javascript async / await和promise

Question

I'm having a hard time understanding how async/await works.I have to make a program which contains three functions: func1 func2 and concatenated . func1 takes a string as an argument and returns the same string after 5 seconds of delay, func2 is an async function which also takes a string as an argument and returns the same string. concatenated is a function that takes two strings (s1,s2) as arguments and uses the above two functions( (func1(s1) and func2(s2)) ) to return their concatenated result after 5 seconds. So if we pass ("hello"," world") to concatenated it should return hello world . My code is:

function func1(x) {
return new Promise(resolve => {
setTimeout(() => {
  resolve(x);
    }, 5000);
  });
 }

async function func2(x) {
const a = await func1(x);
return a;
}

function concatenated(a,b){
  const c = func2(a).then(result =>{console.log(result)});
  const d = func2(b).then(result =>{console.log(result)});
  return (c+d) ;
} 

concatenated("hello"," world")

This code only gives me:
hello world

How can I correct this?

Answer 1

The problem is that your return statement from the concatenated function will be run synchronously . This also means that c and d will still be promises.

A possible solution would be:

async function concatenated(a,b){
  const c = await func2(a);
  const d = await func2(b);

  return (c+d);
} 

concatenated("hello", " world").then(result => {
    console.log(result); // hello world
})

Notice that an async function will always return a promise.

Answer 2

You can get the result after 5 seconds like this:

 function func1(x) { return new Promise(resolve => { setTimeout(() => { resolve(x); }, 5000); }); } async function func2(x) { const a = await func1(x); return a; } async function concatenated(a,b){ const [c,d] = await Promise.all([func2(a), func2(b)]) return c+d; } (async function main() { const ret = await concatenated("hello"," world") console.log(ret) })() 

Using JavaScript async/await syntax, you can write the asynchronous code like the synchronous style. No need promise.then , no need callback. It's a little different from other languages, eg Go, Java

Answer 3

You seem to be misunderstanding console logs. A regular console.log will always put a newline, which is why you are seeing hello world on two lines rather than one. Assuming you are using Node.js, you can use the following to write to the console without a newline to achieve your desired outcome:

process.stdout.write(result);