Range.selectNodeContents()

selectNodeContents(referenceNode)

range = document.createRange();
referenceNode = document.getElementsByTagName("div")[0];
range.selectNodeContents(referenceNode);

<p id="p"><b>Use the buttons below</b> to select or deselect the contents of this paragraph.</p>
<button id="select-button">Select paragraph</button>
<button id="deselect-button">Deselect paragraph</button>

const p = document.getElementById('p');
const selectButton = document.getElementById('select-button');
const deselectButton = document.getElementById('deselect-button');

selectButton.addEventListener('click', e => {
  // Clear any current selection
  const selection = window.getSelection();
  selection.removeAllRanges();

  // Select paragraph
  const range = document.createRange();
  range.selectNodeContents(p);
  selection.addRange(range);
});

deselectButton.addEventListener('click', e => {
  const selection = window.getSelection();
  selection.removeAllRanges();
});