Request feedback for simple pomodoro and calculator


I would like request for a code review and feedback for my simple pomodoro and calculator web pages because I want to improve my coding ability. Thank you very much.

Basic calculator
GitHub: https://github.com/cabpacedilla/boyet/blob/master/calculator
Codepen: https://codepen.io/claive-alvin-polo-acedilla/pen/wvBJENg

function calculate(inputString)
{
var result;

Pomodoro
GitHub: https://github.com/cabpacedilla/boyet/blob/master/pomodoro.html
Codepen: https://codepen.io/claive-alvin-polo-acedilla/pen/GRRBMMz

function addBreakResumeTimes()              
  {                                             
    var resumeInterval = document.getElementById("breakInterval").value;
...