method-signature-style

Enforces using a particular method signature syntax.

There are two ways to define an object/interface function property.

// method shorthand syntax
interface T1 {
  func(arg: string): number;
}

// regular property with function type
interface T2 {
  func: (arg: string) => number;
}

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.

Options

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.

❌ Incorrect

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;
}

✅ Correct

interface T1 {
  func: (arg: string) => number;
}
type T2 = {
  func: (arg: boolean) => void;
};
// this is equivalent to the overload
interface T3 {
  func: ((arg: number) => void) &
    ((arg: string) => void) &
    ((arg: boolean) => void);
}

method

Examples of code with method option.

❌ Incorrect

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

✅ Correct

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.

Attributes

• ✅ Recommended
• 🔧 Fixable
• 💭 Requires type information