How to check for undefined in JavaScript?

What is the most appropriate way to test if a variable is undefined in JavaScript? I've seen several possible ways:

if (window.myVariable) 


if (typeof(myVariable) != undefined) 


if (myVariable) 
//This throws an error if undefined. Should this be in Try/Catch? 
Text Copied From
on : Mon, Aug 02, 2010 Views : 1608226

Accepted Answer

If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example.

// global scope var theFu;
// theFu has been declared, but its value is undefined typeof theFu;
// undefined 

But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in operator for a more robust check.

theFu in window;
// true theFoo in window;
// false 

If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator.

if (typeof myVar != 'undefined') 

The typeof operator is guaranteed to return a string. Direct comparisons against undefined are troublesome as undefined can be overwritten.

window.undefined = omg;
 omg == undefined 
// true 

As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable.

if (window.myVar) will also include these falsy values, so it's not very robust:

 false 0  NaN null undefined 

Thanks to @CMS for pointing out that your third case - if (myVariable) can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError.

// abc was never declared. if (abc) 

// ReferenceError: abc is not defined 

The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,

// or it's a property that can throw an error Object.defineProperty(window, myVariable, 

 throw new Error(W00t?);
   set: undefined  
 if (myVariable) 

// Error: W00t? 
Text Copied From
on : Mon, Aug 02, 2010

Ask Question

If you want to ask question you must LogIn or SignUp
Login SignUp

Quick Links



Total Followers
Study Group Created
Study Group Joined
Following Teacher
Following Organization
Blog Articles Added
Questions Asked
Questions Answered
Jobs Posted
Total Members in Group
Questions asked by members
Tasks added in this Group

Please wait..


Login to Open ESchool OR Create your account    Login   SignUp