Optional and Default Parameters

Optional and Default Parameters

In TypeScript, every parameter is assumed to be required by the function. This doesn’t mean that it can’t be given null or undefined, but rather, when the function is called the compiler will check that the user has provided a value for each parameter. The compiler also assumes that these parameters are the only parameters that will be passed to the function. In short, the number of arguments given to a function has to match the number of parameters the function expects.

function buildName(firstName: string, lastName: string) {
  return firstName + " " + lastName;
}

let result1 = buildName("Bob");          // error, too few parameters
let result2 = buildName("Bob", "Adams", "Sr.");  // error, too many parameters
let result3 = buildName("Bob", "Adams");     // ah, just right

In JavaScript, every parameter is optional, and users may leave them off as they see fit. When they do, their value is undefined. We can get this functionality in TypeScript by adding a ? to the end of parameters we want to be optional. For example, let’s say we want the last name parameter from above to be optional:

function buildName(firstName: string, lastName?: string) {
  if (lastName)
    return firstName + " " + lastName;
  else
    return firstName;
}

let result1 = buildName("Bob");          // works correctly now
let result2 = buildName("Bob", "Adams", "Sr.");  // error, too many parameters
let result3 = buildName("Bob", "Adams");     // ah, just right

Any optional parameters must follow required parameters. Had we wanted to make the first name optional rather than the last name, we would need to change the order of parameters in the function, putting the first name last in the list.

In TypeScript, we can also set a value that a parameter will be assigned if the user does not provide one, or if the user passes undefined in its place. These are called default-initialized parameters. Let’s take the previous example and default the last name to "Smith".

function buildName(firstName: string, lastName = "Smith") {
  return firstName + " " + lastName;
}

let result1 = buildName("Bob");          // works correctly now, returns "Bob Smith"
let result2 = buildName("Bob", undefined);     // still works, also returns "Bob Smith"
let result3 = buildName("Bob", "Adams", "Sr.");  // error, too many parameters
let result4 = buildName("Bob", "Adams");     // ah, just right

Default-initialized parameters that come after all required parameters are treated as optional, and just like optional parameters, can be omitted when calling their respective function. This means optional parameters and trailing default parameters will share commonality in their types, so both

function buildName(firstName: string, lastName?: string) {
  // ...
}

and

function buildName(firstName: string, lastName = "Smith") {
  // ...
}

share the same type (firstName: string, lastName?: string) => string. The default value of lastName disappears in the type, only leaving behind the fact that the parameter is optional.

Unlike plain optional parameters, default-initialized parameters don’t need to occur after required parameters. If a default-initialized parameter comes before a required parameter, users need to explicitly pass undefined to get the default initialized value. For example, we could write our last example with only a default initializer on firstName:

function buildName(firstName = "Will", lastName: string) {
  return firstName + " " + lastName;
}

let result1 = buildName("Bob");          // error, too few parameters
let result2 = buildName("Bob", "Adams", "Sr.");  // error, too many parameters
let result3 = buildName("Bob", "Adams");     // okay and returns "Bob Adams"
let result4 = buildName(undefined, "Adams");   // okay and returns "Will Adams"