Interface Properties and Methods

Interfaces can describe both data properties and callable methods. The method contract includes parameter and return types.

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

An implementing object must have a callable property compatible with the interface. TypeScript can contextually type the function parameters from the interface. The method body is ordinary JavaScript behavior.

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

interface Counter {
  value: number;
  increment(step: number): number;
}
const counter: Counter = {
  value: 0,
  increment(step) {
    this.value += step;
    return this.value;
  }
};
console.log(counter.increment(3));
console.log(counter.increment(2));

Verified output

3
5

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.

counter starts with value 0. The first call uses this.value += 3 and returns 3; the second increments by 2 and returns 5.

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.

Do not make increment a number property and expect to call it. When a method relies on this, extracting it and calling it without the receiver can fail.

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.

Build an interface Light with isOn and toggle(): boolean and implement it with an object.

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.

Interface Properties and Methods: Interfaces can describe both data properties and callable methods.

counter starts with value 0. The first call uses this.value += 3 and returns 3; the second increments by 2 and returns 5.

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