We can also extend a previously existing class. To refer to the original constructor, use super(), and to refer to the parent's method, use super.method(); see the redefinition of .fullName() here:

// Source file: src/class_persons.js

class Developer extends Person {
    constructor(first, last, language) {
        super(first, last);
        this.language = language;
    }

    fullName() {
        // redefines the original method
        return `${super.fullName()}, ${this.language} dev`;
    }
}

let dd = new Developer("John", "Doe", "JS");
console.log(dd); // Developer {first: "John", last: "Doe", language: "JS"}
console.log(dd.initials()); // "JD"
console.log(dd.fullName()); // "John Doe, JS dev"

You are not limited to extending your own classes; you can also extend the JS ones, too:

// Source file: src/class_persons.js

class ExtDate extends Date {
    fullDate() {
        const months = [
            "JAN",
            "FEB",
            "MAR",
            "APR",
            "MAY",
            "JUN",
            "JUL",
            "AUG",
            "SEP",
            "OCT",
            "NOV",
            "DEC"
        ];

        return (
            months[this.getMonth()] + 
            " " +
            String(this.getDate()).padStart(2, "0") +
            " " +
            this.getFullYear()
        );
    }
}

console.log(new ExtDate().fullDate()); // "MAY 01 2018"

If you don't need a special constructor, you can omit it; the parent's constructor will be called by default.