Multiple inheritance using classes

Question

Is it possible to extend only some specific part from multiple classes? Example:

class Walker {
  walk() {
    console.log("I am walking");
  }
  // more functions
}
class Runner {
  run() {
    console.log("I am running");
  }
  // more functions
}

// Make a class that inherits only the run and walk functions

Answer 1

Use composition:

class Walker {
  walk() {
    console.log("I am walking");
  }
// more functions
}
class Runner {
  run() {
    console.log("I am running");
  } 
  // more functions
}
class Composite {
  run() {
    new Runner().run();
  }
  walk() {
    new Walker().walk();
  }
}

Answer 2

You can pick and choose which methods from other classes you want to add to an existing class as long as your new object has the instance data and methods that the methods you are adding expect to be on that object.

class Walker {
  constructor() {}
  walk() {
    console.log("I am walking");
  }
  // more functions
}

class Runner {
  constructor() {}
  run() {
    console.log("I am running");
  }
  // more functions
}

class Participant {
  constructor() {}
}

// add methods from other classes to the Participant class
Participant.prototype.run = Runner.prototype.run;
Participant.prototype.walk = Walker.prototype.walk;

Keep in mind that methods are just functions that are properties on the prototype. So, you can assign any functions to the prototype that you want as long as the object you put them on has the right supporting instance data or other methods that those newly added methods might need.

---

Of course, if we understood more of your overall problem, you may also be able to solve your problem with more classical inheritance, but you can't use inheritance to solve it the exact way you asked to solve it.

Standard inheritance inherits all the methods of the base class. If you just want some of them, you will have to modify the prototype object manually to put just the methods you want there.

In the future, you will get a better set of answers if you describe the problem you're trying to solve rather than the solution you're trying to implement. That lets us understand what you're really trying to accomplish and allows us to offer solutions you haven't even thought of yet.

Answer 3

Having a look on the provided example, I immediately would suggest "... please consider decomposition."

For the given code it was extracting both behaviors "walk" and "run". Thus a Walker might end up as a hypothetical Person withWalkingAbility, a Runner then likewise becomes a Person withRunningAbility. All the // more functions within the originally provided Walker and Runner code then could be implemented by a Person class.

var
  withSelfAwareWalkingAbility = (function mixinFactory () { // does create a closure ...
    function walking() {                                    //
      console.log("I'm walking");                           //
    }                                                       //
    return function abilityMixin () {                       //
      this.walk = walking; // shared code.                  // ... but shares its
    };                                                      // *ability* implementation.
  }()),

  withSelfAwareRunningAbility = (function mixinFactory () { // does create a closure ...
    function running() {                                    //
      console.log("I'm running");                           //
    }                                                       //
    return function abilityMixin () {                       //
      this.run = running;                                   // ... but shares its
    };                                                      // *ability* implementation.
  }());


class Person {
  constructor() {
    // `Person` specific instance slots.
  }
  // `Person` specific prototypal methods.
}

A first big win was that one now is in control of where to place the composition, either at prototype level or within the constructor. It depends on a Walker's / Runner's final design and can be answered best by the OP her/himself.

class Walker extends Person {
  constructor() {
    super();
    withSelfAwareWalkingAbility.call(this); // composition at object level / instantiation time.
  }
}

... versus ...

class Runner extends Person {
  constructor() {
    super();
  }
}
withSelfAwareRunningAbility.call(Runner.prototype); // composition at a `Runner`'s `prototype` slot.

var
  withSelfAwareWalkingAbility = (function mixinFactory () {
    function walking() {
      console.log("I'm walking");
    }
    return function abilityMixin () {
      this.walk = walking;
    };
  }()),

  withSelfAwareRunningAbility = (function mixinFactory () {
    function running() {
      console.log("I'm running");
    }
    return function abilityMixin () {
      this.run = running;
    };
  }());


class Person {
  constructor() {
    // `Person` specific instance slots.
  }
  // `Person` specific prototypal methods.
}


class Walker extends Person {
  constructor() {
    super();
    withSelfAwareWalkingAbility.call(this);
  }
}

class Runner extends Person {
  constructor() {
    super();
  }
}
withSelfAwareRunningAbility.call(Runner.prototype);


var
  walker = (new Walker),
  runner = (new Runner);

console.log('walker : ', walker); // Walker {walk: function}
console.log('(walker instanceof Walker) ? ', (walker instanceof Walker)); // true
console.log('(walker instanceof Person) ? ', (walker instanceof Person)); // true
console.log('(walker instanceof Runner) ? ', (walker instanceof Runner)); // false
walker.walk(); //  "I'm walking"
console.log('(walker.hasOwnProperty("walk")) ? ', walker.hasOwnProperty("walk")); // true

console.log('\n');

console.log('runner : ', runner); // Runner {}
console.log('(runner instanceof Runner) ? ', (runner instanceof Runner)); // true
console.log('(runner instanceof Person) ? ', (runner instanceof Person)); // true
console.log('(runner instanceof Walker) ? ', (runner instanceof Walker)); // false
runner.run(); //  "I'm running"
console.log('(runner.hasOwnProperty("run")) ? ', runner.hasOwnProperty("run")); // false

