javascript - Node.js 中使用 setImmediate() 和 process.nextTick() 的意义

Question

我正在学习 Node.js，但很难理解 setImmediate() 和 process.nextTick() 的工作原理。为了清楚地理解这些概念，我有三个程序，一个简单的 Node.js 程序，一个使用 setImmediate() ，一个使用 process.nextTick() 。显然这三个人的行为方式不同。如果有人解释这些程序在底层操作上有何不同，并解释使用这两个计时器进行有效 Node.js 编程的重要性，那将会非常有帮助。

程序 1:(没有 setImmediate() 和 process.nextTick())

var fs = require('fs');

function logCar(car, callback){
  console.log("Saw a %s", car);
  if(cars.length){
    callback();
  }
}
function logCars(cars){
  var car = cars.pop();
  logCar(car, function(){
    logCars(cars);
  });
}
var cars = ["Ferrari", "Porsche", "Bugatti", 
            "Lamborghini", "Aston Martin"];
logCars(cars);

var fileCheck = function(){
    console.log('entered file operation');
    fs.stat("fileio.js", function(err,stats){
    if(stats)
    console.log("file exists");
    });
    }
    
    fileCheck();

结果:

Saw a Aston Martin
Saw a Lamborghini
Saw a Bugatti
Saw a Porsche
Saw a Ferrari
entered file operation
file exists

程序 1:(使用 setImmediate())

var fs = require('fs');

function logCar(car, callback){
  console.log("Saw a %s", car);
  if(cars.length){
    setImmediate(function(){
      callback();
    });
  }
}
function logCars(cars){
  var car = cars.pop();
  logCar(car, function(){
    logCars(cars);
  });
}
var cars = ["Ferrari", "Porsche", "Bugatti", 
            "Lamborghini", "Aston Martin"];
logCars(cars);

var fileCheck = function(){
    console.log('entered file operation');
    fs.stat("fileio.js", function(err,stats){
    if(stats)
    console.log("file exists");
    });
    }
    
    fileCheck();

结果:

Saw a Aston Martin
entered file operation
Saw a Lamborghini
file exists
Saw a Bugatti
Saw a Porsche
Saw a Ferrari

程序 2:(使用 process.nextTick())

var fs = require('fs');

function logCar(car, callback){
  console.log("Saw a %s", car);
  if(cars.length){
    process.nextTick(function(){
      callback();
    });
  }
}
function logCars(cars){
  var car = cars.pop();
  logCar(car, function(){
    logCars(cars);
  });
}
var cars = ["Ferrari", "Porsche", "Bugatti", 
            "Lamborghini", "Aston Martin"];
logCars(cars);

var fileCheck = function(){
    console.log('entered file operation');
    fs.stat("fileio.js", function(err,stats){
    if(stats)
    console.log("file exists");
    });
    }
    
    fileCheck();

结果:

Saw a Aston Martin
entered file operation
Saw a Lamborghini
Saw a Bugatti
Saw a Porsche
Saw a Ferrari
file exists

Answer 1

使用 process.nextTick() 延迟的回调在触发任何其他 I/O 事件之前运行。

使用setImmediate()，执行将排在队列中已存在的任何 I/O 事件之后。