JavaScript null vs undefined Explained: Differences, Use Cases & Best Practices

everything you need to know about null and undefined in JavaScript, including definitions, type categories, initialization, comparisons, use cases, best practices, and common pitfalls.

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1. Definitions

• undefined indicates a variable has been declared but not initialized.
• null is an assignment value representing intentional absence of any object value.

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2. Type & Category

• undefined has type undefined.
• null has type object (a legacy JavaScript quirk).

js
1console.log(typeof undefined); // 'undefined'
2console.log(typeof null);      // 'object'

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3. Declaration & Initialization

js
1let foo;
2console.log(foo); // undefined
3
4let bar = null;
5console.log(bar); // null

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4. Equality Comparisons

• Loose equality == considers null and undefined equal.

js
1console.log(null == undefined);  // true

• Strict equality === differentiates them.

js
1console.log(null === undefined); // false

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5. Use Cases & Best Practices

Scenario	Use
Uninitialized variables	undefined
Resetting or clearing values	null
Function return when no result	null
API responses absence of data	null

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6. Common Pitfalls

• typeof null returns object, which can be misleading.
• Using undefined unintentionally signals missing value.
• JSON.stringify omits undefined but serializes null.

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7. Summary Table

Feature	undefined	null
Type	undefined	object
Value	uninitialized variable	explicit empty value
== comparison	equal to null	equal to undefined
=== comparison	not equal to null	not equal to undefined
JSON behavior	omitted in JSON.stringify	serialized as null

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Use undefined for uninitialized or missing values.
Use null when you explicitly want to denote “no value.”
❌ Avoid treating undefined and null interchangeably; choose based on intent.