How do I check if a variable is undefined or null?

You can check with if (variable === null) or if (variable === undefined) separately. For a combined check, use if (variable == null) which catches both null and undefined because of type coercion. Alternatively, use if (variable === null || variable === undefined). Modern JavaScript also lets you use the nullish coalescing operator (??) for defaults.

When should I use null instead of undefined?

Use null when you want to explicitly clear a value or indicate that something is intentionally empty. For example, when resetting a form field or before loading an object. Undefined should be left to JavaScript's default for uninitialised variables, missing array elements, or absent function return values. It keeps your code readable and unambiguous.

Can I set a variable to undefined manually?

Yes, you can write variable = undefined, but it's generally discouraged. Overriding undefined can lead to confusion because it removes the distinction between JavaScript's default behaviour and your intentions. Instead, use null to express that you're deliberately setting a variable to nothing. This makes your code clearer and aligns with common best practices in the UK dev community.

What does typeof null return?

In JavaScript, typeof null returns "object". This is a well-known quirk from the language's early days that has never been fixed for backward compatibility. It means you can't rely on typeof alone to check for null. Instead, use strict equality like value === null to confirm it's null.

JavaScript Undefined vs Null: The Complete Guide for 2026

17 August 2026

Learn the difference between undefined and null in JavaScript, with examples and best practices for UK developers in 2026.

Read this page in: Deutsch

What is undefined in JavaScript?

In JavaScript, undefined is a primitive value automatically assigned to variables that have been declared but not initialised. When you declare let name; without a value, name is undefined. It also appears when you try to access an object property that doesn't exist or when a function doesn't return a value. It is a type of its own. For UK developers, think of it as the default state for anything that hasn't had a value assigned yet. This is different from null, which you choose to assign. Understanding this distinction helps you write cleaner code, especially when working with APIs or data from external sources where missing values need explicit handling. Keep this in mind.

What is null in JavaScript?

null is also a primitive value, but it represents an intentional absence of value. It is often used to reset or clear a variable, or to indicate that an object is deliberately empty. Unlike undefined, null is not assigned automatically; you must explicitly set it. For example, let result = null; signals that you expect a value later. In the UK job market, interviewers often ask about this distinction to gauge your understanding of JavaScript's quirks. Null is the developer's way of saying 'nothing here, but I meant it'. It can be helpful when building forms or optional fields, where a null value means the user didn't provide an answer, versus undefined which might mean the field wasn't even considered.

Key differences between undefined and null

The main difference is origin: undefined is default, null is intentional. When you check types, typeof undefined returns "undefined", while typeof null returns "object" – a historical quirk you should remember. Loose equality (==) treats them as equal, but strict equality (===) does not. So undefined == null is true, but undefined === null is false. This can trip up developers who forget to use strict equality. Also, undefined implies a variable exists but has no value, whereas null implies you explicitly set it to empty. In practice, many UK devs prefer to use null for deliberate empty state and leave undefined alone, letting JavaScript handle its default behaviour.

Common pitfalls and how to avoid them

One common pitfall is thinking undefined and null are interchangeable. Always use strict equality to check for either, or use a combined check like value == null (which catches both) if you don't need to distinguish. Another issue arises when accessing nested object properties; you might get undefined and assume it's fine, but it could mask a bug. A useful technique is to use optional chaining (?.) to avoid errors. Also, be careful with API responses from UK government services or banks that might return null for missing data; you need to handle both. To stay safe, write a utility function like isNil(value) that checks for both null and undefined, reducing repetition and errors.

Best practices for using undefined and null in your code

Adopt a consistent approach across your codebase. Many teams decide that undefined means 'not yet assigned' and null means 'intentionally empty'. This matches the ECMAScript spec. For example, when initialising variables that will hold objects, set them to null rather than undefined. For function parameters, rely on undefined to trigger default values. If you're working on a large project in London or Manchester, coding standards often enforce strict equality checks. Document your choices in your team's style guide. Finally, avoid setting a variable to undefined manually; it can lead to confusing behaviour and hides the distinction. Let JavaScript do its thing and reserve null for explicit empties.

FAQ

With loose equality (==), null equals undefined, so null == undefined returns true. However, with strict equality (===), they are not equal, meaning null === undefined returns false. Always use strict equality to avoid unexpected results in your JavaScript code. The difference matters because null is intentional, while undefined is default.

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