JS private variable

example.js

function Container(param) {
    let secret = 3;
    let that = this;
    this.member = param;

    function dec() {
        if (secret > 0) {
            secret -= 1;
            return true;
        } else {
            return false;
        }
    }

    this.service = function () {
        return dec() ? that.member : null;
    };
}