Functions

TypeScript Parameter Type Annotations

Function parameters may be given type annotations with the same syntax as variable declarations – a colon next to the name. The type annotations ensure that the parameters are of the correct type.

function greet(noun: string) {
  console.log(`Hello, ${noun}!`);
}

greet('World'); // Prints: Hello, World  

greet(2020); // Argument of type 'number' is not assignable to parameter of type 'string'.

TypeScript Optional Parameter

Sometimes we would like to skip providing values to function calls. We can declare some parameters in a function to be optional if a value is not provided for all arguments in a function. We do this by adding a question mark symbol, ?, after the parameter name before the colon, :.

function greet(name?: string) {
  console.log(`Hello, ${ name || 'stranger' }!`);
}

greet(); // Prints: Hello, stranger!

TypeScript Default Parameters

If we assign a function parameter to have a default value, TypeScript will infer the parameter type to be the same as the default value’s type.

function exponentiation(power = 1) {
  console.log(4 ** power);
}

exponentiation(); // Prints: 4

exponentiation(4); // Prints: 256

exponentiation(true); // Error: Argument of type 'true' is not assignable to parameter of type 'number | undefined'.

TypeScript Inferring Return Types

By looking at the types of the values in a function’s return statement, TypeScript can infer the return type of a function.

function factOrFiction() {
  return Math.random() >= .5 ? 'true' : 'false';
}

const myAnswer : boolean = factOrFiction(); // Type 'string' is not assignable to type 'boolean'

TypeScript Void Return Type

If a function does not return any value, then you can specify void as a return type using type annotation.

function sayHello(): void { 
    console.log('Hello!')
}

TypeScript Explicit Return Types

We can be explicit about what type a function returns by adding type annotation (: followed by the type) after a function’s closing parenthesis, ).

function trueOrFalse(value: boolean): boolean {
  if (value) {
    return true;
  }

  return 'false'; // Typescript Error: Type 'string' is not assignable to type 'boolean'.
}