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Even more aggressively trying to preload your next page load

22 January 2018 2 comments   Javascript, Web development

In 2014 I tried out an experiment to "Aggressively prefetching everything you might click". It was received with mixed reviews. Today, 4 years later, I stand by that experiment/solution and I even like it so much that I've decided to extend it.

How it works

The gist of the solution is that if you mouse hover over an internal link, with a 200ms delay, an XHR request is made to that URL as a simple GET. Suppose the XHR finishes loading in, say 300ms, and you eventually click the link, by the time it tries to load it, it loads it straight from your browser cache. You get that "instant load" feel and it makes navigating the site more enjoyable. Suppose that you're really fast with your mouse/trackpad and you click the link faster than 500ms (but slower than 200ms) the XHR request gets automatically cancelled by the browser. When your browser loads the new page, it basically has to start from scratch. No harm done. Just not as fast.

Sure, there is a chance that you hover over a link, and stay hovering for more than 200ms but then decide to not click on it. Then the XHR preload was a waste of resources.
But!! If you even have a mouse cursor, the chances that you're on a WiFi connected laptop.

None of this "kicks in" when you're on a mobile device. The onMouseOver event won't trigger. And, I dare to say that only on mobile devices does it strongly matter to reduce the stuff the client has to download. So what's the harm of forcing your laptop to download a couple of extra kilobytes? If you hover over the link, the chances are, after all, that you will click the link.

Even more aggressive

Today I decided to step it up even more. Now, after the HTML has been downloaded, the HTML downloaded is scanned with a regular expression for image URLs that sit on my CDN (where I host all images with far-future cache headers). The first 5 image URLs are preloaded so that when you eventually make that link click, not only is the page load instant, but most images are too.

What do you think? Too aggressive or genius?

Before hovering
Before hovering over the "About" link

After hovering
After hovering over the "About" link

Now, if I go ahead and make the click, the HTML load will be instant and the first 3 images will be instant too.

Show me the code!

It ain't pretty but it works: prefetcher.js

Yes, it's jQuery and I'm OK with that. Yes, the CDN domain name is hardcoded and if this was a work project I'd never do that. Heck, the ultimate reason I'm blogging about this is ultimately to share/teach. When you build something similar you can do it more robustly.

minimalcss 0.6.2 now strips all unused font faces

22 January 2018 0 comments   Node, Javascript, Web development

minimalcss is a Node API and cli app to analyze the minimal CSS needed for initial load. One of it's killer features is that all CSS parsing is done the "proper way". Meaning, it's reduced down to an AST that can be iterated over, mutated and serialized back to CSS as a string.

Thanks to this, together with my contributors @stereobooster and @lahmatiy, minimalcss can now figure out which @font-face rules are redundant and can be "safely" removed. It can make a big difference on web performance. Either because it prevents expensive network requests of downloading some or downloading base64 encoded fonts.

For example, this very blog uses Semantic UI which is a wonderful CSS framework. But it's quite expensive and contains a bunch of base64 encoded fonts. The Ratings module uses a @font-face rule that weighes about 15KB.

Sure, you don't have to download and insert semanticui.min.css in your HTML but it's just sooo convenient. Especially when there's tools like minimalcss that allows you to be "lazy" but get that perfect first load web performance thing.
So, the CSS when doing a search looks like this:

126KB of CSS (gzipped) transferred and 827KB of CSS parsed.

Let's run this through minimalcss instead:

$ minimalcss.js --verbose -o /tmp/ ""
$ ls -lh /tmp/
-rw-r--r--  1 peterbe  wheel    27K Jan 22 09:59 /tmp/
$ head -n 14 /tmp/
Generated 2018-01-22T14:59:05.871Z by minimalcss.
Took 4.43 seconds to generate 26.85 KB of CSS.
Based on 3 stylesheets totalling 827.01 KB.
Options: {
  "urls": [
  "debug": false,
  "loadimages": false,
  "withoutjavascript": false,
  "viewport": null

And let's simulate it being gzipped:

$ gzip /tmp/
$ ls -lh /tmp/
-rw-r--r--  1 peterbe  wheel   6.0K Jan 22 09:59 /tmp/

Wow! Instead of downloading 27KB you only need 6KB. CSS parsing isn't as expensive as JavaScript parsing but it's nevertheless a saving of 827KB - 27KB = 800KB of CSS for the browser to not have to worry about. That's awesome!

