JavaScript SDK Design Guide


This guide gives you an introduction to developing a JavaScript SDK on desktop and mobile web in different platforms and browsers (<99.99% I might skip some browsers), for those developed for non-browser supports (hardware, embedded, node/io js) are excluded in this document and will be considered in the future.

Since I didn't find out a better documentation for the JavaScript SDK, I'm here to gather and note down the knowledge of my personal experiences. JavaScript-SDK-Design is not just about SDK only, it's the connection between user and browser machine. The more native we write, the more we think, we do care about the performances and differences between platforms and browsers.

Feel free to edit or you can drop me suggestions on the issue list.



What is SDK

I know it's very common, but it is.

"Short for software development kit, a programming package that enables a programmer to develop applications for a specific platform. Typically an SDK includes one or more APIs, programming tools, and documentation."

Design Philosophy

It depends on your purpose of your SDK service and usage, but there must be native, short, fast, clean, readable and testable.

Written in native JavaScript code, compiler language like Livescript, Coffeescript, Typescript and others are not recommend. There must be a better way to write your own JavaScript code in native faster than others.

Please don't involve jQuery in your SDK unless it's really important, you can have other jQuery-like libraries, zepto.js, for the DOM manipulation. Or if you need the HTTP ajax request, use another light library like window.fetch.

Once every SDK version released, make sure that it can be fitted into older and newer SDK version in the future. Hence, remember to write your Documentation for your SDK, comment in your code, unit test and user scenario test.


Based on the book Third-Party JavaScript

In which case, you should design a JavaScript SDK for your application?

  1. Embedded widgets - Small interactive applications embedded on the publisher's web page (Disqus, Google Maps, Facebook Widget)
  2. Analytics and metrics - For gathering intelligence about visitors and how they interact with the publisher's website (GA, Flurry, Mixpanel)
  3. Web service API wrappers - For developing client-side applications that communicate with external web services. (Facebook Graph API)

In what case we should use SDK in JavaScript environment? Suggest one.

Include the SDK

You are suggested to use Asynchronous Syntax for your script loaded. We want to optimize the user experience on the website as we don't want our SDK library to interfere the main web loaded.

Asynchronous Syntax

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

Target on the modern browser, you can use async.

<script async src=""></script>

Traditional Syntax

<script type="text/javascript" src=""></script>


Here's the simple graph to show the differentiate between Asynchronous and Traditional Syntax.





Asynchronous and deferred JavaScript execution explained
You should avoid and minimize the use of blocking JavaScript, especially external scripts that must be fetched before they can be executed. Scripts that are necessary to render page content can be inlined to avoid extra network requests, however the inlined content needs to be small and must execute quickly to deliver good performance. Scripts that are not critical to initial render should be made asynchronous or deferred until after the first render.

Problem of Asynchronous

When you are using Asynchronous, you cannot execute your SDK function which script written within the page.

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName('some arguments');

The result will lead to undefined because the SDKName() execute before the script loaded, therefore we should do some tricks and make sure the script execute successfully. The event will store in the SDKName.q queue array, your SDK should handle and execute SDKName.q and reinitial the namespace SDKName.

  (function () {
    // add a queue event here
    SDKName = SDKName || function () {
      (SDKName.q = SDKName.q || []).push(arguments);
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName('some arguments');

Or using [].push

  (function () {
    // add a queue event here
    SDKName = window.SDKName || (window.SDKName = []);
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName.push(['some arguments']);


There are other different ways to include a script

Import in ES2015

import "your-sdk";

Modular include a Script

For full source code, see this awesome tutorial. Loading JavaScript Modules

module('sdk.js',['sdk-track.js', 'sdk-beacon.js'],function(track, beacon) {
  // sdk definitions, split into local and global/exported definitions
  // local definitions
  // exports

// you should contain this "module" method
(function () {

  var modules = {}; // private record of module data

  // modules are functions with additional information
  function module(name,imports,mod) {

    // record module information
    window.console.log('found module '+name);
    modules[name] = {name:name, imports: imports, mod: mod};

    // trigger loading of import dependencies
    for (var imp in imports) loadModule(imports[imp]);

    // check whether this was the last module to be loaded
    // in a given dependency group

  // function loadModule
  // function loadedModule

  window.module = module;

SDK Versioning

Please avoid using your special case for version like brand-v<timestamp>.js brand-v<datetime>.js brand-v1-v2.js, it may cause the developer who use the SDK on confusing which is the latest version.

