Check if object exists in JavaScript

There are a lot of half-truths here, so I thought I make some things clearer.

Actually you can't accurately tell if a variable exists (unless you want to wrap every second line into a try-catch block).

The reason is Javascript has this notorious value of undefined which strikingly doesn't mean that the variable is not defined, or that it doesn't exist undefined !== not defined

var a;
alert(typeof a); // undefined (declared without a value)
alert(typeof b); // undefined (not declared)

So both a variable that exists and another one that doesn't can report you the undefined type.

As for @Kevin's misconception, null == undefined. It is due to type coercion, and it's the main reason why Crockford keeps telling everyone who is unsure of this kind of thing to always use strict equality operator === to test for possibly falsy values. null !== undefined gives you what you might expect. Please also note, that foo != null can be an effective way to check if a variable is neither undefined nor null. Of course you can be explicit, because it may help readability.

If you restrict the question to check if an object exists, typeof o == "object" may be a good idea, except if you don't consider arrays objects, as this will also reported to be the type of object which may leave you a bit confused. Not to mention that typeof null will also give you object which is simply wrong.

The primal area where you really should be careful about typeof, undefined, null, unknown and other misteries are host objects. They can't be trusted. They are free to do almost any dirty thing they want. So be careful with them, check for functionality if you can, because it's the only secure way to use a feature that may not even exist.

Two ways:

typeof for local variables

You can test for a local object using typeof:

if (typeof object !== "undefined") {}

window for global variables

You can test for a global object (one defined on the global scope) by inspecting the window object:

if (window.FormData) {}

You can use:

if (typeof objectName == 'object') {
    //do something
}

If that's a global object, you can use if (!window.maybeObject)

If you care about its existence only ( has it been declared ? ), the approved answer is enough :

if (typeof maybeObject != "undefined") {
   alert("GOT THERE");
}

If you care about it having an actual value, you should add:

if (typeof maybeObject != "undefined" && maybeObject != null ) {
   alert("GOT THERE");
}

As typeof( null ) == "object"

e.g. bar = { x: 1, y: 2, z: null}

typeof( bar.z ) == "object" 
typeof( bar.not_present ) == "undefined" 

this way you check that it's neither null or undefined, and since typeof does not error if value does not exist plus && short circuits, you will never get a run-time error.

Personally, I'd suggest adding a helper fn somewhere (and let's not trust typeof() ):

function exists(data){
   data !== null && data !== undefined
}

if( exists( maybeObject ) ){
    alert("Got here!"); 
}

You could use "typeof".

if(typeof maybeObject != "undefined")
    alert("GOT HERE");

I used to just do a if(maybeObject) as the null check in my javascripts.

if(maybeObject){
    alert("GOT HERE");
}

So only if maybeObject - is an object, the alert would be shown. I have an example in my site.

https://sites.google.com/site/javaerrorsandsolutions/home/javascript-dynamic-checkboxes

The thread was opened quite some time ago. I think in the meanwhile the usage of a ternary operator is the simplest option:

maybeObject ? console.log(maybeObject.id) : ""

I've just tested the typeOf examples from above and none worked for me, so instead I've used this:

    btnAdd = document.getElementById("elementNotLoadedYet");
    if (btnAdd) {
       btnAdd.textContent = "Some text here";
    } else {
      alert("not detected!");
    }

Apart from checking the existence of the object/variable you may want to provide a "worst case" output or at least trap it into an alert so it doesn't go unnoticed.

Example of function that checks, provides alternative, and catch errors.

function fillForm(obj) {
  try {
    var output;
    output = (typeof obj !== 'undefined') ? obj : '';
    return (output);
  } 
  catch (err) {
    // If an error was thrown, sent it as an alert
    // to help with debugging any problems
    alert(err.toString());
    // If the obj doesn't exist or it's empty 
    // I want to fill the form with ""
    return ('');
  } // catch End
} // fillForm End

I created this also because the object I was passing to it could be x , x.m , x.m[z] and typeof x.m[z] would fail with an error if x.m did not exist.

I hope it helps. (BTW, I am novice with JS)

for me this worked for a DOM-object:

if(document.getElementsById('IDname').length != 0 ){
   alert("object exist");
}

You can use for if not exist any key or obj

 if (Object.keys(obj).length !==0){
      Whatever
 }

This verifies if Obj exist without any key, if you need to verify some existence key if (Object.keys(obj).includes('key'))

if (n === Object(n)) {
   // code
}

You could also try the && operator.

key && value

In this case if the key returns any value that is not:

• null
• NaN
• 0
• false
• undefined
• empty string ("" or '' or ``)

it will return key, else it will return value. In other words if the value is true (exists) it returns key else it returns value.

set Textbox value to one frame to inline frame using div alignmnt tabbed panel. So first of all, before set the value we need check selected tabbed panels frame available or not using following codes:

Javascript Code :

/////////////////////////////////////////
<script>

function set_TextID()
            {
                try
                    {
                        if(!parent.frames["entry"])
                            {
                            alert("Frame object not found");    
                            }
                            else
                            {
                                var setText=document.getElementById("formx").value;
                                parent.frames["entry"].document.getElementById("form_id").value=setText;
                            }
                            if(!parent.frames["education"])
                            {
                            alert("Frame object not found");    

                            }
                            else
                            {
                                var setText=document.getElementById("formx").value;
                                parent.frames["education"].document.getElementById("form_id").value=setText;
                            }
                            if(!parent.frames["contact"])
                            {
                            alert("Frame object not found");    

                            }
                            else
                            {
                                var setText=document.getElementById("formx").value;
                                parent.frames["contact"].document.getElementById("form_id").value=setText;
                            }

                        }catch(exception){}
                }

</script>

zero and null are implicit pointers. If you arn't doing arithmetic, comparing, or printing '0' to screen there is no need to actually type it. Its implicit. As in implied. Typeof is also not required for the same reason. Watch.

if(obj) console.log("exists");

I didn't see request for a not or else there for it is not included as. As much as i love extra content which doesn't fit into the question. Lets keep it simple.

if (maybeObject !== undefined)
  alert("Got here!");