Type Guards — typeof, instanceof, in, custom type predicates

A Type Guard is a runtime expression or function that performs a check on a variable to guarantee its specific type to the TypeScript compiler within a conditional block.

1. The typeof Type Guard

Use typeof to narrow primitive types (string, number, boolean, symbol, bigint, undefined, function).

2. The instanceof Type Guard

Use instanceof to narrow values created by class constructor functions or built-in classes (Date, RegExp, HTMLInputElement, custom classes).

3. The in Operator Type Guard

Use the in operator to check if a specific property name exists on an un-narrowed object union:

4. Custom User-Defined Type Predicates (arg is Type)

To create reusable type guard helper functions, define a return type using a Type Predicate: paramName is TargetType.

5. Interactive Code Playground

Test type guards below:

6. Summary Comparison Table

| Predicate (is) | Complex domain structures | function isUser(x: any): x is User |