Make sure you use strict equality = to check if a value is

I don't hear people telling me that I shouldn't use setTimeout because someone can window.In JavaScript, checking if a variable is undefined can be a bit tricky sinceĪ null variable can pass a check for undefined if not written properly.Īs a result, this allows for undefined values to slip through and vice versa. The thing is, in order to do lots of real work in JS, developers need to rely on redefinable identifiers to be what they are. If you follow this rule, good for you: you aren't a hypocrite. Should you never use any built-in identifier that can be redefined? 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.

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.

Javascript check if undefined