return value after a promise [duplicate]

Question

This question already has an answer here:

• setting a variable to get return from call back function using promise 2 answers
• How to return value from an asynchronous callback function? [duplicate] 3 answers

I have a javascript function where I want to return the value that I get after the return method. Easier to see than explain

function getValue(file){
    var val;
    lookupValue(file).then(function(res){
       val = res.val;
    }
    return val;
}

What is the best way to do this with a promise. As I understand it, the return val will return before the lookupValue has done it's then, but the I can't return res.val as that is only returning from the inner function.

Answer 1

Use a pattern along these lines:

function getValue(file) {
  return lookupValue(file);
}

getValue('myFile.txt').then(function(res) {
  // do whatever with res here
});

(although this is a bit redundant, I'm sure your actual code is more complicated)

Answer 2

The best way to do this would be to use the promise returning function as it is, like this

lookupValue(file).then(function(res) {
    // Write the code which depends on the `res.val`, here
});

The function which invokes an asynchronous function cannot wait till the async function returns a value. Because, it just invokes the async function and executes the rest of the code in it. So, when an async function returns a value, it will not be received by the same function which invoked it.

So, the general idea is to write the code which depends on the return value of an async function, in the async function itself.