94 lines
2.0 KiB
Plaintext
94 lines
2.0 KiB
Plaintext
---
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description: 'Enforce that `this` is used when only `this` type is returned.'
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---
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import Tabs from '@theme/Tabs';
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import TabItem from '@theme/TabItem';
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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-return-this-type** for documentation.
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[Method chaining](https://en.wikipedia.org/wiki/Method_chaining) is a common pattern in OOP languages and TypeScript provides a special [polymorphic `this` type](https://www.typescriptlang.org/docs/handbook/2/classes.html#this-types) to facilitate it.
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Class methods that explicitly declare a return type of the class name instead of `this` make it harder for extending classes to call that method: the returned object will be typed as the base class, not the derived class.
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This rule reports when a class method declares a return type of that class name instead of `this`.
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```ts
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class Animal {
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eat(): Animal {
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// ~~~~~~
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// Either removing this type annotation or replacing
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// it with `this` would remove the type error below.
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console.log("I'm moving!");
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return this;
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}
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}
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class Cat extends Animal {
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meow(): Cat {
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console.log('Meow~');
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return this;
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}
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}
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const cat = new Cat();
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cat.eat().meow();
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// ~~~~
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// Error: Property 'meow' does not exist on type 'Animal'.
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// because `eat` returns `Animal` and not all animals meow.
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```
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## Examples
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<Tabs>
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<TabItem value="❌ Incorrect">
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```ts
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class Foo {
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f1(): Foo {
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return this;
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}
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f2 = (): Foo => {
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return this;
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};
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f3(): Foo | undefined {
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return Math.random() > 0.5 ? this : undefined;
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}
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}
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```
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</TabItem>
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<TabItem value="✅ Correct">
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```ts
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class Foo {
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f1(): this {
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return this;
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}
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f2() {
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return this;
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}
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f3 = (): this => {
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return this;
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};
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f4 = () => {
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return this;
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};
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}
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class Base {}
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class Derived extends Base {
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f(): Base {
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return this;
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}
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}
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```
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</TabItem>
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</Tabs>
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## When Not To Use It
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If you don't use method chaining or explicit return values, you can safely turn this rule off.
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