How To Define Functions In TypeScript

Here's a quick rundown on functions in TypeScript. 💁‍♂️

Parameters

Parameters should always be followed by type annotations. The exception to this rule are default values as TypeScript is able to infer the type from the values themselves. Is a parameter optional? Mark it with a ? or else your linter will throw an error when calling the function.

function printName(firstName: string, lastName = 'Smith', middleName?: string) { 
  if (middleName) console.log(firstName, middleName, lastName) 
  else console.log(firstName, lastName) 
}

Return value

While TypeScript can infer a function's return type, it's good practice 👍 to explicitly state it as it serves as a form of self-documentation. The type annotation is written after the parentheses and before the function body.

If the function doesn't return anything, type void is used.

function getNumber(): number { return 6 }
function sayHello(): void { console.log('Hello!') }

Arrow functions

In arrow functions, type annotations follow the same syntax as in function declarations.

const isNumber5 = (value: number): boolean => value === 5