Methods / Functions

To make code reusable, we can wrap commonly used snippets into functions. In TypeScript, we define a function with the function keyword. The basic syntax:

function functionName(parameters) {
  // function body
}

Another way to define a function is the arrow function. The basic syntax:

const functionName = (parameters) => {
  // function body
}

Both do the same job, but arrow functions have a few shortcuts: for example, you can leave out the return keyword and the curly braces. A simple example:

const add = (a: number, b: number) => a + b;
// is the same as
function add(a: number, b: number) {
  return a + b;
}

In real projects, you can mix and match the two as you see fit.

To make code easier to read, we can add type annotations to functions. For example:

function equal(a: number, b: number): boolean {
  return a === b;
}
const equal = (a: number, b: number): boolean => {
  return a === b;
}

Here we added type annotations to equal: it takes two number parameters and returns a boolean. Note that the return type : boolean is optional in TS, so leaving it out doesn’t mean the function returns nothing. We strongly recommend adding type annotations to functions, because they make your code easier to read and maintain.