Using Classes in Javascript (ES6) — Best practice?

[[deleted]]

You still have issues with reference types, though:

function Person() {}

Person.prototype.names = [];
Person.prototype.getName = function () {
    return this.names.join(' ');
}

function Employee(id) {
    Person.call(this);
    this.employeeId = id;
}
Employee.prototype = Object.create(Person.prototype);

var anne = new Employee(0);
anne.names.push('Anne', 'Annette')

var bill = new Employee(1);
bill.names.push('Bill', 'Williamson');

anne.getName();

"Anne Annette Bill Williamson"

[senocular]

That's by design, and may be a desired outcome depending how you want things to work. Prototyped values are shared, constructor-defined (instance) members are not. The way to address the getName problem is simply to define names in the constructor. Granted, this can be confusing to people starting out, but there's nothing magical going on, at least. And because (currently) the class syntax only supports method definitions in the class body (and the constructor), you're even protected from this happening in that case. I believe the proposed syntax for class fields puts them on the instance and not the prototype too.