Proper Case (Convert string to Proper/Title Case)

// convert string to proper case
String.prototype.toProperCase = function () {
    return this.replace(/\w\S*/g, function(txt){return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();});
};

Usage:

'your name'.toProperCase();
Output: 'Your Name'

Snake Case (Convert studly case ‘YourName’ to ‘your_name’)

// convert string to snake case
String.prototype.toSnakeCase = function () {
    return this.replace(/(.)([A-Z])/g, "$1_$2").toLowerCase();
};

Usage:

'YourName'.toSnakeCase();
Output: 'your_name'

Is Date (Check if string is a Date)

// check if string is a valid date
String.prototype.isDate = function () {
    var dateFormat;

    if (toString.call(this) === '[object Date]') {
        return true;
    }
    if (typeof this.replace === 'function') {
        this.replace(/^\s+|\s+$/gm, '');
    }

    dateFormat = /(^\d{1,4}[\.|\\/|-]\d{1,2}[\.|\\/|-]\d{1,4})(\s*(?:0?[1-9]:[0-5]|1(?=[012])\d:[0-5])\d\s*[ap]m)?$/;

    if (dateFormat.test(this)) {
        return !!new Date(this).getTime();
    }

    return false;
};

Usage:

'12-03-1990'.isDate();
Output: true

Is Time (Check if string is a Time)

// check if string is a time
String.prototype.isTime = function () {
    var isValid = /^([0-1]?[0-9]|2[0-3]):([0-5][0-9])(:[0-5][0-9])?$/.test(this);
    return isValid;
};

Usage:

'14:36'.isTime();
Output: true

Is DateTime (Check if string is DateTime, to make it working, copy the above Date & Time Prototype)

// check if string is a time
String.prototype.isDateTime = function () {
    var date = this.split(" ");

    if (date[0].isDate() && date[1].isTime()) {
        return true;
    }

    return false;
};

Usage:

'20-10-1990 14:36'.isDateTime();
Output: true

Month Long Name

// getting month long name from Date object
Date.prototype.getMonthName = function(lang) {
    lang = lang && (lang in Date.locale) ? lang : 'en';
    return Date.locale[lang].month_names[this.getMonth()];
};

Month Short Name

// getting month short name from Date object
Date.prototype.getMonthNameShort = function(lang) {
    lang = lang && (lang in Date.locale) ? lang : 'en';
    return Date.locale[lang].month_names_short[this.getMonth()];
};

Date Locale(Multilingual Support)

Date.locale = {
    en: {
        month_names: ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'],
        month_names_short: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    }
};

Usage:

var current_month = new Date().getMonthName(lang);
var current_short_name = new Date().getMonthNameShort(lang);

lang parameter is optional

Contains (Check if given element exists in Array, similar to jQuery inArray)

Array.prototype.contains = function(obj) {
    var i = this.length;
    while (i--) {
        if (this[i] === obj) {
            return true;
        }
    }
    return false;
}

Usage:

['Red', 'Yellow', 'Green', 'Blue'].contains('Green');
Output: true

Random (Pick a random element from array)

Array.prototype.random = function () {
    return this[Math.floor((Math.random()*this.length))];
}

Usage:

[1, 2, 3, 4].random();

Set Object in LocalStorage/SessionStorage

Storage.prototype.setObject = function(key, value) {
    this.setItem(key, JSON.stringify(value));
}

Usage:

localStorage.setObject('numbers', [1, 2, 3, 4]);

Get Object from LocalStorage/Session

Storage.prototype.getObject = function(key) {
    var value = this.getItem(key);
    return value && JSON.parse(value);
}

Usage:

localStorage.getObject('numbers');