93 lines
2.1 KiB
Plaintext
93 lines
2.1 KiB
Plaintext
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---
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description: 'Enforce using function types instead of interfaces with call signatures.'
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/prefer-function-type** for documentation.
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TypeScript allows for two common ways to declare a type for a function:
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- Function type: `() => string`
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- Object type with a signature: `{ (): string }`
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The function type form is generally preferred when possible for being more succinct.
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This rule suggests using a function type instead of an interface or object type literal with a single call signature.
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## Examples
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<!--tabs-->
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### ❌ Incorrect
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```ts
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interface Example {
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(): string;
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}
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```
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```ts
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function foo(example: { (): number }): number {
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return example();
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}
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```
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```ts
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interface ReturnsSelf {
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// returns the function itself, not the `this` argument.
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(arg: string): this;
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}
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```
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### ✅ Correct
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```ts
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type Example = () => string;
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```
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```ts
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function foo(example: () => number): number {
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return bar();
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}
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```
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```ts
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// returns the function itself, not the `this` argument.
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type ReturnsSelf = (arg: string) => ReturnsSelf;
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```
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```ts
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function foo(bar: { (): string; baz: number }): string {
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return bar();
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}
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```
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```ts
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interface Foo {
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bar: string;
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}
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interface Bar extends Foo {
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(): void;
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}
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```
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```ts
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// multiple call signatures (overloads) is allowed:
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interface Overloaded {
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(data: string): number;
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(id: number): string;
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}
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// this is equivelent to Overloaded interface.
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type Intersection = ((data: string) => number) & ((id: number) => string);
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```
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## When Not To Use It
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If you specifically want to use an interface or type literal with a single call signature for stylistic reasons, you can avoid this rule.
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This rule has a known edge case of sometimes triggering on global augmentations such as `interface Function`.
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These edge cases are rare and often symptomatic of odd code.
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We recommend you use an [inline ESLint disable comment](https://eslint.org/docs/latest/use/configure/rules#using-configuration-comments-1).
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See [#454](https://github.com/typescript-eslint/typescript-eslint/issues/454) for details.
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