CODEWITHSUNDEEP
angulartypescriptpromise
blenzcoffee
blenzcoffee

Reputation: 891

resolving promise from another function

How does promise chaining inside a function works? In the following code:

     promiseA() {
            return anotherPromise().then( res => {
                   // resolution of promise A
             })
     };


      promiseB() {
             promiseA().then( res => {
                     // resolution of promise B
              })
      }

I tested this code and I get that resolution of B is always done after A is resolved. However, I can't find any documentation. Will resolution of promise B always happens after resolution of Promise A done, or is my test just one case of race condition?

Upvotes: 1

Views: 465

Answers (2)

Jamiec
Jamiec

Reputation: 136134

Yes, it will always resolve A first before B. If you re-wrote your code without the indirection of separate methods what you have is simply:

anotherPromise().then(...A...).then(...B...)

And that should make it clear that the code inside the A then must always run before the code inside the B then.

A simple test confirms. You can run this a million times and A will always resolve before B:

function anotherPromise(){
  return new Promise((resolve,reject) => {
  	setTimeout(resolve,1000);
  });
}

function promiseA(){
   return anotherPromise().then( res => {
      console.log("Resolve promiseA");
      return res;
  });
}

function promiseB(){
    return promiseA().then(res => {
    	console.log("Resolve promiseB");
      return res;
    })
}

promiseB();

Upvotes: 2

Ajay
Ajay

Reputation: 4971

You have invoked the Promise A inside Promise B, so by logic Promise A should be resolved before Promise B.

Promise.then( // resolved first )
       .then( // resolved second )

Upvotes: 0

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