8- Asynchronous progression

This article focuses on the following aspects:

event loop
Promise the advanced
async/await
Microtask/macro task
Scenario question – Connection of promise then and catch
Scenario problem -async/await syntax
Scenario question – Order of Promise and setTimeout
Scenario questions – plus async/await order questions

Essay question

1 Please describe the mechanism of event loop(event loop/event polling), and draw figure 2. What are macro tasks and micro tasks, and what is the difference between them? 3 What are the three states of Promise? How does it change?Copy the code

Please describe the mechanism of event loop(event loop/event polling)
What are macro tasks and micro tasks, and what is the difference between them?
What are the three states of a Promise? How does it change?
Scenario question – Connection of promise then and catch

/ / 1
Promise.resolve().then(() = >{
    console.log(1);
}).catch(() = >{
    console.log(2);
}).then(() = >{
    console.log(3);
})
// Print 1, 3

/ / the second question
Promise.resolve().then(() = >{
    console.log(1);
    throw new Error('error1')
}).catch(() = >{
    console.log(2);
}).then(() = >{
    console.log(3);
})
// Print 1, 2, 3

/ / the third topic
Promise.resolve().then(() = >{
    console.log(1);
    throw new Error('error1')
}).catch(() = >{
    console.log(2);
}).catch(() = >{
    console.log(3);
})
// Print 1, 2
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Scenario problem -async/await syntax

async function fn(){
    return 100;
}
(async function(){
    const a = fn();/ /?? Return a promise value of 100
    const b = await fn(); / /?? Return value 100 (await equals promise then)}) () (async function(){
    console.log('start');
    const a = await 100;
    console.log('a',a);
    const b = await Promise.resolve(200);
    console.log('b',b);
    const c = await Promise.resolve(300);
    console.log('c',c);
    console.log('end'); }) ()// What is printed out after execution?
// Just print start A :100 b:200
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Scenario question – Order of Promise and setTimeout

console.log(100) setTimeout(()=>{ console.log(200) }) Promise.resolve().then(()=>{ console.log(300) }) console.log(400) / / 100 400 300 200Copy the code

Scenario questions – plus async/await order questions

async function async1(){
    console.log('async1 start'); / / 2
    await async2();
    // await as callback content -- microtasks
    console.log('async1 end') / / 6
}
async function async2(){
    console.log('async2') / / 3
}
console.log('script start') / / 1

setTimeout(function(){ / / macro task
    console.log('setTimeout') / / 8
},0)

async1();

//**** When a promise is initialized, the callback passed in is executed immediately
new Promise(function(resolve){
    console.log('promise1') / / 4
    resolve()
}).then(function(){ Micro / / task
    console.log('promise2') / / 7
})

console.log('script end')/ / 5

// The event loop-call stack is cleared.
// Perform microtasks
// (try to trigger DOM rendering)
// Trigger Event loop to execute macro task
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Event loop(Event loop/Event polling)

JS runs in a single thread
Asynchrony is implemented based on callbacks
The Event loop is how asynchronous callbacks are implemented
Review: How is JS executed?
- From front to back, line by line
- If a line of execution fails, stop the execution of the following code
- Execute synchronous code first, and then execute asynchronous code

The sample

console.log('Hi')
setTimeout(function cb(){
    console.log('cb1')
},5000)
console.log('Bye')
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The event loop process

Synchronizes code and executes it line by line on the Call Stack
In the case of asynchrony, it “records” and waits for an opportunity (timing, network request, etc.).
When the time is right, move to the Callback Queue
If the Call Stack is empty (that is, the synchronized code is finished) the Event Loop starts working
Search the Callback Queue and move it to the Call Stack if any exists
And then continue to search (like perpetual motion machines)

DOM events and Event loops

JS is single threaded
Asynchronous (setTimeout, Ajax, etc.) uses callbacks, based on event loops
DOM events also use callbacks, based on event loops

console.log('Hi')
setTimeout(function cb(){
    console.log('cb1')
},5000)
console.log('Bye')
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<button id="btn1">submit</button>
<script>
console.log('Hi'The $()'#btn1').click(function(e){
    console.log('button clicked')})console.log('Bye')
</script>
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Macrotasks and micro tasks

What are macro tasks and what are micro tasks
Event loop and DOM rendering
The difference between microtasks and macro tasks

Code demo

console.log(100) setTimeout(()=>{ console.log(200) }) Promise.resolve().then(()=>{ console.log(300) }) console.log(400) // Answer 100, 400, 300, 200Copy the code

Macro and micro tasks

Macro task: setTimeout and setInterval, Ajax, DOM events
Microtasks: Promise async/await
Microtasks are executed earlier than macro tasks

