Get the value of a promise and assign to variable

user_2402135 0
Viewed 38134

utility.fetchInfo() returns a Promise object. I need to be able to get the value of this Promise object and assign the value of it to a variable that I can then use later on in my code.

At the moment, I can happily print the value of result to the console, but I need to be able to assign this value to myVal. So far, I've tried a lot of things and nothing has worked.

var myVal = utility.fetchInfo().then(result => console.log(result));

Thanks

promise node.js
4 Answers

What @dhilt said but do your stuff inside the promise callback function:

utility.fetchInfo().then(result => { 
  doSomethingWith(result);
});

Or use async/await if you are using a recent version of Node or Babel:

async function myFunction () {
    var myVal = await utility.fetchInfo();
}
user_8944200

Just do an assignment within the callback's body

utility.fetchInfo().then(result => { myVal = result; });

Depends on the situation, for example, if there's a big piece of async logic, it may be better to extract it in a separate function: 

let myVal; // undefined until myAsyncFunc is called

const myAsyncFunc = (result) => { 
   console.log(result);
   myVal = result;
   // ...
};

utility.fetchInfo().then(myAsyncFunc); // async call of myAsyncFunc
user_32119320

When you return the value to another file/function and you are getting undefined because another function where you are using that value is not waiting for the value to be assigned to the variable. For example there are two function

function1(){
   var x=desired_value
}

function2(){
   var y = x
}

now function1 might take some time to assign desire value to var x and java script ll not wait for this, it ll start running function2 and when you read the value of y you will get undefined.

To solve this problem we can use java script promise/await/async, return value from async function as resolve. refer below example

async function1(){  //function1 should be async here for this to work
   return new Promise(function(resolve,reject){
       resolve(desired_value)
});
}

or above function can be writtern as

 async function1(){  //function1 should be async here for this to work

        return Promise.resolve(desired_value)
    }

Now we can use this value in another function by calling the function1 and use await keyword

function2(){
   var y = await function1();
}

now it ll wait for function1 to complete before assigning its value to y.

user_32706930

I think what you are looking for is some fancy ES7 syntax:

var myVal = (async () => {
    var data = await utility.fetchInfo();
    return data;
})();

(async () => {
    console.log(await myVal);
})();

Keep in mind console.log(myVal) will end up with Promise { <pending> }, So instead you would use

(async () => {
    console.log(await myVal);
})();

which would return your desired output.

user_143716450
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