TypeScript: Typing functions

A function can accept values as parameters, and return values after its execution.

So we must have 2 types: the type of the parameters values, and the type of the return value.

This is how a function accepts arguments of a specific type:

const multiply = (a: number, b: number) => {
  return a * b

Now try writing multipl( in your editor, you will see VS Code suggests you the types of the function as you type it:

This is really handy especially when using functions you didn’t write, maybe coming from a library you imported.

If you watch closely, TypeScript already inferred the return type is number.

In this case you don’t need to, but here is how functions can explicitly declare their return value:

const multiply = (a: number, b: number): number => {
  return a * b

Lessons in this unit:

0: Introduction
1: Your first TypeScript program
2: Types
3: ▶︎ Typing functions
4: The editor helps you with type errors
5: Running TypeScript code
6: Valid types
7: Type aliases and interfaces
8: Union types
9: Typing arrays with generics
10: The DX of editing TypeScript
11: There's more...
Want to learn more? Check out our courses