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TypeScript in Depth · Type Basics · Lesson 2 of 16

Function Type Annotations

Concept

Function Type Annotations

You can type both function parameters and return values, making function contracts explicit.

By the end of this lesson
  • Explain the core idea behind Function Type Annotations.
  • Predict output before running a change.
  • Test one realistic and one unusual input.
  • Use the result to make the next decision.
How to study this page
  1. 1. Read one concept.
  2. 2. Change the example.
  3. 3. Run, compare, and explain.
  4. 4. Complete the challenge below.
ExampleRunnable
function add(a: number, b: number): number {
  return a + b;
}
Try it Yourself »
Self-check before continuing

Without looking at the example, describe what changes when you modify one input in Function Type Annotations. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Write a typed function greet(name: string): string.

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Console

Output will appear here...