12. Null and Undefined

• undefined is a type that represents an uninitialized or missing value.
• null is a type that represents an intentional absence of any object value.

Example 1: Assigning undefined and null

let u: undefined = undefined;  // OK, u is assigned undefined
let n: null = null;            // OK, n is assigned null

Key Points:

• A variable of type undefined can only be assigned the value undefined.
• A variable of type null can only be assigned the value null.

Example 2: null and undefined with Strict Null Checks

If strictNullChecks is enabled in TypeScript, null and undefined are not automatically assignable to other types like string or number.

let x: number = null;  // Error if `strictNullChecks` is enabled
let y: string = undefined;  // Error if `strictNullChecks` is enabled