noUselessConstructor

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Diagnostic Category: lint/complexity/noUselessConstructor

JavaScript (and super languages)

Since: v1.0.0

Sources:

Same as: no-useless-constructor
Same as: @typescript-eslint/no-useless-constructor

Disallow unnecessary constructors.

ES2015 provides a default class constructor if one is not specified. As such, providing an empty constructor or one that delegates into its parent is unnecessary.

The rule ignores:

decorated classes;
constructors with at least one parameter property;
private and protected constructors.

Caveat

This rule reports on constructors whose sole purpose is to make a parent constructor public. See the last invalid example.

Examples

Invalid

class A {
    constructor (a) {}
}

code-block.js:2:5 lint/complexity/noUselessConstructor  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This constructor is unnecessary.
  
    1 │ class A {
  > 2 │     constructor (a) {}
      │     ^^^^^^^^^^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ Unsafe fix: Remove the unnecessary constructor.
  
    1 1 │   class A {
    2   │ - ····constructor·(a)·{}
    3 2 │   }
    4 3 │

class B extends A {
    constructor (a) {
        super(a);
    }
}

code-block.ts:2:5 lint/complexity/noUselessConstructor  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This constructor is unnecessary.
  
    1 │ class B extends A {
  > 2 │     constructor (a) {
      │     ^^^^^^^^^^^^^^^^^
  > 3 │         super(a);
  > 4 │     }
      │     ^
    5 │ }
    6 │ 
  
  ℹ Unsafe fix: Remove the unnecessary constructor.
  
    1 1 │   class B extends A {
    2   │ - ····constructor·(a)·{
    3   │ - ········super(a);
    4   │ - ····}
    5 2 │   }
    6 3 │

class C {
    /**
     * Documented constructor.
     */
    constructor () {}
}

code-block.js:5:5 lint/complexity/noUselessConstructor  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This constructor is unnecessary.
  
    3 │      * Documented constructor.
    4 │      */
  > 5 │     constructor () {}
      │     ^^^^^^^^^^^^^^^^^
    6 │ }
    7 │ 
  
  ℹ Unsafe fix: Remove the unnecessary constructor.
  
    1 1 │   class C {
    2   │ - ····/**
    3   │ - ·····*·Documented·constructor.
    4   │ - ·····*/
    5   │ - ····constructor·()·{}
    6 2 │   }
    7 3 │

class A {
    protected constructor() {
        this.prop = 1;
    }
}

class B extends A {
    // Make the parent constructor public.
    constructor () {
        super();
    }
}

code-block.js:2:5 parse ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ ‘protected’ modifier can only be used in TypeScript files
  
    1 │ class A {
  > 2 │     protected constructor() {
      │     ^^^^^^^^^
    3 │         this.prop = 1;
    4 │     }

Valid

class A {
    constructor (prop) {
        this.prop = prop;
    }
}

class B extends A {
    constructor () {
        super(5);
    }
}

class C {
    // Empty constructor with parameter properties are allowed.
    constructor (private prop: number) {}
}

class D {
  constructor(public arg: number){}
}

class F extends D {
  // constructor with default parameters are allowed.
  constructor(arg = 4) {
    super(arg)
  }
}

@Decorator
class C {
    constructor (prop: number) {}
}