Neatest (and fast) way to remove top lines from a textarea

Question

I have a webpage that displays last 1000 lines of a logfile then updates via AJAX every x seconds loading new content (if any) and appending to textare

Answer 1

Something like this (demo linked below is probably more useful):

HTML

More lines



Lines: 0

JavaScript

var $log = $('#log'),
    $button = $('#clickme'),
    $numLines = $('#numLines'),
    MAX_LINES = 5000,
    lorem_ipsum = ' Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
    lineCounter = 0;

$button.click(function()
{
    $log.val($log.val() + generateMoreLines()).change();
});

$log.change(function ()
{
    var text = tail(MAX_LINES, $log.val());
    $log.val(text);
    $numLines.text(countNewlines(text));
});

function generateMoreLines()
{
    var buf = [];
    for (var i = 0; i < 1000; i++)
    {
        buf.push(lineCounter++ + lorem_ipsum);
    }
    return buf.join('\n');
}

function countNewlines(haystack)
{
    return count('\n', haystack);
}

function count(needle, haystack)
{
    var num = 0,
        len = haystack.length;
    for (var i=0; i < len; i++)
    {
        if (haystack.charAt(i) === needle)
        {
             num++;
        }
    }
    return num;
}

function tail(limit, haystack)
{
    var lines = countNewlines(haystack) + 1;
    if (lines > limit)
    {
        return haystack
            .split('\n')
            .slice(-limit)
            .join('\n');
    }
    return haystack;
}

The newline handling isn't perfect (do you count all occurrences of '\n'? What if the string starts or ends with '\n'? etc.).

Demo: http://jsfiddle.net/mattball/3ghjm/