method-signature-style.md

Pattern: Inconsistent method signature format

Issue: -

Description

TypeScript allows two ways to define a method signature in an interface or type: method shorthand (method(arg: string): void) and property function type (method: (arg: string) => void). Mixing these styles reduces readability and affects type checking behavior, as method signatures use different variance rules than function properties in strict mode.

Examples

Example of incorrect code when preferring property style:

interface Example {
  func(arg: string): number;
  
  other(arg: boolean): void;
  
  multi(arg: number): void;
  multi(arg: string): void;
}

type Shape = {
  draw(x: number): void;
  paint(color: string): void;
}

Example of correct code when preferring property style:

interface Example {
  func: (arg: string) => number;
  
  other: (arg: boolean) => void;
  
  // Multiple signatures combined with intersection type
  multi: ((arg: number) => void) & ((arg: string) => void);
}

type Shape = {
  draw: (x: number) => void;
  paint: (color: string) => void;
}

Example of incorrect code when preferring method style:

interface Example {
  func: (arg: string) => number;
  
  other: (arg: boolean) => void;
}

type Shape = {
  draw: (x: number) => void;
  paint: (color: string) => void;
}

Example of correct code when preferring method style:

interface Example {
  func(arg: string): number;
  
  other(arg: boolean): void;
}

type Shape = {
  draw(x: number): void;
  paint(color: string): void;
}