Use Semantic Versioning to define your SDK Version in the form "MAJOR.MINOR.PATCH". Version in v1.0.0 v1.5.0 v2.0.0 is easier for them to trace and search for the changelog documentation.

Normally, we can have different ways to state the SDK version, it depends on your service and design.

Using Query String path.

Using the Folder Naming.

Using hostname (subdomain).

For the further development, you are advised to use stable unstable alpha latest experimental version.

Changelog Document

You should notice that your SDK user will not know if you upgrade your SDK without announcement. Remember to write a changelog to document your major, minor and even bug fix change. It will be a good developer experience if we can trace the changing API for the SDK. - Keep a Changelog (Github Repo)

Each version should have:

[Added] for new features.
[Changed] for changes in existing functionality.
[Deprecated] for once-stable features removed in upcoming releases.
[Removed] for deprecated features removed in this release.
[Fixed] for any bug fixes.
[Security] to invite users to upgrade in case of vulnerabilities.

In addition, commit-message-emoji uses emoji to explain the commit's changes itself.


You should not define more than one global namespace in your SDK and prevent using the common word for your namespace to avoid collision with other libraries.

On your SDK mainland, you should use (function () { ... })() to wrap all your source.

This is an increasingly common practice, employed by many popular JavaScript libraries (jQuery, Node.js, etc.). This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries. #

To avoid namespace collision

Learning this from Google Analytics, you can define your namespace by changing the value ga

(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),

From OpenX experience, support a parameter to request the namespace.

<script src="http://your_domain/sdk?namespace=yourcompany"></script>

Storage Mechanism


The domain scope of using cookie is quite complex while involving the subdomain and path.

For path=/, you have a cookie first=value1 in domain, another cookie second=value2 in domain

You have a cookie first=value1 in domain, cookie second=value2 in domain path and cookie third=value3 in domain,

Check Cookie Writable

Given a domain (Default as current hostname), check whether the cookie is writable.

var checkCookieWritable = function(domain) {
    try {
        // Create cookie
        document.cookie = 'cookietest=1' + (domain ? '; domain=' + domain : '');
        var ret = document.cookie.indexOf('cookietest=') != -1;
        // Delete cookie
        document.cookie = 'cookietest=1; expires=Thu, 01-Jan-1970 00:00:01 GMT' + (domain ? '; domain=' + domain : '');
        return ret;
    } catch (e) {
        return false;

Check Third Party Cookie Writable

It's impossible to check only using client side JavaScript, you need a server to do that. (Example)

Write Read Remove Cookie Code

Snippet Code for write/read/remove cookie script.

var cookie = {
    write: function(name, value, days, domain, path) {
        var date = new Date();
        days = days || 730; // two years
        path = path || '/';
        date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
        var expires = '; expires=' + date.toGMTString();
        var cookieValue = name + '=' + value + expires + '; path=' + path;
        if (domain) {
            cookieValue += '; domain=' + domain;
        document.cookie = cookieValue;
    read: function(name) {
        var allCookie = '' + document.cookie;
        var index = allCookie.indexOf(name);
        if (name === undefined || name === '' || index === -1) return '';
        var ind1 = allCookie.indexOf(';', index);
        if (ind1 == -1) ind1 = allCookie.length;
        return unescape(allCookie.substring(index + name.length + 1, ind1));
    remove: function(name) {
        if ( {
            this.write(name, '', -1, '/');


It's important to know that JavaScript is not possible to write Session, please refer to the server side team to implement Session.

A page session lasts for as long as the browser is open and survives over page reloads and restores. Opening a page in a new tab or window will cause a new session to be initiated.


Stores data with no expiration date, storage limit is far larger (at least 5MB) and information is never transferred to the server.

