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var s = 'this-is-a-very-very-very-long-file-name.jpg';

console.log(truncate(s, 100)); //this-is-a-very-very-very-long-file-name.jpg
console.log(truncate(s, 10)); //this-is-a-[...].jpg
console.log(truncate(s, 4)); //this[...].jpg

function truncate(n, len) {
    var ext = n.substring(n.lastIndexOf(".") + 1, n.length).toLowerCase();
    var filename = n.replace('.' + ext,'');
    if(filename.length <= len) {
        return n;
    }
    filename = filename.substr(0, len) + (n.length > len ? '[...]' : '');
    return filename + '.' + ext;
};