prefer-return-this-type.md

prefer-return-this-type

Enforce that this is used when only this type is returned.

Method chaining is a common pattern in OOP languages and TypeScript provides a special polymorphic this type. If any type other than this is specified as the return type of these chaining methods, TypeScript will fail to cast it when invoking in subclass.

class Animal {
  eat(): Animal {
    console.log("I'm moving!");
    return this;
  }
}

class Cat extends Animal {
  meow(): Cat {
    console.log('Meow~');
    return this;
  }
}

const cat = new Cat();
// Error: Property 'meow' does not exist on type 'Animal'.
// because `eat` returns `Animal` and not all animals meow.
cat.eat().meow();

// the error can be fixed by removing the return type of `eat` or use `this` as the return type.
class Animal {
  eat(): this {
    console.log("I'm moving!");
    return this;
  }
}

class Cat extends Animal {
  meow(): this {
    console.log('Meow~');
    return this;
  }
}

const cat = new Cat();
// no errors. Because `eat` returns `Cat` now
cat.eat().meow();

Rule Details

Examples of code for this rule:

❌ Incorrect

class Foo {
  f1(): Foo {
    return this;
  }
  f2 = (): Foo => {
    return this;
  };
  f3(): Foo | undefined {
    return Math.random() > 0.5 ? this : undefined;
  }
}

✅ Correct

class Foo {
  f1(): this {
    return this;
  }
  f2() {
    return this;
  }
  f3 = (): this => {
    return this;
  };
  f4 = () => {
    return this;
  };
}

class Base {}
class Derived extends Base {
  f(): Base {
    return this;
  }
}

Options

// .eslintrc.json
{
  "rules": {
    "@typescript-eslint/prefer-return-this-type": "warn"
  }
}

This rule is not configurable.

When Not To Use It

If you don't use method chaining or explicit return values, you can safely turn this rule off.

Attributes

• Configs:
  • ✅ Recommended
  • 🔒 Strict
• 🔧 Fixable
• 💭 Requires type information