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useShorthandFunctionType

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Diagnostic Category: lint/style/useShorthandFunctionType

Since: v1.5.0

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.

interface Example {
(): string;
}
code-block.ts:2:3 lint/style/useShorthandFunctionType  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Use a function type instead of a call signature.

1 │ interface Example {
> 2 │ (): string;
^^^^^^^^^^^
3 │ }
4 │

Types containing only a call signature can be shortened to a function type.

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();
}
code-block.ts: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 │ }

Types containing only a call signature can be shortened to a function type.

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 }

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);