how to highlight text in pdf code example

Example: pdf highlighter

function getSelectionCoords() {
    var pdfViewer = window.appPdfViewer;    
    var pageIndex = pdfViewer.currentPageNumber - 1; 
    var page = pdfViewer.getPageView(pageIndex);
    var pageRect = page.canvas.getClientRects()[0];
    var selectionRects = window.getSelection().getRangeAt(0).getClientRects();
    var viewport = page.viewport;
    var selectionRectsList = Object.values(selectionRects);
    var selected = selectionRectsList.map(function (r) {
      return viewport.convertToPdfPoint(r.left - pageRect.x, r.top - pageRect.y).concat(
         viewport.convertToPdfPoint(r.right - pageRect.x, r.bottom - pageRect.y)); 
    });
    return {page: pageIndex, coords: selected};
}



function showHighlight() {
    var selected = getSelectionCoords();
    var pageIndex = selected.page; 
    var pdfViewer = window.appPdfViewer;  
    var page = pdfViewer.getPageView(pageIndex);
    var pageElement = page.canvas.parentElement;
    var viewport = page.viewport;

    selected.coords.forEach(function (rect) {

      // Individual Seletions...
      var x1 = Math.min(bounds[0], bounds[2]);
      var y1 = Math.min(bounds[1], bounds[3]);
      var width =  Math.abs(bounds[0] - bounds[2]);
      var hight = Math.abs(bounds[1] - bounds[3]);
      var el = createRectDiv([x1, y1, width, hight]);
      pageElement.appendChild(el);

    });
   
}

function createRectDiv(boundBox){
    var randomColor = Math.floor(Math.random()*16777215).toString(16);
    console.log(randomColor);
    var el = document.createElement('div');
    el.setAttribute('class', 'hiDiv')
    el.setAttribute('style', 'position: absolute; background-color: #'+randomColor+'; opacity: 0.5;' + 
    'left:' + boundBox[0] + 'px; top:' + boundBox[1] + 'px;' +
    'width:' + boundBox[2] + 'px; height:' + boundBox[3] + 'px;');
    return el;
}


window.addEventListener('mouseup', function() {

    var length = window.getSelection().toString().length;
    if(length > 0){
        showHighlight();
    }else{
        // Clear All ?!
    }
  console.log();
});