test_page.js

class ButtonTextPair {
    constructor(btnId, btnTextId, clickMsg = 'Well, this was a nice test!') {
        this.btnId = btnId;
        this.btnTextId = btnTextId;
        this.clickMsg = clickMsg;

        this.initBtn();
    }

    get btn() {
        return document.getElementById(this.btnId);
    }

    get btnText() {
        return document.getElementById(this.btnTextId);
    }

    initBtn() {
        this.clearMsg();
        this.btn.addEventListener('mousedown', e => this.writeMsg());
        this.btn.addEventListener('mouseup', e => this.clearMsg());
        this.btn.addEventListener('mouseout', e => this.clearMsg());
    }

    clearMsg() {
        this.btnText.className = 'test_button_text_empty';
        this.btnText.innerHTML = '';
    }

    writeMsg() {
        this.btnText.className = 'test_button_text';
        this.btnText.innerHTML = this.clickMsg;
    }
}

var pair1 = new ButtonTextPair('btn1', 'btn1_text', 'Well, this was a nice test.');
var pair1 = new ButtonTextPair('btn2', 'btn2_text', 'This is fun.');