TypeScript: How to declare optional parameters in functions?

To declare a parameter of a function as optional in TypeScript, we need to add ? after the parameter name. For example, if a parameter name is id and we want it to be optional then we need to write it as id?.

Let’s understand it better by declaring a simple greet function that takes an argument message and logs it.

function greet(message: string) {
  console.log(message);
}

greet("Hello"); // Hello

Here, we need to pass message of type string or Typescript will complain about it.

function greet(message: string) {
  console.log(message);
}

greet(); // Expected 1 arguments, but got 0. An argument for 'message' was not provided.

TypeScript will throw an error, Expected 1 arguments, but got 0. An argument for 'message' was not provided.

If we want to make message an optional parameter, we can rename it to message?

function greet(message?: string) {
  if (message) {
    console.log(message);
  } else {
    console.log("Hi");
  }
}

greet("Hello"); // Hello
greet(); // Hi

In the above example, message is now an optional parameter. Even if we do not pass an argument in greet() function, TypeScript will not complain. If the message is present we will see the message getting printed else the text Hi will be displayed.