It's good to know that each localStorage from http and https in the same domain aren't shared. You can create an iframe inside the website and use postMessage to pass the value to others. HOW TO?

Check LocalStorage Writable

window.localStorage is not support in every browser, SDK should check available before using it.

var testCanLocalStorage = function() {
   var mod = 'modernizr';
   try {
       localStorage.setItem(mod, mod);
       return true;
   } catch (e) {
       return false;

Session Storage

Stores data for one session (data is lost when the tab is closed).

Check SessionStorage Writable

var checkCanSessionStorage = function() {
  var mod = 'modernizr';
  try {
    sessionStorage.setItem(mod, mod);
    return true;
  } catch (e) {
    return false;


In client browser, there are events load unload on off bind .... Here's some polyfills for you to handle all different platforms.

Document Ready

Please do make sure that the entire page is finished loading(ready) before starting execution the SDK functions.

// handle IE8+
function ready (fn) {
    if (document.readyState != 'loading') {
    } else if (window.addEventListener) {
        // window.addEventListener('load', fn);
        window.addEventListener('DOMContentLoaded', fn);
    } else {
        window.attachEvent('onreadystatechange', function() {
            if (document.readyState != 'loading')

DOMContentLoaded - fired when the document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading

load event can be used to detect a fully-loaded page

Information from JS Tip -

Message Event

It's about the cross-origin communication between iframe and window, read the API documentation.

// in the iframe
parent.postMessage("Hello"); // string

// ==========================================

// in the iframe's parent
// Create IE + others compatible event handler
var eventMethod = window.addEventListener ? "addEventListener" : "attachEvent";
var eventer = window[eventMethod];
var messageEvent = eventMethod == "attachEvent" ? "onmessage" : "message";

// Listen to message from child window
eventer(messageEvent,function(e) {
  // e.origin , check the message origin
  console.log('parent received message!:  ',;

The post message data should be String, for more advanced used in json, use JSON String. Although the modern browser do support Structured Clone Algorithm on the parameter, but not the 100% browser.

Orientation Change

Detect device orientation change

window.addEventListener('orientationchange', fn);

Get Orientation Rotate Degree

window.orientation; // => 90, -90, 0

Screen portrait-primary, portrait-secondary, landscape-primary, landscape-secondary (Experimental)

var orientation = screen.orientation || screen.mozOrientation || screen.msOrientation;

Disable Scroll

In web page, use CSS style overflow: hidden, in some mobile web, this css doesn't work, use javascript event.

document.addEventListener('touchstart', function(e){ e.preventDefault(); }); // prevent scroll
// or 
document.body.addEventListener('touchstart', function(e){ e.preventDefault(); }); // prevent scroll
// use move if you need some touch event
document.addEventListener('touchmove', function(e){ e.preventDefault(); }); // prevent scroll


The communication between our SDK and Server is using Ajax Request, as we know we can use jQuery ajax http request to communicate with Server, but there's a better solution to implement it.

Image Beacon

Use the Image Beacon to ask the browser to perform a method GET Request to get an Image.

Remember to add timestamp (Cache Buster) to prevent caching in browser.

(new Image()).src = '';

Some notice for GET Query String, there is the limit of length which is 2048(Basically It depends on different browser and server). You should do some tricks to handle if exceed length limit.

if (length > 2048) {
    // do Multiple Post (form)
} else {
    // do Image Beacon

You may have the problem on encodeURI or encodeURIComponent, it's better if you understand them. See below.

