Typing Callbacks & Higher-Order Functions

A Higher-Order Function (HOF) is a function that accepts one or more functions as arguments (callbacks) or returns a function as its result.

TypeScript allows you to write strongly-typed callbacks and higher-order functions using Function Type Expressions or Generics.

1. Typing Callback Parameters

To type a function that accepts a callback, declare the callback parameter with a function type signature (arg1: Type1, ...) => ReturnType.

2. Typing Functions Returning Functions

When a Higher-Order Function returns a new function, specify the return type signature explicitly:

3. Generic Higher-Order Functions

To create reusable Higher-Order Functions (like map, filter, memoize, or pipe), use TypeScript Generics:

4. Handling Optional Parameters in Callbacks

Rule for Callbacks: When writing callback type definitions, do NOT mark callback parameters as optional (?) unless the callback function itself is expected to be called without those parameters.

5. Interactive Code Playground

Test higher-order function typing below:

6. Common Pitfalls & Edge Cases

Void Return Type in Callbacks: A callback parameter typed as () => void means the caller will ignore any returned value, NOT that the callback is forbidden from returning a value.

7. Summary Table

| Callback Parameters | Do NOT use ? optional flags on callback arguments |