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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.

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.

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/method-signature-style": "warn"
  }
};

Options​

This rule accepts an options string of the following possible values:

type Options = "property" | "method";

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.

The default is "property".

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;
}

method​

Examples of code with method option.

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

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.

Resources​