For the image load success/error callback

var img = new Image();
img.src = '';
img.onload = successCallback;
img.onerror = errorCallback;

Single Post

Use the native form element method POST to send a key value.

var form = document.createElement('form');
var input = document.createElement('input'); = 'none';
form.setAttribute('method', 'POST');
form.setAttribute('action', ''); = 'username';
input.value = 'attacker';



Multiple Post

The Service is often complex, we need to send more data through method POST.

function requestWithoutAjax( url, params, method ){

    params = params || {};
    method = method || "post";

    // function to remove the iframe
    var removeIframe = function( iframe ){

    // make a iframe...
    var iframe = document.createElement('iframe'); = 'none';

    iframe.onload = function(){
        var iframeDoc = this.contentWindow.document;

        // Make a invisible form
        var form = iframeDoc.createElement('form');
        form.method = method;
        form.action = url;

        // pass the parameters
        for( var name in params ){
            var input = iframeDoc.createElement('input');
            input.type = 'hidden';
   = name;
            input.value = params[name];

        // remove the iframe
        setTimeout( function(){
        }, 500);

requestWithoutAjax('url/to', { id: 2, price: 2.5, lastname: 'Gamez'});


When you need to generate a content within the page, you can use iframe to embed your html.

var iframe = document.createElement('iframe');
var body = document.getElementsByTagName('body')[0]; = 'none';
iframe.src = '';
iframe.onreadystatechange = function () {
    if (iframe.readyState !== 'complete') {
iframe.onload = loadCallback;


Remove extra margin from INSIDE an iframe

<iframe src="..."

Putting html content into an iframe

<iframe id="iframe"></iframe>

  var html_string= "content <script>alert(location.href);</script>";
  document.getElementById('iframe').src = "data:text/html;charset=utf-8," + escape(html_string);
  // alert data:text/html;charset=utf-8.....
  // access cookie get ERROR

  var doc = document.getElementById('iframe').contentWindow.document;;
  // alert "top window url"

  var iframe = document.createElement('iframe');
  iframe.src = 'javascript:;\'' + encodeURI('<html><body><script>alert(location.href);</body></html>') + '\'';
  // iframe.src = 'javascript:;"' + encodeURI((html_tag).replace(/\"/g, '\\\"')) + '"';
  // alert "about:blank"

Script jsonp

This is the case that your server needs to response JavaScript code and let the client browser execute it. Just include the JS script link.

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '/yourscript?some=parameter&callback=jsonpCallback';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

Know more about jsonp

  1. JSONP only works in GET HTTP request.
  2. JSONP lacks error handling, means you cannot detect case in response status code 404, 500 and so on.
  3. JSONP requests are always asynchronous.
  4. Beware of CSRF attack.
  5. Cross domain communication. Script response side (server-side) don't need to care about CORS.


Look at the documentation.

This method addresses the needs of analytics and diagnostics code that typically attempt to send data to a web server prior to the unloading of the document. Sending the data any sooner may result in a missed opportunity to gather data. However, ensuring that the data has been sent during the unloading of a document is something that has traditionally been difficult for developers.

Send POST beacon through the API. It's cool.

navigator.sendBeacon("/log", analyticsData);


Writing XMLHttpRequest is not a good idea. I assume that you don't want to waste time on battling with IE or other browsers. Here's some polyfills or code you can try

  1. window.fetch - A window.fetch JavaScript polyfill.
  2. got - Simplified HTTP/HTTPS requests
  3. microjs - list of ajax lib
  4. more

Fragment Identifier

Also known as hash mark #. Remember that request with hash mark at the end is not pass within http request.

For example, you are in the page

// Sending a request with a parameter url which contains current url
(new Image()).src = '';

// actual request will be without #
(new Image()).src = '';

// Solved, encodeURIComponent(url);
(new Image()).src = '' + encodeURIComponent('');

Maximum Number of Connection

Check the maximum number of browser's request connection. browserscope

max number of connection

Component of URI

It's important to know if your SDK needs to parse the location url.

                  /                    \
              userinfo                host                          resource
               __|___                ___|___                 __________|___________
              /      \              /       \               /                      \
         username  password     hostname    port     path & segment      query   fragment
           __|___   __|__    ______|______   |   __________|_________   ____|____   |
          /      \ /     \  /             \ / \ /                    \ /         \ / \
    \_/                     \ / \       \ /    \__________/ \     \__/
     |                       |   \       |           |       \      |
  scheme               subdomain  \     tld      directory    \   suffix
                                   \____/                      \___/
                                      |                          |
                                    domain                   filename

Parsing URI

Here's a simplest by using the native URL() Interface but it doesn't support all the browsers also not standard yet.

var parser = new URL('');
parser.hostname; // => ""

For the browser which doesn't have the URL() Interface, try DOM createElement('a').

var parser = document.createElement('a');
parser.href = "";
parser.hostname; // => ""


Simulating Multiple Domains

You don't need to register different domain names to simulate multiple domain, you can just edit your operating system's hosts file.

