Una forma corta y eficiente.
Código
<!DOCTYPE html> <html> <input type="number" placeholder="input" id="inpcalc"></input> <button type="button" class="bcalc">7</button> <button type="button" class="bcalc">8</button> <button type="button" class="bcalc">9</button> <button type="button" class="bcalc">+</button> <button type="button" class="bcalc">4</button> <button type="button" class="bcalc">5</button> <script> var botones = document.querySelectorAll(".bcalc"); var input = document.querySelector("#inpcalc"); for(var i = 0; i < botones.length; ++i) { botones[i].addEventListener("click", function(){alert("Boton Número " + i)}); } </script> </body> </html>