null

Definition

null is a primitive value that represents the intentional absence of any object value. It is one of JavaScript's primitive values and is treated as falsy for boolean operations.

typeof null         // "object" (this is a bug/legacy behavior)
null === null       // true
null == undefined   // true (loose equality)
null === undefined  // false (strict equality)

Using null

null is typically used to indicate that a variable intentionally has no value or that an object reference is intentionally empty.

let user = null          // User not set yet
let data = null          // Data not loaded yet

null vs undefined

Aspect	null	undefined
Type	object (bug)	undefined
Intent	Intentional absence of value	Variable not assigned
Default	Must be explicitly set	Default for uninitialized vars
Usage	Explicitly set to "no value"	Automatically assigned

Common Use Cases

Resetting Values

let data = fetchData()
// ... use data ...
data = null             // Clear reference

Optional Parameters

function processUser(user) {
  if (user === null) {
    return "No user provided"
  }
  return `Processing ${user.name}`
}

Object Properties

const person = {
  name: "Alice",
  age: null            // Age not provided
}

Type Checking

// Check for null
value === null         // true if null
value !== null         // true if not null

// Check for null or undefined
value == null          // true if null or undefined
value === null || value === undefined  // explicit check

Nullish Coalescing

The nullish coalescing operator (??) provides a default value when a variable is null or undefined:

const name = username ?? "Guest"        // "Guest" if username is null/undefined
const value = input ?? null             // null if input is null/undefined

JSON Serialization

null is preserved in JSON:

JSON.stringify({ name: null })         // '{"name":null}'
JSON.parse('{"name":null}')            // { name: null }

Best Practices

1. Use null for intentional absence: When you want to explicitly indicate "no value".

2. Use undefined for uninitialized: When a variable hasn't been assigned yet.

3. Be consistent: Choose one approach and stick with it in your codebase.

4. Use nullish coalescing: Prefer ?? over || when you only want to check for null/undefined:

// Only checks null/undefined
const value = input ?? "default"

// Checks all falsy values (including 0, "", false)
const value = input || "default"