Optional, Default, and Rest Parameters#

Functions often need to be flexible in how many arguments they accept. TypeScript provides three powerful features to handle this: optional parameters, default parameters, and rest parameters. These allow you to write versatile functions while keeping type safety intact.

Optional Parameters

Optional parameters let callers omit arguments. Mark a parameter optional by adding a ? after its name.

function greet(name?: string): string {
  return `Hello, ${name ?? "Guest"}!`;
}
 
console.log(greet()); // Hello, Guest!
console.log(greet("Alice")); // Hello, Alice!

Note: Optional parameters must come after all required parameters.

Default Parameters

Default parameters provide a fallback value if the argument is omitted or undefined.

function multiply(a: number, b: number = 1): number {
  return a * b;
}
 
console.log(multiply(5)); // 5
console.log(multiply(5, 2)); // 10

Default parameters are implicitly optional and can simplify your function logic.

Rest Parameters

Rest parameters collect all remaining arguments into an array.

function sum(...numbers: number[]): number {
  return numbers.reduce((total, n) => total + n, 0);
}
 
console.log(sum(1, 2, 3)); // 6
console.log(sum(4, 5)); // 9

Rest parameters must be the last parameter in the function signature.

Combining Parameter Types

You can combine required, optional, default, and rest parameters:

function buildName(
  firstName: string,
  lastName?: string,
  ...titles: string[]
): string {
  let fullName = firstName;
  if (lastName) fullName += ` ${lastName}`;
  if (titles.length) fullName += `, ${titles.join(" ")}`;
  return fullName;
}
 
console.log(buildName("John")); // John
console.log(buildName("John", "Doe")); // John Doe
console.log(buildName("John", "Doe", "PhD", "Esq.")); // John Doe, PhD Esq.

Best Practices

• Place required parameters first, then optional/default, then rest parameters.
• Use default parameters to avoid manual checks for undefined.
• Avoid mixing optional and default parameters in confusing ways.
• Document your function parameters clearly.

Common Pitfalls

• Placing optional parameters before required ones causes errors.
• Forgetting that rest parameters must be last.
• Using undefined explicitly can override default parameters unintentionally.

Next Steps

Next, explore function overloading in TypeScript to define multiple function signatures for more complex behaviors.