$ sudo vim /etc/hosts

Add the following entries

# refer to localhost

Then you can access the page and

Developer Tools

Use the web debugging tools from browser vendors when debugging your SDK JavaScript code - Chrome Developer Tools Safari Developer Tools Firebug. Developer tools also short for DevTools.

The DevTools provide web developers deep access into the internals of the browser and their web application. Use the DevTools to efficiently track down layout issues, set JavaScript breakpoints, and get insights for code optimization.

Console Logs

For testing expected output text and other general debugging, Console Logs can be used throught the browser API console.log(). There are various typeways to format and output your messages, find out more Console API.

screen shot 2015-06-15 at 3 50 23 pm

Debugging Proxy

Sometimes debugging proxy will give you a hand on testing your SDK. Debug traffic, modify cookies, headers, cache, edit http request/response, SSL Proxying, ajax debugging and more.

Here's some software you can try


BrowserSync makes your tweaking and testing faster by synchronising file changes and interactions across multiple devices. It’s wicked-fast and totally free.

It really helps a lot if you need to test your SDK result in multiple cross devices. Try it =)

Tips and Tricks


Sometimes, we don't want our developers include all the SDK source, we just need to do a simple 1x1 pixel request (for example: return a request when landing on thank you/last page). All we need to do is ask the developer to include an image file with our url link.

<img height="1" width="1" alt="" style="display:none" src="" />

Page Visibility API

Sometimes, your SDK wants to detect if the user has your page in focus. Try the polyfills visibly.js and visibilityjs.

Document Referrer

Use document.referrer to get the url of current previous page. But remember that this referrer is "Browser Referrer" not the "Human Known Referrer". If you click the browser back button, for example pageA -> pageB -> pageC -> (back button) pageB, current pageB's referrer is pageA, not pageC.

Console Logs Polyfill

It's not a real polyfill, just make sure that calling console.log API doesn't throw error event to client side.

if (typeof console === "undefined") {
    var f = function() {};
    console = {
        log: f,
        debug: f,
        error: f,
        info: f

EncodeURI or EncodeURIComponent

Understand the difference between escape() encodeURI() encodeURIComponent() here.

Remember that using encodeURI() and encodeURIComponent() has exactly 11 characters difference. These characters are: # $ & + , / : ; = ? @ more discussion.

encodeuri or encodeuricomponent


As the title said, you might not need jquery. It's really useful if you are looking for some utilities code - AJAX EFFECTS, ELEMENTS, EVENTS, UTILS

You Don't Need jQuery

Free yourself from the chains of jQuery by embracing and understanding the modern Web API and discovering various directed libraries to help you fill in the gaps.

Useful Tips

  1. Selecting Elements
  2. DOM Manipulation

Load Script with Callback

It's similar to asynchrnous script loading with addition callback event

function loadScript(url, callback) {
  var script = document.createElement('script');
  script.async = true;
  script.src = url;

  var entry = document.getElementsByTagName('script')[0];
  entry.parentNode.insertBefore(script, entry);

  script.onload = script.onreadystatechange = function () {
    var rdyState = script.readyState;

    if (!rdyState || /complete|loaded/.test(script.readyState)) {

      // detach the event handler to avoid memory leaks in IE (
      script.onload = null;
      script.onreadystatechange = null;

Once Function

Implementation of the function once

Every so often you have a function which you only want to run once. Oftentimes these functions are in the form of event listeners which may be difficult to manage. Of course if they were easy to manage, you'd just remove the listeners but that's a perfect world and sometimes you simply want the ability to only allow a function to be called once. Here's the JavaScript function to make that possible!

