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method-signature-style

Enforce using a particular method signature syntax.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

TypeScript provides two ways to define an object/interface function property:

interface Example {
  // method shorthand syntax
  func(arg: string): number;

  // regular property with function type
  func: (arg: string) => number;
}

The two are very similar; most of the time it doesn't matter which one you use. However, when TypeScript's strictFunctionTypes option is enabled, there is an important difference: methods are always bivariant in their arguments, while function properties are contravariant. This means that switching from method syntax to property syntax (or vice versa) can cause TypeScript to report new type errors or stop reporting existing ones.

A good practice is to use the TypeScript's strict option (which implies strictFunctionTypes) which enables correct typechecking for function properties only (method signatures get old behavior).

TypeScript FAQ:

A method and a function property of the same type behave differently. Methods are always bivariant in their argument, while function properties are contravariant in their argument under strictFunctionTypes.

See the reasoning behind that in the TypeScript PR for the compiler option.

eslint.config.mjs
export default tseslint.config({
  rules: {
    "@typescript-eslint/method-signature-style": "error"
  }
});

Try this rule in the playground ↗

Options​

This rule accepts the following options:

type Options = [
  /** The method signature style to enforce using. */
  | 'method'
  /** The method signature style to enforce using. */
  | 'property',
];

const defaultOptions: Options = ['property'];

This rule accepts one string option:

  • "property": Enforce using property signature for functions. Use this to enforce maximum correctness together with TypeScript's strict mode.
  • "method": Enforce using method signature for functions. Use this if you aren't using TypeScript's strict mode and prefer this style.

property​

Examples of code with property option.

interface T1 {
  func(arg: string): number;
}
type T2 = {
  func(arg: boolean): void;
};
interface T3 {
  func(arg: number): void;
  func(arg: string): void;
  func(arg: boolean): void;
}
Open in Playground

method​

Examples of code with method option.

interface T1 {
  func: (arg: string) => number;
}
type T2 = {
  func: (arg: boolean) => void;
};
Open in Playground

When Not To Use It​

If you don't want to enforce a particular style for object/interface function types, and/or if you don't use strictFunctionTypes, then you don't need this rule.

You may also want to avoid this rule if you're introducing it to a large existing codebase with strictFunctionTypes enabled. Running the autofix across many files at once could introduce multiple TypeScript errors, making it difficult to identify which specific change caused each error. In such cases, consider adopting the rule incrementally or reviewing changes file-by-file.

However, keep in mind that inconsistent style can harm readability in a project. We recommend picking a single option for this rule that works best for your project.

Resources​