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.

Questions about optional, default, or rest parameters? Share your thoughts or examples in the comments!