nl2br() equivalent in javascript [duplicate]

Take a look at nl2br on php.js which seems exactly what you’re looking for. Basically, it’s:

function nl2br (str, is_xhtml) {
    if (typeof str === 'undefined' || str === null) {
        return '';
    }
    var breakTag = (is_xhtml || typeof is_xhtml === 'undefined') ? '<br />' : '<br>';
    return (str + '').replace(/([^>\r\n]?)(\r\n|\n\r|\r|\n)/g, '$1' + breakTag + '$2');
}

EDIT:
your example using nl2br() may be changed like this:

$('#TextArea').keypress(function(evt){
        $('#TextArea1').html(nl2br($('#TextArea').val()));
    });

(note that this updates #TextArea1 on every keypress and doesn’t change the value of #TextArea wich is what I think you’re looking for, but I might be misunderstanding)

EDIT2:
If you want to get the behaviour of your old function (with inserting <br/>s to #TextArea) do this:

$('#TextArea').keypress(function(evt){
        $('#TextArea').html(nl2br($('#TextArea').val())); // replace linebreaks first
        $('#TextArea1').html($('#TextArea').val()); // copy to #TextArea1
    });