Функция для кнопок

const groups = document.querySelectorAll('.group'); 
for (let i = 0; i < groups.length; i++) { 
  groups[i].addEventListener('click', function() { 
    console.log(this.innerHTML); 
  }); 
} 
 
document.getElementById('all').addEventListener('click', function() { 
  console.log('All click'); 
  const groups = document.querySelectorAll('.group'); 
  for (let i = 0; i < groups.length; i++) { 
    groups[i].click(); 
  } 
});

<button id="all">All</button><br/> 
<button class="group">1</button> 
<button class="group">2</button> 
<button class="group">3</button> 
<button class="group">4</button> 
<button class="group">5</button> 
<button class="group">6</button>

let buttons = ['#btn1', '#btn2', "#btn3"]; 
let startBtn = document.querySelector("#start"); 
 
//назначаем событие кнопкам. 
buttons.forEach((item)=>{ 
 document.querySelector(item).onclick = () => { 
  console.log("Click "+ item) 
 } 
}) 
 
//Назначаем событие на кнопку "start all" 
startBtn.onclick = () => { 
  console.log("START ALL"); 
  buttons.forEach(item=>document.querySelector(item).onclick()); 
 }

<button id="btn1">btn</button> 
<button id="btn2">btn</button> 
<button id="btn3">btn</button> 
<button id="start">start all</button>