return value after a promise

javascript node.js promise q
pedalpete picture pedalpete · Apr 9, 2014 · Viewed 111k times · Source

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

SomeKittens picture SomeKittens · Apr 9, 2014

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)

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