useShorthandFunctionType

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TypeScript and TSX

Summary

Rule available since: v1.5.0
Diagnostic Category: lint/style/useShorthandFunctionType
This rule is recommended, which means is enabled by default.
This rule has a safe fix.
The default severity of this rule is information.
Sources:
- Same as @typescript-eslint/prefer-function-type

Description

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

Invalid

1
interface Example {
2
  (): string;
3
}

code-block.ts:2:3 lint/style/useShorthandFunctionType  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ 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 │

1
function foo(example: { (): number }): number {
2
  return example();
3
}

code-block.ts:1:25 lint/style/useShorthandFunctionType  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ 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

1
type Example = () => string;

1
function foo(example: () => number): number {
2
  return bar();
3
}

1
// returns the function itself, not the `this` argument.
2
type ReturnsSelf2 = (arg: string) => ReturnsSelf;

1
interface Foo {
2
  bar: string;
3
}
4
interface Bar extends Foo {
5
  (): void;
6
}

1
// multiple call signatures (overloads) is allowed:
2
interface Overloaded {
3
  (data: string): number;
4
  (id: number): string;
5
}
6
// this is equivalent to Overloaded interface.
7
type Intersection = ((data: string) => number) & ((id: number) => string);

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "style": {
5
        "useShorthandFunctionType": "error"
6
      }
7
    }
8
  }
9
}