114 lines
2.7 KiB
Plaintext
114 lines
2.7 KiB
Plaintext
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---
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description: 'Enforce using a particular method signature syntax.'
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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/method-signature-style** for documentation.
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TypeScript provides two ways to define an object/interface function property:
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```ts
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interface Example {
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// method shorthand syntax
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func(arg: string): number;
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// regular property with function type
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func: (arg: string) => number;
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}
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```
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The two are very similar; most of the time it doesn't matter which one you use.
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A good practice is to use the TypeScript's `strict` option (which implies `strictFunctionTypes`) which enables correct typechecking for function properties only (method signatures get old behavior).
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TypeScript FAQ:
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> A method and a function property of the same type behave differently.
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> Methods are always bivariant in their argument, while function properties are contravariant in their argument under `strictFunctionTypes`.
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See the reasoning behind that in the [TypeScript PR for the compiler option](https://github.com/microsoft/TypeScript/pull/18654).
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## Options
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This rule accepts one string option:
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- `"property"`: Enforce using property signature for functions. Use this to enforce maximum correctness together with TypeScript's strict mode.
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- `"method"`: Enforce using method signature for functions. Use this if you aren't using TypeScript's strict mode and prefer this style.
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The default is `"property"`.
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### `property`
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Examples of code with `property` option.
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='"property"'
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interface T1 {
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func(arg: string): number;
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}
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type T2 = {
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func(arg: boolean): void;
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};
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interface T3 {
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func(arg: number): void;
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func(arg: string): void;
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func(arg: boolean): void;
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}
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```
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#### ✅ Correct
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```ts option='"property"'
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interface T1 {
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func: (arg: string) => number;
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}
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type T2 = {
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func: (arg: boolean) => void;
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};
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// this is equivalent to the overload
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interface T3 {
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func: ((arg: number) => void) &
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((arg: string) => void) &
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((arg: boolean) => void);
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}
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```
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### `method`
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Examples of code with `method` option.
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='"method"'
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interface T1 {
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func: (arg: string) => number;
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}
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type T2 = {
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func: (arg: boolean) => void;
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};
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```
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#### ✅ Correct
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```ts option='"method"'
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interface T1 {
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func(arg: string): number;
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}
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type T2 = {
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func(arg: boolean): void;
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};
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```
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## When Not To Use It
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If you don't want to enforce a particular style for object/interface function types, and/or if you don't use `strictFunctionTypes`, then you don't need this rule.
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However, keep in mind that inconsistent style can harm readability in a project.
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We recommend picking a single option for this rule that works best for your project.
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