// Copy from DWB
function once(fn, context) { 
    var result;

    return function() { 
        if(fn) {
            result = fn.apply(context || this, arguments);
            fn = null;

        return result;

// Usage
var canOnlyFireOnce = once(function() {

canOnlyFireOnce(); // "Fired!"
canOnlyFireOnce(); // nada

Pixel Ratio Density

If you stuck on the term pixel, ratio, density, dimension, what while developing mobile web, try understanding the video, it may help.

Device pixel ratio - Mobile Web Development
Mobile device pixels - Mobile Web Development

Get Style Value

Get inline-style value

<span id="black" style="color: black"> This is black color span </span>
    document.getElementById('black').style.color; // => black

Get Real style value

#black {
    color: red !important;

<span id="black" style="color: black"> This is black color span </span>

    document.getElementById('black').style.color; // => black

    // real
    var black = document.getElementById('black');
    window.getComputedStyle(black, null).getPropertyValue('color'); // => rgb(255, 0, 0)


Check if Element in Viewport

You can find out more here.

function isElementInViewport (el) {

    //special bonus for those using jQuery
    if (typeof jQuery === "function" && el instanceof jQuery) {
        el = el[0];

    var rect = el.getBoundingClientRect();

    return ( >= 0 &&
        rect.left >= 0 &&
        rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /*or $(window).height() */
        rect.right <= (window.innerWidth || document.documentElement.clientWidth) /*or $(window).width() */

Check if Element is Visible

var isVisible: function(b) {
    var a = window.getComputedStyle(b);
    return 0 === a.getPropertyValue("opacity") || "none" === a.getPropertyValue("display") || "hidden" === a.getPropertyValue("visibility") || 0 === parseInt(, 10) || "none" === || "hidden" === ? false : true;

var element = document.getElementById('box');
isVisible(element); // => false or true

Get Viewport Size

var getViewportSize = function() {
    try {
        var doc = top.document.documentElement
          , g = (e = top.document.body) && top.document.clientWidth && top.document.clientHeight;
    } catch (e) {
        var doc = document.documentElement
          , g = (e = document.body) && document.clientWidth && document.clientHeight;
    var vp = [];
    doc && doc.clientWidth && doc.clientHeight && ("CSS1Compat" === document.compatMode || !g) ? vp = [doc.clientWidth, doc.clientHeight] : g && (vp = [doc.clientWidth, doc.clientHeight]);
    return vp;

// return as rray [viewport_width, viewport_height]


Misspelling Of Referrer

Do you know that why the HTTP Request Header having the field name referer not referrer

According to the Wikipedia

The misspelling of referrer originated in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the field into the HTTP specification. The misspelling was set in stone by the time of its incorporation into the Request for Comments standards document RFC 1945; document co-author Roy Fielding has remarked that neither "referrer" nor the misspelling "referer" were recognized by the standard Unix spell checker of the period. "Referer" has since become a widely used spelling in the industry when discussing HTTP referrers; usage of the misspelling is not universal, though, as the correct spelling "referrer" is used in some web specifications such as the Document Object Model.


Someone asks for the template/boilerplate of the SDK, here some examples for you.

Book/Nice to Read

  1. Third-Party JavaScript
  2. JQuery Plugin
  3. LightningJS


  1. What is Software Development Kit
  2. A window.fetch JavaScript polyfill.
  3. POST Request
  4. Semantic VersioningVersioning 2.0.0
  5. HTTP API design guide extracted from work on the Heroku Platform API
  6. Understanding URIs
  7. URI Parsing with JavaScript
  8. Modernizr: the feature detection library for HTML5/CSS3
  9. HTML5 Web Storage
  10. Check if third-party cookies are enabled
  11. Introduction to Analytics.js - Universal Analytics Web Tracking
  12. Facebook Conversion Tracking Pixel
  13. What is the maximum length of a URL
  15. What is a Polyfill?
  16. Asynchronous and deferred JavaScript execution explained
  17. generate random UUIDs
  18. DOMContentLoaded and Load Event
  19. Must See JavaScript Dev Tools That Put Other Dev Tools to Shame
  20. Learn the art of writing quality READMEs.

(inspired by http-api-design)