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HTML 

              
                <div class='resizable'>
  <div class='resizers'>
    <div class='resizer top-left'></div>
    <div class='resizer top-right'></div>
    <div class='resizer bottom-left'></div>
    <div class='resizer bottom-right'></div>
  </div>
</div>
!

CSS 

              
                body,
html {
  background: black;
}
.resizable {
  background: white;
  width: 100px;
  height: 100px;
  position: absolute;
  top: 100px;
  left: 100px;
}

.resizable .resizers{
  width: 100%;
  height: 100%;
  border: 3px solid #4286f4;
  box-sizing: border-box;
}

.resizable .resizers .resizer{
  width: 10px;
  height: 10px;
  border-radius: 50%; /*magic to turn square into circle*/
  background: white;
  border: 3px solid #4286f4;
  position: absolute;
}

.resizable .resizers .resizer.top-left {
  left: -5px;
  top: -5px;
  cursor: nwse-resize; /*resizer cursor*/
}
.resizable .resizers .resizer.top-right {
  right: -5px;
  top: -5px;
  cursor: nesw-resize;
}
.resizable .resizers .resizer.bottom-left {
  left: -5px;
  bottom: -5px;
  cursor: nesw-resize;
}
.resizable .resizers .resizer.bottom-right {
  right: -5px;
  bottom: -5px;
  cursor: nwse-resize;
}
!

JS 

              
                /*Make resizable div by Hung Nguyen*/
function makeResizableDiv(div) {
  const element = document.querySelector(div);
  const resizers = document.querySelectorAll(div + ' .resizer')
  const minimum_size = 20;
  let original_width = 0;
  let original_height = 0;
  let original_x = 0;
  let original_y = 0;
  let original_mouse_x = 0;
  let original_mouse_y = 0;
  for (let i = 0;i < resizers.length; i++) {
    const currentResizer = resizers[i];
    currentResizer.addEventListener('mousedown', function(e) {
      e.preventDefault()
      original_width = parseFloat(getComputedStyle(element, null).getPropertyValue('width').replace('px', ''));
      original_height = parseFloat(getComputedStyle(element, null).getPropertyValue('height').replace('px', ''));
      original_x = element.getBoundingClientRect().left;
      original_y = element.getBoundingClientRect().top;
      original_mouse_x = e.pageX;
      original_mouse_y = e.pageY;
      window.addEventListener('mousemove', resize)
      window.addEventListener('mouseup', stopResize)
    })
    
    function resize(e) {
      if (currentResizer.classList.contains('bottom-right')) {
        const width = original_width + (e.pageX - original_mouse_x);
        const height = original_height + (e.pageY - original_mouse_y)
        if (width > minimum_size) {
          element.style.width = width + 'px'
        }
        if (height > minimum_size) {
          element.style.height = height + 'px'
        }
      }
      else if (currentResizer.classList.contains('bottom-left')) {
        const height = original_height + (e.pageY - original_mouse_y)
        const width = original_width - (e.pageX - original_mouse_x)
        if (height > minimum_size) {
          element.style.height = height + 'px'
        }
        if (width > minimum_size) {
          element.style.width = width + 'px'
          element.style.left = original_x + (e.pageX - original_mouse_x) + 'px'
        }
      }
      else if (currentResizer.classList.contains('top-right')) {
        const width = original_width + (e.pageX - original_mouse_x)
        const height = original_height - (e.pageY - original_mouse_y)
        if (width > minimum_size) {
          element.style.width = width + 'px'
        }
        if (height > minimum_size) {
          element.style.height = height + 'px'
          element.style.top = original_y + (e.pageY - original_mouse_y) + 'px'
        }
      }
      else {
        const width = original_width - (e.pageX - original_mouse_x)
        const height = original_height - (e.pageY - original_mouse_y)
        if (width > minimum_size) {
          element.style.width = width + 'px'
          element.style.left = original_x + (e.pageX - original_mouse_x) + 'px'
        }
        if (height > minimum_size) {
          element.style.height = height + 'px'
          element.style.top = original_y + (e.pageY - original_mouse_y) + 'px'
        }
      }
    }
    
    function stopResize() {
      window.removeEventListener('mousemove', resize)
    }
  }
}

makeResizableDiv('.resizable')
!
999px

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