CODEWITHSUNDEEP
promisees6-promise
Garvit
Garvit

Reputation: 55

Javascript Promise resolve method prints in console. How?

Please have a look at the below javascript code. When I run this code in browser console window the below mentioned output is received. I am new to javascript and promises. Please help me understand how "resolved" got printed in browser console?







    function resolveAfter2Seconds() {
      return new Promise(resolve => {
        setTimeout(() => {
          resolve('resolved');
        }, 2000);
      });
    }
     
    async function asyncCall() {
      console.log('calling');
      const result = await resolveAfter2Seconds();
      console.log(result);
    }
    asyncCall();

OUTPUT in browser console:

calling

resolved

Now my question is why "resolved" got printed in console? As of what I know is then() method should have executed when resolve() method is called.

Upvotes: 2

Views: 4114

Answers (1)

Yousaf
Yousaf

Reputation: 29314

await operator is used to wait for the Promise to settle. It returns the fulfilled value of the Promise or the value itself if its not a Promise.

In your code, since resolveAfter2Seconds() function returns a Promise, so in the following statement

const result = await resolveAfter2Seconds();

javascript waits for the Promise to settle and then assigns its fulfillment value, i.e. 'resolved' to the result constant. After that, you log the result on the console which is why 'resolved' gets logged on the console.

Comments above each statement in the asyncCall function will help you understand the output.

async function asyncCall() {
  // log 'calling' on the console
  console.log('calling');   

  // wait for the promise to resolve and then
  // assign the fulfillment value to 'result'                   
  const result = await resolveAfter2Seconds(); 

  // log the value with which promise returned
  // by 'resolveAfter2Seconds' function resolved
  console.log(result);                         
}

As of what I know is then() method should have executed when resolve() method is called.

I can't see where you have called .then() method in your code. .then() method is invoked if you register any callback calling .then() method on any Promise.

Instead of using async-await syntax, you could change your code as shown below to see that callback registered using .then() is invoked after 2 seconds.

function resolveAfter2Seconds() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('resolved');
    }, 2000);
  });
}

// store the promise returned by 'resolveAfter2Seconds' function
const p = resolveAfter2Seconds();

// register a callback function to be invoked after 2
// seconds with the resolved value
p.then(resolvedValue => {
  console.log('callback invoked');
  console.log('Promise resolved with the value of: ' + resolvedValue);
}).catch(error => console.log(error));

Its clear from your question that you are confused about the output of the code because it uses async-await syntax instead of promise-chaining. async-await is syntactic sugar over the regular promise code that uses .then() method calls.

I suggest that you visit the following links to understand how async-await syntax works. Once you understand that, you will be able to understand the output.

Upvotes: 4

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