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Another win of decomposing behavior into smaller or even atomic "composable units of reuse"[^1] comes from the easiness of composing them again into bigger tailored mixins/traits like ...

function withSelfAwareWalkingAndRunningAbility() {
  withSelfAwareWalkingAbility.call(this);
  withSelfAwareRunningAbility.call(this);
}

[^1]: SCG, University of Bern: »Talents: Dynamically Composable Units of Reuse«

Note ... there is now a second part of this answer providing the example from above again, but with a real trait approach backed by a library

Answer 4

Note ... I had to split my previous answer into 2 separate parts since for quite a while it is not anymore that easy referencing outside hosted javascript code. Due to that I now do provide the necessary trait library as minified inline code within the next given example ...

Having on hand natively implemented Talents or Traits one day in JavaScript, will make this kind of composition even more flexible/powerful ...

var
  withWalkingAbility = (function mixinFactory () {
    function walking() {
      return "walking";
    }
    return function abilityMixin () {
      this.walk = walking;
    };
  }()),

  withRunningAbility = Trait.create(function trait (use, applicator) {
    function running() {
      return "running";
    }
    applicator(function () {
      this.run = running;
    });
  });

var
  withSelfAwareness = (function mixinFactory () {
    function reflectThySelf() {
      return "I'm";
    }
    return function awarenessMixin () {
      this.reflect = reflectThySelf;
    };
  }()),

  withDepersonalization = Trait.create(function trait (use, applicator) {
    function objectify() {
      return "It's";
    }
    applicator(function () {
      this.objectify = objectify;
    });
  });

var
  withSelfAwareWalkingAndRunningAbility = Trait.create(function trait (use, applicator) {

    use(withWalkingAbility, withRunningAbility)
      .apply.all();

    applicator(function () {

      this.walk = (function (proceed) {
        return function () {

          console.log([this.reflect(), proceed.call(this)].join(' '))
        };
      }(this.walk));

      this.run = (function (proceed) {
        return function () {

          console.log([this.reflect(), proceed.call(this)].join(' '))
        };
      }(this.run));

    }).requires([

      "reflect"

    ]);
  }),
  withDepersonalizedRunningAbility = Trait.create(function trait (use, applicator) {

    use(withRunningAbility)
      .apply(withRunningAbility);

    applicator(function () {

      this.run = (function (proceed) {
        return function () {

          console.log([this.objectify(), proceed.call(this)].join(' '))
        };
      }(this.run));

    }).requires([

      "objectify"

    ]);
  });



class Human {
  constructor() {
    // `Human` specific instance slots.
  }
  // `Human` specific prototypal methods.
}
withSelfAwareness.call(Human.prototype);


class Animal {
  constructor() {
    withDepersonalization.call(this);
    // other `Animal` specific instance slots.
  }
  // `Animal` specific prototypal methods.
}


class HumanWalkerAndRunner extends Human {
  constructor() {
    super();
  }
}
withSelfAwareWalkingAndRunningAbility.call(HumanWalkerAndRunner.prototype);


class AnimalRunner extends Animal {
  constructor() {
    super();
    withDepersonalizedRunningAbility.call(this);
  }
}


var
  humanWalkerAndRunner = (new HumanWalkerAndRunner),
  animalRunner = (new AnimalRunner);

console.log('humanWalkerAndRunner : ', humanWalkerAndRunner); // HumanWalkerAndRunner {}
console.log('(humanWalkerAndRunner instanceof HumanWalkerAndRunner) ? ', (humanWalkerAndRunner instanceof HumanWalkerAndRunner)); // true
console.log('(humanWalkerAndRunner instanceof Human) ? ', (humanWalkerAndRunner instanceof Human)); // true
humanWalkerAndRunner.walk(); //  "I'm walking"
humanWalkerAndRunner.run(); //  "I'm running"
console.log('(humanWalkerAndRunner.hasOwnProperty("walk")) ? ', humanWalkerAndRunner.hasOwnProperty("walk")); // false
console.log('(humanWalkerAndRunner.hasOwnProperty("run")) ? ', humanWalkerAndRunner.hasOwnProperty("run")); // false

console.log('\n');

console.log('animalRunner : ', animalRunner); // AnimalRunner {objectify: function, run: function}
console.log('(animalRunner instanceof AnimalRunner) ? ', (animalRunner instanceof AnimalRunner)); // true
console.log('(animalRunner instanceof Animal) ? ', (animalRunner instanceof Animal)); // true
animalRunner.run(); //  "It's running"
console.log('(animalRunner.hasOwnProperty("run")) ? ', animalRunner.hasOwnProperty("run")); // true

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