What is the most appropriate way to check if a variable is undefined in JavaScript?

General Discussions
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1

I’ve come across several methods to JavaScript check if undefined:

  1. if (window.myVariable)
  2. if (typeof(myVariable) != "undefined")
  3. if (myVariable) — but this throws an error if the variable is undefined.

Should I use a try/catch block for the last one, or is there a better, more reliable way to check if a variable is undefined?

2

Honestly, the most reliable javascript check if undefined, especially when you’re not even sure if the variable was ever declared, is using typeof.

if (typeof myVariable !== "undefined") {
  // variable exists
}

This approach won’t throw a ReferenceError, so it’s your safest bet when you’re just trying to avoid crashing the app over a missing variable. Super handy during defensive coding or when integrating with unpredictable external scripts.

3

Yup, typeof is the safest play. But if you’re already sure the variable’s declared in your scope, you can clean it up a bit. My go-to javascript check if undefined in that case:

if (myVariable === undefined) {
  // yep, it's undefined
}

It’s more readable and intentional, especially when you’re debugging or writing code in a well-defined block. Just remember, it’ll break if myVariable was never declared, so use it with scoped confidence.

4

Totally agree with both. I’d just add that if you’re working with deeply nested objects (think API responses), then optional chaining is a modern lifesaver for javascript check if undefined cases.

if (myObj?.someProp === undefined) {
  // someProp is either missing or undefined
}

It prevents runtime errors while still giving you clarity. Especially useful when dealing with data structures that aren’t always consistent—makes your code cleaner and safer without too much overhead