Optional Parameters

Adding ? to a parameter makes the corresponding argument optional at the call site. Inside the function, the parameter may be undefined.

What you will learn: the purpose of optional parameters, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

When strictNullChecks is enabled, an optional string parameter has an effective string | undefined type. Narrow it before calling string methods. An optional parameter must follow required parameters unless using an alternative API shape.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function greet(name: string, title?: string): string {
  if (title !== undefined) {
    return `Hello, ${title} ${name}`;
  }
  return `Hello, ${name}`;
}
console.log(greet("Asha"));
console.log(greet("Asha", "Dr."));

Verified output

Hello, Asha
Hello, Dr. Asha

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

The first call omits title, so it is undefined and the shorter greeting returns. The second supplies Dr.; the check succeeds and the more formal greeting returns.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Calling title.toUpperCase() without checking for undefined is unsafe; optional does not mean empty string.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Add an optional suffix to a formatName function. Test calls with and without the suffix.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Optional Parameters: Adding ? to a parameter makes the corresponding argument optional at the call site.

The first call omits title, so it is undefined and the shorter greeting returns. The second supplies Dr.; the check succeeds and the more formal greeting returns.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.