Event loop and DOM rendering

const $p1 = $('<p>A piece of writing</p>');
const $p2 = $('<p>A piece of writing</p>');
const $p3 = $('<p>A piece of writing</p>'); $('#container') .append($p1) .append($p2) .append($p3) console.log('length',$('#container').children().length) //3 Alert (' This call to stack is over, DOM structure has been updated, but rendering has not yet been triggered ') //(Alert will block JS execution and DOM rendering to check the effect)Copy the code

const $p1 = $('<p>A piece of writing</p>');
const $p2 = $('<p>A piece of writing</p>');
const $p3 = $('<p>A piece of writing</p>'); $(' # container). Append ($(p1), append ($(p2), append ($p3) micro / / task: Resolve (). Then ()=>{console.log('length1', $('#container').children().length)//3 alert('Promise then')//DOM renders? --//NO}) // SetTimeout (()=>{console.log('length2', $('#container').children().length)//3 alert('setTimeout') //DOM rendered? ---YES })Copy the code

Explain from the Event loop why microtasks fire before rendering rather than macro tasks

By the browser is the macro task: setTimeout and setInterval, Ajax, DOM events
Microtasks are specified by ES6 syntax: Promise async/await

Microtasks and macro tasks – Summary

What are the macro tasks? What are microtasks? Why do microtasks trigger earlier
Relationships between microtasks, macro tasks, and DOM rendering
Microtasks, macro tasks, and DOM rendering, in the event loop process
Describe the Event loop mechanism (graphable)
- Review DOM rendering relationships for yourself
- The different processing of microtasks and macro tasks in the Event loop process

Promise

Three states
The manifestation and change of state
Effects of then and catch on state
- Review 7- Asynchronous basics

// Image loading
function loadImg(src){
    return new Promise((resolve,reject) = >{
        const img = document.createElement('img');
        img.onload = () = >{
            resolve(img)
        }
        img.onerror = () = >{
            const err = new Error('Image load failed${src}`)
            reject(err)
        }
        img.src = src
    })
}

const url = 'https://img.xxx';
loadImg(url).then(img= >{
    console.log(img.width);
    return img;
}).then(img= >{
    console.log(img.height)
}).catch(err= >console.error(err))

// Fix callback hell:
const url1 = 'https://img1.xxx';
const url2 = 'https://img2.xxx';

loadImg(url1).then(img1= >{
    console.log(img.width);
    return img1;
}).then(img1= >{
    console.log(img1.height)
    return loadImg(url2);
}).then(img2= >{
    console.log(img2.width)
    return img2;
}).then(img2= >{
    console.log(img2.height)
}).catch(err= >console.error(err))

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Three states

Resolved resolved resolved rejected
Resolved or Pending –> Rejected
Irreversible change
Performance of state
- Pending state, which does not trigger then and catch
- Resolved state, which triggers subsequent THEN callbacks
- The Rejected state triggers the subsequent catch callback

const p1 = new Promise((resolve,reject) = >{})console.log('p1',p1) //pending

const p2 = new Promise((resolve,reject) = >{
    setTimeout(() = >{
        resolve()
    })
})
console.log('p2',p2)//pending
setTimeout(() = >console.log('p2-setTimeout',p2))//resolved


const p3 = new Promise((resolve,reject) = >{
    setTimeout(() = >{
        resolve()
    })
})
console.log('p3',p3)//pending
setTimeout(() = >console.log('p3-setTimeout',p3))//reject

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Then and catch change state

Then check resolve and rejected if there is an error
If catch returns resolve, and if failed, return Rejected

const p1 = Promise.resolve().then(() = >{
    return 100
})
console.log('p1',p1) // Resolved to trigger subsequent then callback
p1.then(() = >{
    console.log('124')})const p2 = Promise.resolve().then(() = >{
    throw new Error('then error')})console.log('p2',p2) // Rejected triggers subsequent catch callbacks
p2.then(() = >{
    console.log('456')
}).catch(err= >{
    console.log('err100',err)
})
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const p3 = Promise.reject('my error').catch(err= > {
    console.error(err)
})
console.log('p3',p3) // Resolved note: Trigger then callback
p3.then(() = >{
    console.log(100)})const p4 = Promise.reject('my error').catch(err= >{
    throw new Error('catch err')})console.log('p4',p4)  // Rejected triggers the catch callback
p4.then(() = >{
    console.log(200)
}).catch(() = >{
    console.log('some err')})Copy the code

The print result is as follows:

async/await

Asynchronous callback callback hell
Promise then catch chain calls, but also based on callback functions
Async /await is synchronous syntax and eliminates callback functions completely

