How to get callback from calling function inside variable

Question

I have some Javascript code that is similar to the following:

var Requests = {
  get: function(search) {
    // HTTP request happens, returns data.
  }
}

I'm trying to invoke this somewhere else, like the following:

Requests.get('thing');

Although this is returning undefined because it's not waiting for the http request result. I'm wondering how I can defer this request until it's successful, or somehow check for a callback in the invoking code. Thanks!

Answer 1

You need a promise. https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise

var Requests = {
     get: function(search){
        return new Promise(resolve,reject){
           //if it does what you want
           if(data) resolve(data)
           else reject(error)
        }
     }
}

And then

Requests.get('thing').then('do something else')

The other option is to use await/async but that is not fully supported yet. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

Answer 2

The function can return a promise:

var Requests = {
  get: function(search) {
    return new Promise(function(resolve, reject){
       // call resolve in case of success, and reject in case of error
    })
  }
}

Requests.get('thing').then(
  function(response){
   // success
  },
  function(error){
   // something went wrong
  }
);

In the "then" method of a Promise object, you need to pass the two functions used when the promise is resolved or rejected.

For more information on Promises: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise .