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();
}
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10 Answers
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.
3 Comments
PhiLho
Isn't a test like if (myObject == null) (or undefined) enough, since that's precisely the point of the simple equality? Or I am missing something?
rjzii
@PhiLho - Depending upon the object type, the object could be null, or it could be undefined. JavaScript treats them as two different things.
Šime Vidas
@Rob @PhiLho This:
x == null is equivalent to this: x === null || x === undefined (under the condition that the global variable undefined hasn't been modified). Therefore, instead of your expression you can just write myObject == null. It's not only smaller, but also more robust since it's indifferent to assignments to the variable undefined.if(typeof(o) != 'object') o = loadObject();
1 Comment
Alana Storm
Remember that typeof returns "object" for nulls, so you'll probably want something like [if(null !== x && 'object' == typeof(x)){]
myObject = myObject || loadObject();
Comments
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(); }
Comments
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.
Comments
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!");
}
Comments
You can also just use a shortcut if(obj)
Comments
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();
}
Comments
typeof(obj) would return "object" for an object of a class among other possible values.
Comments
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.
