Selecting text in an element (akin to highlighting with your mouse)

Plain Javascript

function selectText(nodeId) {
    const node = document.getElementById(nodeId);

    if (document.body.createTextRange) {
        const range = document.body.createTextRange();
        range.moveToElementText(node);
        range.select();
    } else if (window.getSelection) {
        const selection = window.getSelection();
        const range = document.createRange();
        range.selectNodeContents(node);
        selection.removeAllRanges();
        selection.addRange(range);
    } else {
        console.warn("Could not select text in node: Unsupported browser.");
    }
}

const clickable = document.querySelector('.click-me');
clickable.addEventListener('click', () => selectText('target'));

<div id="target"><p>Some text goes here!</p><p>Moar text!</p></div>
<p class="click-me">Click me!</p>

Here is a working demo. For those of you looking for a jQuery plugin, I made one of those too.

jQuery (original answer)

I have found a solution for this in this thread. I was able to modify the info given and mix it with a bit of jQuery to create a totally awesome function to select the text in any element, regardless of browser:

function SelectText(element) {
    var text = document.getElementById(element);
    if ($.browser.msie) {
        var range = document.body.createTextRange();
        range.moveToElementText(text);
        range.select();
    } else if ($.browser.mozilla || $.browser.opera) {
        var selection = window.getSelection();
        var range = document.createRange();
        range.selectNodeContents(text);
        selection.removeAllRanges();
        selection.addRange(range);
    } else if ($.browser.safari) {
        var selection = window.getSelection();
        selection.setBaseAndExtent(text, 0, text, 1);
    }
}

This thread (dead link) contains really wonderful stuff. But I'm not able to do it right on this page using FF 3.5b99 + FireBug due to "Security Error".

Yipee!! I was able to select whole right hand sidebar with this code hope it helps you:

    var r = document.createRange();
    var w=document.getElementById("sidebar");  
    r.selectNodeContents(w);  
    var sel=window.getSelection(); 
    sel.removeAllRanges(); 
    sel.addRange(r); 

PS:- I was not able to use objects returned by jquery selectors like

   var w=$("div.welovestackoverflow",$("div.sidebar"));
   
   //this throws **security exception**

   r.selectNodeContents(w);

For any tag one can select all text inside that tag by this short and simple code. It will highlight the entire tag area with yellow colour and select text inside it on single click.

document.onclick = function(event) {
    var range, selection;
event.target.style.backgroundColor = 'yellow';
        selection = window.getSelection();
        range = document.createRange();
        range.selectNodeContents(event.target);
        selection.removeAllRanges();
        selection.addRange(range);
};

Have a look at the Selection object (Gecko engine) and the TextRange object (Trident engine.) I don't know about any JavaScript frameworks that have cross-browser support for this implemented, but I've never looked for it either, so it's possible that even jQuery has it.

lepe - That works great for me thanks! I put your code in a plugin file, then used it in conjunction with an each statement so you can have multiple pre tags and multiple "Select all" links on one page and it picks out the correct pre to highlight:

<script type="text/javascript" src="../js/jquery.selecttext.js"></script>
<script type="text/javascript">
  $(document).ready(function() { 
        $(".selectText").each(function(indx) {
                $(this).click(function() {                 
                    $('pre').eq(indx).selText().addClass("selected");
                        return false;               
                    });
        });
  });

My particular use-case was selecting a text range inside an editable span element, which, as far as I could see, is not described in any of the answers here.

The main difference is that you have to pass a node of type Text to the Range object, as described in the documentation of Range.setStart():

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.

The Text node is the first child node of a span element, so to get it, access childNodes[0] of the span element. The rest is the same as in most other answers.

Here a code example:

var startIndex = 1;
var endIndex = 5;
var element = document.getElementById("spanId");
var textNode = element.childNodes[0];

var range = document.createRange();
range.setStart(textNode, startIndex);
range.setEnd(textNode, endIndex);

var selection = window.getSelection();
selection.removeAllRanges();
selection.addRange(range);

Other relevant documentation:
Range
Selection
Document.createRange()
Window.getSelection()

According to the jQuery documentation of select():

Trigger the select event of each matched element. This causes all of the functions that have been bound to that select event to be executed, and calls the browser's default select action on the matching element(s).

There is your explanation why the jQuery select() won't work in this case.