check if JS object exists

Question

I need to discover at runtime if an object with a given name exists, but this name is stored in a variable as a string.

For example, in my Javascript I have an object named test, then at runtime I write the word "text" in a box, and I store this string in a variable, let's name it input. How can I check if a variable named with the string stored in input variable exist?

I would add a && window["string"] !== null to that to circumvent the typeof null being object — Alex K.
– Alex K., Commented May 28, 2012 at 16:26

Eddie Groves · Accepted Answer · 2014-07-25 04:24:34Z

7

If the object is in the global scope;

var exists = (typeof window["test"] !== "undefined");

edited Jul 25, 2014 at 4:24

Eddie Groves

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answered May 28, 2012 at 16:25

Alex K.

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Comments

Amadan · Accepted Answer · 2012-05-28 16:24:12Z

1

If you're in a browser (i.e. not Node):

var varName = 'myVar';
if (typeof(window[varName]) == 'undefined') {
  console.log("Variable " + varName + " not defined");
} else {
  console.log("Variable " + varName + " defined");
}

However, let me say that I would find it very hard to justify writing this code in a project. You should know what variables you have, unless you expect people to write plugins to your code or something.

answered May 28, 2012 at 16:24

Amadan

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Comments

Niet the Dark Absol · Accepted Answer · 2012-05-28 16:24:37Z

1

if( window[input])...

All global variables are properties of the window object. [] notation allows you to get them by an expression.

answered May 28, 2012 at 16:24

Niet the Dark Absol

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4 Comments

Amadan Over a year ago

Will give incorrect result for variables that are defined but have falsy values.

Niet the Dark Absol Over a year ago

@Amadan Since we're testing for objects, and no object (not even an empty one) has a falsy value, this is perfectly acceptable.

Amadan Over a year ago

Oh, missed the part about it having to be an object.

Niet the Dark Absol Over a year ago

No problem, it's fair enough. Strictly speaking it should be checking its type against undefined, but this is a case of circumstances altering requirements.

Zach · Accepted Answer · 2012-05-28 16:25:09Z

1

If you want to see whether it exists as a global variable, you can check for a member variable on the window object. window is the global object, so its members are globals.

if (typeof window['my_objname_str'] != 'undefined')
{
    // my_objname_str is defined
}

answered May 28, 2012 at 16:25

Zach

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Comments

quevedo · Accepted Answer · 2016-06-05 05:55:50Z

0

I'm writing my code but not allways can know if a method exists. I load my js files depending on requests. Also I have methods to bind objects "via" ajax responses and need to know, if they must call the default callbacks or werther or not particular a method is available. So a test like :

function doesItExist(oName){ 
   var e = (typeof window[oName] !== "undefined"); 
   return e;
}

is suitable for me.

edited Jun 5, 2016 at 5:55

answered Jun 5, 2016 at 3:48

quevedo

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