Node.js 事件循环详解

// 执行宏任务
setImmediate(() => {
    console.log('10-setImmediate');
});

// 执行 nextTick 队列中的所有回调后，然后再处理微任务队列中的回调
Promise.resolve().then(() => {
    console.log('5-promise 1 start');

    Promise.resolve().then(() => {
        console.log('8-promise in promise');
    })

    // 注意
    // process.nextTick() 会在每次事件循环的微任务执行中。如果在 Promise 回调中又调用了 process.nextTick()，它不会立即执行，会等其他微任务执行完成后，再执行
    process.nextTick(() => {
        console.log('9-nextTick in promise');
    })
    // 微任务执行
    console.log('6-promise 1 end')
})

// 当前执行栈中的所有同步代码执行完后，Node.js 会先执行 nextTick 队列中的回调
process.nextTick(() => {
    console.log('2-nextTick 1 start');

    Promise.resolve().then(() => {
        console.log('7-promise in nextTick');
    })

    // process.nextTick() 会在每次事件循环的 任何微任务之前执行。如果在 process.nextTick() 中又调用了 process.nextTick()，它会立即执行，不会让出控制权给其他微任务
    process.nextTick(() => {
        console.log('4-nextTick in nextTick');
    })
    console.log('3-nextTick 1 end');
})

// 同步代码执行
console.log('1-global');
// 1-global
// 2-nextTick 1 start
// 3-nextTick 1 end
// 4-nextTick in nextTick
// 5-promise 1 start
// 6-promise 1 end
// 7-promise in nextTick
// 8-nextTick in promise
// 9-promise in promise
// 10-setImmediate

setTimeout(() => {
    console.log('16-setTimeout 100');

    setTimeout(() => {
        console.log('21-setTimeout 100 - 0');
        process.nextTick(() => {
            console.log('22-nextTick in setTimeout 100 - 0');
        })
    }, 0)

    // 这次setImmediate比setTimeout(0)先执行的原因是：流程从timers向后走到check队列，已经有了setImmediate的回调，立即执行。
    setImmediate(() => {
        console.log('19-setImmediate in setTimeout 100');
        process.nextTick(() => {
            console.log('20-nextTick in setImmediate in setTimeout 100');
        })
    });

    process.nextTick(() => {
        console.log('17-nextTick in setTimeout100');
    })

    Promise.resolve().then(() => {
        console.log('18-promise in setTimeout100');
    })
}, 100)

const fs = require('fs')
fs.readFile('./1.poll.js', () => {
    console.log('14-poll 1');
    process.nextTick(() => {
        console.log('15-nextTick in poll');
    })
})

setTimeout(() => {
    console.log('7-setTimeout 0');

    process.nextTick(() => {
        console.log('8-nextTick in setTimeout');
    })
}, 0)

setTimeout(() => {
    console.log('9-setTimeout 1');

    Promise.resolve().then(() => {
        console.log('11-promise in setTimeout1');
    })

    process.nextTick(() => {
        console.log('10-nextTick in setTimeout1');
    })
}, 1)

setImmediate(() => {
    console.log('12-setImmediate');

    process.nextTick(() => {
        console.log('13-nextTick in setImmediate');
    })
});

process.nextTick(() => {
    console.log('2-nextTick 1');

    process.nextTick(() => {
        console.log('3-nextTick 2');
    })
})

console.log('1-global');

Promise.resolve().then(() => {
    console.log('4-promise 1');

    process.nextTick(() => {
        console.log('6-nextTick in promise');
    })

    // 注意，这里比 nextTick 先执行
    Promise.resolve().then(() => {
        console.log('5-promise in promise');
    })
})

/** 执行顺序如下
 1-global
 2-nextTick 1
 3-nextTick 2
 4-promise 1
 5-promise in promise
 6-nextTick in promise
 7-setTimeout 0
 8-nextTick in setTimeout
 9-setTimeout 1
 10-nextTick in setTimeout1
 11-promise in setTimeout1
 12-setImmediate
 13-nextTick in setImmediate
 14-poll 1
 15-nextTick in poll
 16-setTimeout 100
 17-nextTick in setTimeout100
 18-promise in setTimeout100
 19-setImmediate in setTimeout 100
 20-nextTick in setImmediate in setTimeout 100
 21-setTimeout 100 - 0
 22-nextTick in setTimeout 100 - 0
 */