makeResizableDiv_1.js

function makeResizableDiv(div) {
  const element = document.querySelector(div);
  const resizers = document.querySelectorAll(div + ' .resizer')
  for (let i = 0;i < resizers.length; i++) {
    const currentResizer = resizers[i];
    currentResizer.addEventListener('mousedown', function(e) {
      currentResizer.addEventListener('mousemove', resize)
    })
    
    function resize(e) {
      if (currentResizer.classList.contains('bottom-right')) {
        element.style.width = e.pageX - element.getBoundingClientRect().left + 'px';
      }
    }
  }
}

makeResizableDiv('.resizable')