// Image loading
function loadImg(src){
    return new Promise((resolve,reject) = >{
        const img = document.createElement('img');
        img.onload = () = >{
            resolve(img)
        }
        img.onerror = () = >{
            const err = new Error('Image load failed${src}`)
            reject(err)
        }
        img.src = src
    })
}

const url = 'https://img.xxx';
loadImg(url).then(img= >{
    console.log(img.width);
    return img;
}).then(img= >{
    console.log(img.height)
}).catch(err= >console.error(err))

// Fix callback hell:
const url1 = 'https://img1.xxx';
const url2 = 'https://img2.xxx'; Define an anonymous function,awaitWant to useasyncPackage execution (async function(){
    const img1 = await loadImg(url1)
    console.log(img1.width)
    
    const img2 = await loadImg(url2)
    console.log(img2.width)
    
})()

// equivalent to the following function

loadImg(url1).then(img1= >{
    console.log(img.width);
    return img2;
}).then(img2= >{
    console.log(img2.width)
})
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Define an anonymous function,awaitWant to useasyncPackage execution (async function(){
    const img1 = await loadImg(url1)
    console.log(img1.width)
    
    const img2 = await loadImg(url2)
    console.log(img2.width)})() (awaitI can call a new oneasync awaitfunctionasync function loadImg1(url1){
    const img1 = await loadImg(url1)
    return img1
}
async function loadImg2(url2){
    const img2 = await loadImg(url2)
    return img2
}
(async function(){
    const img1 = await loadImg1(url1)
    console.log(img1.width)
    
    const img2 = await loadImg2(url2)
    console.log(img2.width)
})()
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Async /await and Promise relationship

Async /await is the ultimate weapon against asynchronous callbacks, but promises and async are not mutually exclusive, they complement each other
The async function is executed and returns a Promise object
Await is equivalent to then of Promise
try… Catch catches exceptions instead of the catch of Promise

async function fn1(){
    //return 100; //return
    return Promise.resolve(100)}const res1 = fn1() // Perform async and return a Promise object
console.log('res1',res1)  / / Promise object
res1.then(data= >{
    console.log('data',data) / / 100
})
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Await equals promise.then

! (async function(){//! Just isolate the preceding statement to avoid the previous statement without '; Resolve (300) const data = await p1 // = promise.then console.log('data',data)})()! Resolve (222) console.log('data1',data1)})() async function fn1(){ //return 100; Return Promise. Resolve (100)}! (async function(){ const data2 = await fn1() console.log('data2',data2) })()Copy the code

Using a try… catch

! (async function(){
    const p4 = Promise.reject('err1') 
    try{
        console.log('intry 1')
        const res = await p4
        console.log(res)
    }catch(err){
        console.log('p4err',err)// try... Catch is equivalent to a promise catch
    }
})()

!(async function(){
    const p5 = Promise.resolve('hello') 
    try{
        console.log('intry 2')
        const res = await p5
        console.log(res)
    }catch(err){
        console.log('p5err',err)// try... Catch is equivalent to a promise catch}})() Prints: intry1
intry 2
p4err err1
hello
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Why async await should be tried.. catch?
- Since await is equivalent to then of Promise, it does not handle catch
- try.. Cat catches exceptions instead of the catch of Promise

The following code

! (async function(){
    const p4 = Promise.reject('err1') / / reject status
    const res = await p4 //await -> then
    console.log('res',res)
})()
// This code cannot catch exceptions
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Nature of asynchrony

Async /await is the ultimate weapon against asynchronous callbacks
JS is still single threaded, asynchronous, and event loop-based
Async /await is just a syntactic candy, but this candy smells good!

Async function async1(){console.log('async1 start') //2 await async2() //await Async function async2(){console.log('async2')//3} console.log('script start') //1 Async1 () console.log('script end')//4 // Execute event loop when code is synchronizedCopy the code

for… of

for… In (and forEach for) is regular synchronous traversal
for… Of is often used for asynchronous traversal

function muti(num){ return new Promise(resolve => { setTimeout(()=>{ resolve(num * num) },1000) }) } const nums = [1,2,3] nums.foreach (async(I) => {const res =await muti[I] console.log(res)}) //for... In synchronous loop, print all results after 1 second! (async function(){ for( let i of nums){ const res = await muti(i); console.log(res) } })() //for... Of is an asynchronous loop that prints a result every secondCopy the code

This article focuses on the following aspects:

Essay question

Event loop(Event loop/Event polling)

The event loop process

DOM events and Event loops

Macrotasks and micro tasks

Macro and micro tasks

Event loop and DOM rendering

Explain from the Event loop why microtasks fire before rendering rather than macro tasks

Microtasks and macro tasks – Summary

Promise

Three states

Then and catch change state

async/await

Async /await and Promise relationship

Nature of asynchrony

for… of

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