Programmatically select text in a contenteditable HTML element?

If you want to select all the content of an element (contenteditable or not) in Chrome, here's how. This will also work in Firefox, Safari 3+, Opera 9+ (possibly earlier versions too) and IE 9. You can also create selections down to the character level. The APIs you need are DOM Range (current spec is DOM Level 2, see also MDN) and Selection, which is being specified as part of a new Range spec (MDN docs).

function selectElementContents(el) {
    var range = document.createRange();
    range.selectNodeContents(el);
    var sel = window.getSelection();
    sel.removeAllRanges();
    sel.addRange(range);
}

var el = document.getElementById("foo");
selectElementContents(el);

In addition to Tim Downs answer, i made a solution that work even in oldIE:

var selectText = function() {
  var range, selection;
  if (document.body.createTextRange) {
    range = document.body.createTextRange();
    range.moveToElementText(this);
    range.select();
  } else if (window.getSelection) {
    selection = window.getSelection();
    range = document.createRange();
    range.selectNodeContents(this);
    selection.removeAllRanges();
    selection.addRange(range);
  }
};

document.getElementById('foo').ondblclick = selectText;​

Tested in IE 8+, Firefox 3+, Opera 9+, & Chrome 2+. Even I've set it up into a jQuery plugin:

jQuery.fn.selectText = function() {
  var range, selection;
  return this.each(function() {
    if (document.body.createTextRange) {
      range = document.body.createTextRange();
      range.moveToElementText(this);
      range.select();
    } else if (window.getSelection) {
      selection = window.getSelection();
      range = document.createRange();
      range.selectNodeContents(this);
      selection.removeAllRanges();
      selection.addRange(range);
    }
  });
};

$('#foo').on('dblclick', function() {
  $(this).selectText();
});

...and who's intereseted in, here's the same for all coffee-junkies:

jQuery.fn.selectText = ->
  @each ->
    if document.body.createTextRange
      range = document.body.createTextRange()
      range.moveToElementText @
      range.select()
    else if window.getSelection
      selection = window.getSelection()
      range = document.createRange()
      range.selectNodeContents @
      selection.removeAllRanges()
      selection.addRange range
    return

Update:

If you want to select the entire page or contents of an editable region (flagged with contentEditable), you can do it much simpler by switching to designMode and using document.execCommand:

There's a good starting point at MDN and a littledocumentation.

var selectText = function () {
  document.execCommand('selectAll', false, null);
};

(works well in IE6+, Opera 9+, Firefoy 3+, Chrome 2+) http://caniuse.com/#search=execCommand

The modern way of doing things is like this. More details on MDN

document.addEventListener('dblclick', (event) => {
  window.getSelection().selectAllChildren(event.target)
})

<div contenteditable="true">Some text</div>

Since all of the existing answers deal with div elements, I'll explain how to do it with spans.

There is a subtle difference when selecting a text range in a span. In order to be able to pass the text start and end index, you have to use a Text node, as described here:

If the startNode is a Node of type Text, Comment, or CDATASection, then startOffset is the number of characters from the start of startNode. For other Node types, startOffset is the number of child nodes between the start of the startNode.

var e = document.getElementById("id of the span element you want to select text in");
var textNode = e.childNodes[0]; //text node is the first child node of a span

var r = document.createRange();
var startIndex = 0;
var endIndex = textNode.textContent.length;
r.setStart(textNode, startIndex);
r.setEnd(textNode, endIndex);

var s = window.getSelection();
s.removeAllRanges();
s.addRange(r);