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Function Types
Functions themselves are values in JavaScript (and by extension, TypeScript), which means they must also have a type, right? You might think:
Oh, easy. They're probably some "
function" type.
Not so fast. Function types are much more specific than that. In TypeScript, a function's type includes information about its parameters and return value.
Defining Function Types
The syntax for a function type looks like this:
(param1: type1, param2: type2, ...) => returnType
For example, a function that takes two numbers and returns a number:
(a: number, b: number) => number;
and both of these functions are of that type:
const add = (a, b) => a + b;
const subtract = (a, b) => a - b;