How can I get console.log to output the getter result instead of the string "[Getter/Setter]"?

You can use console.log(Object.assign({}, obj));

Use console.log(JSON.stringify(obj));

You can define an inspect method on your object, and export the properties you are interested in. See docs here: https://nodejs.org/api/util.html#util_custom_inspection_functions_on_objects

I guess it would look something like:

function Cls() {
    this._id = 0;
    Object.defineProperty(this, 'id', {
        get: function() {
            return this._id;
        },
        set: function(id) {
            this._id = id;
        },
        enumerable: true
    });
};

Cls.prototype.inspect = function(depth, options) {
    return `{ 'id': ${this._id} }`
}

var obj = new Cls();
obj.id = 123;
console.log(obj);
console.log(obj.id);