浅析Event Loop

事件循环是由一系列阶段构成的
setTimeout()会被放在timers阶段
setImmediate()会被放在check阶段
process.nextTick()会在当前阶段结束后紧跟着执行

来判断如下代码的用node.js执行后输出顺序

setTimeout(()=>console.log('setTimeout'),0)
setImmediate(()=>console.log('setImmediate'))
/*
output:
setTimeout
setImmediate
*/
//或
/*output
setImmediate
setTimeout
*/

可以发现执行两个计时器的顺序是非确定性的。
因为不在 I/O 周期（即主模块）内的运行的脚本，它受进程性能的约束。

如果把这两个函数放入I/O循环内调用，setImmediate总是会先被调用

const fs = require('fs');
fs.readFile(__filename, () => {
  setTimeout(() => {
    console.log('setTimeout');
  }, 0);
  setImmediate(() => {
    console.log('setImmediate');
  });
});
/*output
setImmediate
setTimeout
*/

把这两个函数放在setTimeout或setImmediate里调用，setImmediate也总是会被先调用

setImmediate(()=>{
    setTimeout(()=>console.log('setTimeout'),0)
    setImmediate(()=>console.log('setImmediate'))
})
/*output
setImmediate
setTimeout
*/

setTimeout(()=>{
    setTimeout(()=>console.log('setTimeout'),0)
    setImmediate(()=>console.log('setImmediate'))
},0)
/*output
setImmediate
setTimeout
*/

关于process.nextTick()的执行顺序

setTimeout(()=>{
    setTimeout(()=>console.log('setTimeout'),0)
    setImmediate(()=>console.log('setImmediate'))
      process.nextTick(()=>console.log('nextTick'))
},0)
/*output
nextTick
setImmediate
setTimeout
*/

setTimeout(() => {
  setTimeout(() => console.log("setTimeout"), 0);
  setImmediate(() => {
    process.nextTick(() => console.log("nextTick"));
    console.log("setImmediate");
  });
}, 0);
/*output
setImmediate
nextTick
setTimeout
*/

setTimeout(() => {
  setTimeout(() => {
    process.nextTick(() => console.log("nextTick"));
    console.log("setTimeout");
  }, 0);
  setImmediate(() => console.log("setImmediate"));
}, 0);
/*output
setImmediate
setTimeout
nextTick
*/