how to create private method in javascript iojs es6

Question

I am trying iojs, and using the classes new feature.

I just want to create some private method used only in the class itself, but so far cannot work out.

For the following example, I just want the function called "upcase" to be private, how to do it?

class Animal {
    constructor(name) {
        this._name = name;
    }

    // failed here
    function upcase(word) {
        return word.toUpperCase();
    };

    getName() {
        return this.upcase(this._name);
    }

    get name() {
        return this.upcase(this._name);
    }
}

let an = new Animal('abc');
console.log(an.getName());
console.log(an.name);

Cheers, Ron