如何使异步变为同步

Question

I'm still trying to figure out how the async/await works.

 function func1() { console.log('a'); func2(); console.log('b'); } async function func2() { console.log('1'); let value = await new Promise(resolve => setTimeout(function() { resolve('async') } , 2000)); console.log(value) console.log('2'); } func1();

How can I achieve output as below?

a
1
async
2
b

I try to make func2 as async and await the setTimeout, but then they whole func2 become async function. So it will print out this instead:

a
1
b    
async
2

Answer 1

You can't make async into sync

But you can get the output you need

make func1 async and use await

 async function func1() { console.log('a'); await func2(); console.log('b'); } async function func2() { console.log('1'); let value = await new Promise(resolve => setTimeout(function() { resolve('async') } , 2000)); console.log(value) console.log('2'); } func1();

Answer 2

Synchronous means things are happening chronologically one after the other. Asynchronous means things are happening chronologically independent of one another.

When you execute a task synchronously, you wait for it to finish before moving on to another task(one after the other).

When you execute a task asynchronously, you can move on to another task before the prior task finishes. But it can't be exactly guranteed.

So, in your code you've call an async function inside a sync funcion. So, accoding to above explanation, you can not gurantee that after calling async function func2() rest of the func1() will execute. Because func2() is an async funcation which you can't say how much time will it take l to end up its execution process. So, You can't make async become sync.

function func1() {
   console.log('a');
   func2(); // call a asyncronized method inside a syncronized method
   console.log('b');
}

async function func2() {
   console.log('1');
   let value = await new Promise(resolve => setTimeout(function() { resolve('async') } , 2000));
   console.log(value)
   console.log('2');
  }
func1();

But you can call that async function func2() inside func1() using await keyword so at the same time func1() will need to convert into an async function in order to use await keyword. So, in this way your desired output can be generated.

// change func1() as async method
async function func1() {
   console.log('a');
   await func2(); // use await keyword
   console.log('b');
}

async function func2() {
   console.log('1');
   let value = await new Promise(resolve => setTimeout(function() {resolve('async') } , 2000));
   console.log(value)
  console.log('2');
}

func1();

I hope that this will help you to understand the problem.