CODEWITHSUNDEEP
javascriptreturn-valueconditional-statements
Ben Racicot
Ben Racicot

Reputation: 5935

Best way to 'return true' based on conditions from nested functions

This may be a novice question but I am trying to create a function that returns true. However, this is based on what happens within several other functions inside.

function checkGeo(){

    // CHECK FOR GEOLOCATION
    if( "geolocation" in navigator ) {

        navigator.geolocation.getCurrentPosition( function(position){
            sessionStorage.pinlat = position.coords.latitude; 
            sessionStorage.pinlon = position.coords.longitude; 
            // position object is set!
        });

        // position is not defined
        if ( position.coords.latitude && position.coords.longitude ){ 
           return true; 
        }

    }

}

This is the order I want things to happen with my geolocation check but I'm a bit surprised that the nested if is tested before the getCurrentPosition method finishes.

Putting this condition within the getCurrentPosition success function and returning true from there does not make checkGeo return true. How do I check if this asyncronous function has ended and therefore check its results in order to return true?

Upvotes: 0

Views: 448

Answers (3)

Downgoat
Downgoat

Reputation: 14361

Have your function have a finished variable 

function checkGeo(){
    var self = this;
    this.ready  = function () {}
    this.result = false;
    if("geolocation" in navigator) {
        navigator.geolocation.getCurrentPosition(function(position) {
            sessionStorage.pinlat = position.coords.latitude; 
            sessionStorage.pinlon = position.coords.longitude; 
            self.result = (position.coords.latitude && position.coords.longitude);
            self.ready.call(self);
        });
    }
}

Now you can use the function:

var run = new checkGeo();
run.ready = function () {
    alert(this.result); //Both work
    alert(run.result);  //Both work
};

A bit more complicated, but better programming in my opinion.

Upvotes: 1

Guffa
Guffa

Reputation: 700362

The geolocation call is asynchronous, so you can't return the result from the function. When the function ends, you don't yet know the result from the asynchronous call. Returning anything from the callback for the asynchronous call won't make that the return value of the function, because the function has already returned.

You can use callback to report the result back. You have to use the code that checks the position in the callback for the asynchronous call:

function checkGeo(callback){
  if( "geolocation" in navigator ) {
    navigator.geolocation.getCurrentPosition(function(position){
      sessionStorage.pinlat = position.coords.latitude; 
      sessionStorage.pinlon = position.coords.longitude; 
      callback(position.coords.latitude && position.coords.longitude);
    });
  } else {
    callback(false);
  }
}

Usage:

checkGeo(function(exists){
  // here you can use the result
  if (exists) {
    // ...
  }
});

Upvotes: 0

Trott
Trott

Reputation: 70075

position in the anonymous function is not the same as position in the if statement after it. Scope in JavaScript (ignoring ES6 let keyword for simplicity) is by function.

Additionally, if getCurrentPosition() is asynchronous, then you can't rely on the anonymous callback function to run before anything else.

If all you want the return true to signify is that you are trying to get geolocation info without any guarantee that you will be successful, use something more like this:

function checkGeo(){
    var hasGeolocation = false;

    // CHECK FOR GEOLOCATION
    if( "geolocation" in navigator ) {
        hasGeolocation = true;

        navigator.geolocation.getCurrentPosition( function(position){
            sessionStorage.pinlat = position.coords.latitude; 
            sessionStorage.pinlon = position.coords.longitude; 
            // position object is set! but only inside this function.
        });

        return hasGeolocation;
    }
}

On the other hand, if you are trying to have return true indicate that the geolocation was successfully set, then you need to indicate it some other way than the return value of the synchronous function, because you won't know that it will be set (an error might occur, a user might disallow geolocation for your site etc.) until the asynchronous function invokes the callback.

Upvotes: 1

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