How to add mixins to ES6 javascript classes?

Javascript’s object/property system is much more dynamic than most languages, so it’s very easy to add functionality to an object. As functions are first-class objects, they can be added to an object in exactly the same way. Object.assign is the way to add the properties of one object to another object. (Its behaviour is in many ways comparable to _.mixin.)

Classes in Javascript are only syntactic sugar that makes adding a constructor/prototype pair easy and clear. The functionality hasn’t changed from pre-ES6 code.

You can add the property to the prototype:

Object.assign(Test.prototype, mixin);

You could add it in the constructor to every object created:

constructor() {
    this.var1 = 'var1';
    Object.assign(this, mixin);
}

You could add it in the constructor based on a condition:

constructor() {
    this.var1 = 'var1';
    if (someCondition) {
        Object.assign(this, mixin);
    }
}

Or you could assign it to an object after it is created:

let test = new Test();
Object.assign(test, mixin);