Then again, most variables in Javascript can be redefined. Now some people will keel over in pain when they read this, screaming: 'Wait! WAAITTT!!! undefined can be redefined!'Ĭool. I think it is long winded and unnecessary. I do not like typeof myVar = 'undefined'. Warning: Please note that = is used over = and that myVar has been previously declared (not defined).

For example, // or it's a property that can throw an error The other case is when the variable has been defined, but has a getter function which throws an error when invoked. The first is when the variable hasn't been defined which throws a ReferenceError. Thanks to for pointing out that your third case - if (myVariable) can also throw an error in two cases. If (window.myVar) will also include these falsy values, so it's not very robust: window.undefined = 'foo' Īs pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable. If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string: if (typeof myVar != 'undefined')ĭirect comparisons against undefined are troublesome as undefined can be overwritten.

Use the in operator for a more robust check. Var theFu // theFu has been declared, but its value is undefinedīut this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Choosing your preferred method is totally up to you.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. We also learned three methods we can use to check if a variable is undefined. In this article, we learned how to check if a variable is undefined and what causes a variable to be undefined. It looks like this: let myStr Īlso, when you try accessing values in, for example, an array or object that doesn’t exist, it will throw undefined. When a variable is declared or initialized but no value is assigned to it, JavaScript automatically displays 'undefined'. You'll typically assign a value to a variable after you declare it, but this is not always the case. This is not the same as null, despite the fact that both imply an empty state. An undefined variable or anything without a value will always return 'undefined' in JavaScript.

Javascript undefined