        console.log("TIMEOUT "+msg);
  },time);  
}

function fn(name){
  return f;

  function f(){
    var n = name;
    console.log("Next TICK "+n+", ");       
  }
}

fn("ONE")();
myTimeout(500,"AFTER-ONE");
fn("TWO")();
myTimeout(500,"AFTER-TWO");
fn("THREE")();
myTimeout(500,"AFTER-THREE");
fn("FOUR")();

运行上述代码后输出为

rahul@rahul:~/myPractise/PlainNodeJSPractise01/Process$ node practise02.js 
Next TICK ONE, 
Next TICK TWO, 
Next TICK THREE, 
Next TICK FOUR, 
TIMEOUT AFTER-ONE
TIMEOUT AFTER-TWO
TIMEOUT AFTER-THREE

如果您看到两个输出相同。

那么在哪种情况下我需要使用 process.nextTick ？

当我尝试阅读更多内容时，我开始明白如果我需要在 eventloop 为空时立即执行某些功能而不是去“process.nextTick”。

那么它与我的第二种方法有何不同。

请解释或给我一些指示

Answer 1

Node 文档实际上很好地解释了何时以及为何使用 nextTick:

https://nodejs.org/api/process.html#process_process_nexttick_callback_args

它的作用:

This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

以及何时使用:

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred...

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}
definitelyAsync(true, () => {
  foo();
});
bar(); // will now allways be called before foo()