Check if object exists in JavaScript

问题:

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.

回答1:

You can safely use the typeof operator on undefined variables.

If it has been assigned any value, including null, typeof will return something other than undefined. typeof always returns a string.

Therefore

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

回答2:

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.

回答3:

You can use:

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

回答4:

Two ways.

typeof for local variables

You can test for a local object using typeof:

if (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) {}

回答5:

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

回答6:

You could use "typeof".

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

回答7:

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

回答8:

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 != null) {
       btnAdd.textContent = "Some text here";
    } else {
      alert("not detected!");
    }

回答9:

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)

回答10:

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>

回答11:

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

回答12:

You can use the ! operator twice !!:

if (!!maybeObject)
  alert("maybeObject exists");

Or one time ! for not exists:

if (!maybeObject)
  alert("maybeObject does not exist");

What is the !! (not not) operator in JavaScript?

回答13:

for me this worked for a DOM-object:

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

回答14:

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

回答15:

Think it's easiest like this

if(myobject_or_myvar)
    alert('it exists');
else
   alert("what the hell you'll talking about");

回答16:

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...