How to check if value exists in an object in JavaScript

Question

I have a function in my angular factory service to fetch some data. How do I check that a certain value exists inside the object before using it?

here is what I've been trying...

categories.fetch = function(app){
  if(!app.subject.name.length){
    return false;
  }
  var p = Restangular.all('v1/categories').getList({app.subject.name.toUpperCase()});
}

so I just want to check that there is value in app.subject.name before using it in the restanguar call...

Thanks

Answer 1

Your code will retrieve the value of the length property and attempt to convert it to a Boolean for the purposes of the if/then test, but this will throw an error if the value happens to be null .

Also, if your test is simply: app.subject.name , you will get a false positive if the value happens to be a falsy value, like 0 or false , which are both perfectly valid values.

With strings, the simplest test is to check for a non-empty string and non-null. If the value was provided by an end-user, it's a good idea to call .trim() on the string first to remove any leading or trailing spaces that may have been inadvertently added.

 var myObj = { test : 0, testing : null } // This will fail with an error when the value is null /* if(myObj.testing.length){ console.log("The testing property has a value."); } else { console.log("The testing property doesn't have a value."); } */ // This will return a false positive when the value is falsy if(myObj.test){ console.log("The test property has a value."); } else { console.log("The test property doesn't have a value."); // <-- Incorretly reports this } // This explicit test will pass and fail correctly if(myObj.testing !== "" && myObj.testing !== null){ console.log("The testing property has a value."); } else { console.log("The testing property doesn't have a value."); } 

Also, if the value is there, put your code in the if s true branch and don't worry about return false .

categories.fetch = function(app){
  if(app.subject.name !== "" && app.subject.name !== null) {
    var p = 
      Restangular.all('v1/categories').getList({app.subject.name.toUpperCase()});
  }

Answer 2

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property. MDN Docs

Example

var priceOfFood = {
    pizza: 14,
    burger 10
}

priceOfFood.hasOwnProperty('pizza') // true
priceOfFood['pizza'] // 14
priceOfFood.hasOwnProperty('chips') // false
priceOfFood['chips'] // undefined

Answer 3

I know the question does not ask about Lodash, but I manage to do this kind of checking a lot with it and it works flawlessly. In your case it would be something like this:

categories.fetch = function(app){
  if (_.isEmpty(app.subject.name)) {
   return false;
  }
 var p = Restangular.all('v1/categories').getList({app.subject.name.toUpperCase()});
}

If you expect that keys may not be available inside app object you can do it like this:

categories.fetch = function(app){
  if (_.isEmpty(_.get(app, "subject.name"))) {
   return false;
  }
 var p = Restangular.all('v1/categories').getList({app.subject.name.toUpperCase()});
}

Or simply:

categories.fetch = function(app){
  if (!_.get(app, "subject.name")) {
   return false;
  }
 var p = Restangular.all('v1/categories').getList({app.subject.name.toUpperCase()});
}

Answer 4

Simple as that:

if(!app.subject.name){
        return ;
    }