Check if object exists in JavaScript

Question

How do I verify the existence of an object in JavaScript?

The following works:

if (!null)
   alert("GOT HERE");

But this throws an Error:

if (!maybeObject)
   alert("GOT HERE");

The Error:

maybeObject is not defined.

Answer 1

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

Answer 2

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)

Answer 3

for me this worked for a DOM-object:

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

Answer 4

Or, you can all start using my exclusive exists() method instead and be able to do things considered impossible. i.e.:

Things like: exists("blabla"), or even: exists("foreignObject.guessedProperty.guessNext.propertyNeeded") are also possible...

Answer 5

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>

Answer 6

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