The first is when the variable hasn't been defined which throws a ReferenceError. Check the type (Typeof operator): Here you will get what type of variable was that if there is no variable assigned then it will display undefined. However, if (myVariable) can also throw an error in two cases. Syntax: Check by Value (Strict equality Operator): Here you will get whether the variable is assigned a value or not if the variable is not assigned a value it will display undefined. This has been patched in ECMAScript 5th ed., and undefined is not-writable:- if (window.myVar) will also include these falsy values, so it's not very robust: false window.undefined = 'foo' 'foo' = undefined // true If you are keen in finding out whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string as mentioned below:- if (typeof myVar != 'undefined')ĭirect comparisons against undefined are troublesome as undefined can be overwritten.

Use the in operator for a more robust check as mentioned below:- 'theFu' in window // true 'theFoo' in window // false However, this could not be the expected result for some cases, since the variable or property was declared but still, was not initialized. Shown in the example below: // global scope var theFu // theFu has been declared, but its value is undefined typeof theFu // 'undefined' But don't use void(0) directlyĪvoid using void(0) or typeof x = 'undefined' verbatim in code.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. If you compared an undeclared variable to undefined or void(0) you would instead get a ReferenceError. Even if name wasn't declared typeof would still say it's undefined. Note that this is slightly different from the previous options. In modern browsers you can safely compare the variable directly to undefined: if (name = undefined) The way I recommend to check for undefined in JavaScript is using the strict equality operator,, and comparing it to the primitive undefined. How do you check if a value is undefined in JavaScript? The short answer When a variable is declared without being assigned a value its initial value is undefined. Menu How to check for undefined in JavaScript 16 September 2015

Javascript check if undefined