Node 执行顺序 nextTick 异步IO setTimeout setImmediate观察者优先级在每次轮训检

var fs = require('fs');
fs.readFile('1.txt', function(err, data) {
    console.log('异步');
})

setTimeout(function() {
    console.log('setTimeout');
}, 0);


process.nextTick(function() {
    console.log('nextTick-a');
    process.nextTick(function() {
        console.log('nextTick-b');
    })
})

setImmediate(function() {
    console.log('setImmediate-a');
    setImmediate(function() {
        console.log('setImmediate-b');
    })
})
/*
nextTick-a
nextTick-b
setTimeout
异步
setImmediate-a
setImmediate-b
*/

观察者优先级

在每次轮训检查中，各观察者的优先级分别是：

idle观察者 > I/O观察者 > check观察者。

idle观察者：process.nextTick

I/O观察者：一般性的I/O回调，如网络，文件，数据库I/O等

check观察者：setTimeout>setImmediate

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