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Multiple Inheritance

As of July 2017, there is no native support for multiple inheritance. We can, however, implement 'mix-ins':


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//(continues from the example in 8.6.3)var MixedDalmatian = Base => class extends Base {
   jump(){
      console.log(this.name+ ' jumped.');
   }
   *bark(){   // to override bark() in GermanShepherd
      while (true){
         console.log('wok');
         yield;
      }
   }};class Dalmatian extends MixedDalmatian(Dog){};                                                                  // pure breedclass DalShepherd extends MixedDalmatian(GermanShepherd){}  // a crossbreed!var p = new Dalmatian('Spotty',7,'White');p.jump();  // Spotty jumped.var m = new DalShepherd ('Mixy',6,'Grey');m.jump();    //Mixy jumped.var mb = m.bark();mb.next();   // wokm.sleep();   // Mixy has fallen asleep.//Mixy seems to have awaken.//When will Mixy sleep again?

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We can also define a standard class aggregation function:


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var aggregation = (baseClass, ...mixins) => {
    class base extends baseClass {
        constructor (...args) {
            super(...args);
            mixins.forEach((mixin) => {
                copyProps(this,(new mixin));
            });
        }
    }let copyProps = (target, source) => {     // this function copies all properties and symbols,    // filtering out some special ones
        Object.getOwnPropertyNames(source)
       .concat(Object.getOwnPropertySymbols(source))
                   .forEach((prop) => {
                 if (!prop.match(
   /^(?:constructor|prototype|arguments|caller|name|bind|call|apply|toString|length)$/))
       Object.defineProperty(target, prop,
        Object.getOwnPropertyDescriptor(source, prop));
               })
    }mixins.forEach((mixin) => {         // outside contructor() to allow     // aggregation(A,B,C).staticFunction() to be     // called etc.
        copyProps(base.prototype, mixin.prototype);
        copyProps(base, mixin);
    });
    return base;}

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class Person{
   constructor(n){
      this.name=n;
   }}class Male{
   constructor(s='male'){
      this.sex=s;
   }}class Child{
   constructor(a=12){
      this.age=a;
   }
   tellAge(){console.log(this.name+' is '+this.age+' years old.');}}class Boy extends aggregation(Person,Male,Child){}var m = new Boy('Mike');m.tellAge(); // Mike is 12 years old.

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The aggregation function above requires the use of default parameters in the constructors of the subsequent classes in the class list. Furthermore, it will prefer properties and methods of a class that appears later in the class list, when there are duplicate names.