Skip to content
Biome
Docs Blog Playground

useShorthandFunctionType (since v1.5.0)

Diagnostic Category: lint/style/useShorthandFunctionType

Sources:

Enforce using function types instead of object type with call signatures.

TypeScript allows for two common ways to declare a type for a function:

  • Function type: () => string
  • Object type with a signature: { (): string }

The function type form is generally preferred when possible for being more succinct.

This rule suggests using a function type instead of an interface or object type literal with a single call signature.

Examples

Section titled Examples

Invalid

Section titled Invalid
interface Example {
  (): string;
}
style/useShorthandFunctionType.js:2:3 lint/style/useShorthandFunctionType  FIXABLE  ━━━━━━━━━━━━━━━━

  ✖ Use a function type instead of a call signature.
  
    1 │ interface Example {
  > 2 │   (): string;
      │   ^^^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ Types containing only a call signature can be shortened to a function type.
  
  ℹ Safe fix: Alias a function type instead of using an interface with a call signature.
  
    1   │ - interface·Example·{
    2   │ - ··():·string;
    3   │ - }
      1 │ + type·Example·=·()·=>·string
    4 2 │   
  

function foo(example: { (): number }): number {
  return example();
}
style/useShorthandFunctionType.js:1:25 lint/style/useShorthandFunctionType  FIXABLE  ━━━━━━━━━━━━━━━

  ✖ Use a function type instead of a call signature.
  
  > 1 │ function foo(example: { (): number }): number {
      │                         ^^^^^^^^^^
    2 │   return example();
    3 │ }
  
  ℹ Types containing only a call signature can be shortened to a function type.
  
  ℹ Safe fix: Use a function type instead of an object type with a call signature.
  
    1   │ - function·foo(example:·{·():·number·}):·number·{
      1 │ + function·foo(example:·()·=>·number):·number·{
    2 2 │     return example();
    3 3 │   }

Valid

Section titled Valid
type Example = () => string;
function foo(example: () => number): number {
  return bar();
}
// returns the function itself, not the `this` argument.
type ReturnsSelf2 = (arg: string) => ReturnsSelf;
interface Foo {
  bar: string;
}
interface Bar extends Foo {
  (): void;
}
// multiple call signatures (overloads) is allowed:
interface Overloaded {
  (data: string): number;
  (id: number): string;
}
// this is equivalent to Overloaded interface.
type Intersection = ((data: string) => number) & ((id: number) => string);
Section titled Related links