How to check if a variable is loaded in JavaScript?

Question

How do I see if a certain object has been loaded, and if not, how can it be loaded, like the following?

if (!isObjectLoaded(someVar)) {
    someVar= loadObject();
}

Answer 1

If it is an object then you should just be able to check to see if it is null or undefined and then load it if it is.

if (myObject === null || myObject === undefined) {
   myObject = loadObject();
}

Using the typeof function is also an option as it returns the type of the object provided. However, it will return null or undefined if the object has not been loaded so it might boil down a bit to personal preference in regards to readability.

Answer 2

if(typeof(o) != 'object') o = loadObject();

Answer 3

myObject = myObject || loadObject();

Answer 4

I'm not sure what you mean by "loaded"... does the variable object exist and simply doesn't have the type you want? In that case, you'll want something like:

function isObjectType(obj, type) {
    return !!(obj && type && type.prototype && obj.constructor == type.prototype.constructor);
}

and then use if (isObjectType(object, MyType)) { object = loadObject(); }.

If object is not populated with anything before your test (ie - typeof object === 'undefined') then you just need:

if ('undefined' === typeof object) { object = loadObject(); }

Answer 5

You probably want to see if a given object is defined

Especially if its done in an asynchronous thread with a setTimeout to check when it turns up.

  var generate = function()
  { 
      window.foo = {}; 
  }; 
  var i = 0;
  var detect = function()
  {
     if( typeof window.foo == "undefined" ) 
     {
           alert( "Created!"); 
           clearInterval( i );
     }
   };
   setTimeout( generate, 15000 ); 
   i = setInterval( detect, 100 ); 

should in theory detect when window.foo comes into existance.

Answer 6

If by loaded you mean defined, you can check the type of the variable with the typeof function. HOWEVER typeof has a few quirks, and will identify an Object, an Array, and a null as an object

alert(typeof(null));

Identifying a null as a defined object would probably cause your program to fail, so check with something like

if(null !== x && 'object' == typeof(x)){
    alert("Hey, It's an object or an array; good enough!");
}

Answer 7

You can also just use a shortcut if(obj)

Answer 8

If you want to detect a custom object:

// craete a custom object
function MyObject(){

}

// check if it's the right kind of object
if(!(object instanceof MyObject)){
   object = new MyObject();
}

Answer 9

typeof(obj) would return "object" for an object of a class among other possible values.

Answer 10

if (!("someVar" in window)) {
  someVar = loadObject();
}

will tell you whether any JS has previously assigned to the global someVar or declared a top-level var someVar.

That will work even if the loaded value is undefined.