selecttxt.js

const selection = window.getSelection();
const range = document.createRange();
const element = document.getElementById('elmt');

// Collect a range based off the contents of an element.
range.selectNodeContents(element);

// Clear ranges that might be stored in the selection object.
selection.removeAllRanges();
selection.addRange(range);

//Optional
document.execCommand('copy');