Default Parameters

A default parameter supplies a value when the caller omits an argument or explicitly passes undefined.

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

The default expression runs when the argument is undefined, not when it is 0, false, an empty string, or null. TypeScript infers an appropriate parameter type from the default when you do not add an annotation.

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 discount(price: number, rate = 0.1): number {
  return price * (1 - rate);
}
console.log(discount(100));
console.log(discount(100, 0.25));
console.log(discount(100, 0));

Verified output

90
75
100

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 omitted rate becomes 0.1, yielding 90. An explicit 0.25 yields 75; explicit 0 overrides the default and yields 100.

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.

Default parameters do not replace all falsy values: using rate || 0.1 inside the function would incorrectly replace 0.

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.

Change the default rate to 0.2. Test discount(50), discount(50, 0), and discount(50, undefined).

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.

Default Parameters: A default parameter supplies a value when the caller omits an argument or explicitly passes undefined.

The omitted rate becomes 0.1, yielding 90. An explicit 0.25 yields 75; explicit 0 overrides the default and yields 100.

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