Method Overriding

A derived class can override an inherited method to give it more specific behavior.

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

Use the override keyword to ask TypeScript to verify that the base class actually has a matching member. The runtime dispatch still follows JavaScript: a method call on a derived instance uses the derived implementation.

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

class Message {
  format(): string { return "Message"; }
}
class Alert extends Message {
  override format(): string { return `Alert: ${super.format()}`; }
}
const item: Message = new Alert();
console.log(item.format());

Verified output

Alert: Message

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 variable is typed as Message but stores an Alert instance. Calling format uses Alert.format at runtime. The override invokes super.format to reuse the base string, yielding Alert: Message.

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.

An override is not method overloading. Changing an inherited signature incompatibly should fail type checking.

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 a Notice subclass with an overriding format method and compare all three class outputs.

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.

Method Overriding: A derived class can override an inherited method to give it more specific behavior.

The variable is typed as Message but stores an Alert instance. Calling format uses Alert.format at runtime. The override invokes super.format to reuse the base string, yielding Alert: Message.

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