String.prototype.toCamelCase

JavaScript

String.prototype.toCamelCase = function() {
    return this
        .toLowerCase()
        .replace(/[^\w\s\-]/g, '  ')//remove anything thats not a word character, space or dash
        .replace(/[^a-z0-9]/g, '  ')//replace anything else with a space
        .replace(/\s(.)/g, function(match,group) {return group.toUpperCase()})
}
var str = "aa_aa_aa";
var hashtag = "#"+(str.toCamelCase())
console.log(hashtag);