Callable Objects with Properties

What it is and why to use it

JavaScript functions are objects and can also carry properties. A call signature inside an interface describes how such an object is called, alongside its named metadata. This is useful for a configured formatter or callback that needs a readable label.

Common mistakes and limitations

A call signature uses a colon before its result type; it is not a method named call. Metadata is not automatically read-only, so mark it readonly if mutation should be rejected by checking. A type declaration alone does not attach properties.

Run this example

Use TypeScript 5.8 or later with strict checking and an ES2022 target. Save each complete example independently as example.ts, compile it, then run the generated JavaScript with a compatible Node.js installation. Compiler-only options below affect checking; they do not add runtime validation.

Terminal — compile and run

npx tsc example.ts --strict --target ES2022 --module commonjs
node example.js

• function types
• interfaces

Syntax

interface Formatter { (value: number): string; label: string }

Examples

interface Formatter { (value: number): string; label: string }
const format: Formatter = Object.assign(
  (value: number) => value.toFixed(2), { label: "Price" }
);
console.log(format.label + ": " + format(3));

Best practices

• A call signature uses a colon before its result type; it is not a method named call. Metadata is not automatically read-only, so mark it readonly if mutation should be rejected by checking. A type declaration alone does not attach properties.
• Keep external-input validation separate from compile-time type descriptions.

Runtime & versions

Compiled to JavaScript. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

Related TypeScript documentation

Specifications & further reading

• Official TypeScript documentation