By the way, the produced minimal CSS contains a lot of license preamble as left over from the fact that the semanticui.min.css is made up of components. See the gist itself.
Out of the total size of 27KB (uncompressed) 8KB is just the license preambles. minimalcss does not attempt to touch that when it minifies but you could easily add your own little tooling to re-write it, since there's a lot of repetition and save another ~7KB. However, all that repetition compresses well so it might not be worth it.

Display current React version

07 January 2018 0 comments   ReactJS, Javascript

Usually you know what version of React your app is using by opening the package.json, or poking around in node_modules/react/index.js. But perhaps there are many packaging abstractions in between your command line and the server. Especially if you have a continous integration server that builds your static assets and if that CI uses caching. It might get scary.

If you really want to print out what version of React is rendering your app here's one way to do that:

import React from 'react'

class Introspection extends React.Component {
  render() {
    return <div>
      Currently using React {React.version}

Suppose that you want this display to depend on the app being in dev or prod mode:

import React from 'react'

class Introspection extends React.Component {
  render() {
    return <div>
        process.env.NODE_ENV === 'development' ?
        <p>Currently using React {React.version}</p> : null

Note that there's no need to import process.

See this CodeSandbox snippet for a live example.

CSS selector simplifier regular expression in JavaScript

20 December 2017 0 comments   Javascript, Web development

The Problem

I'm working on a project where it needs to evaluate CSS as a string. Basically, it compares CSS selectors against a DOM to see if the CSS selector is used in the DOM.

But CSS has pseudo classes. A common one a lot of people are familiar with is: a:hover { text-decoration: crazy }. So that :hover part is not relevant when evaluating the CSS selector against the DOM. So you chop off the :hover bit and is left with a which you can then look for in the DOM.

But there are some tricks and make this less trivial. Consider, this from Bootstrap 3

a[href^="javascript:"]:after {
    content: "";

In this case we can't simply split on a : character.

Another non-trivial example comes from Semantic UI:

.ui[class*="4:3"].embed {
  padding-bottom: 75%;
.ui[class*="16:9"].embed {
  padding-bottom: 56.25%;
.ui[class*="21:9"].embed {
  padding-bottom: 42.85714286%;

Basically, if you just split the selectors (e.g. a:hover) on the first : and keep everything to the left (e.g. a), with these non-trivial CSS selectors you'd get this:




etc. These CSS selectors will fail. Both Firefox and Chrome seem to swallow any errors but cheerio will raise a SyntaxError and not just that but the problem is that the CSS selector is just the wrong one to look for.

The Solution

The solution has to be to split by the : character when it's not between two quotation marks.

This Stackoverflow post helped me with the regex. It was trivial to extend now my final solution looks like this:

 * Reduce a CSS selector to be without any pseudo class parts.
 * For example, from 'a:hover' return 'a'. And from 'input::-moz-focus-inner'
 * to 'input'.
 * Also, more advanced ones like 'a[href^="javascript:"]:after' to
 * 'a[href^="javascript:"]'.
 * The last example works too if the input was 'a[href^='javascript:']:after'
 * instead (using ' instead of ").
 * @param {string} selector
 * @return {string}
const reduceCSSSelector = selector => {
  return selector.split(

Extra; About regexes

I've been coding for about 20 years and would like to think I know my way around writing regular expressions in various languages. However, I'm also eager to admit that I often fumble and rely on googling/stackoverflow more than actually understanding what the heck I'm doing. That's why I found this comment so amusing:

Thank you! Didn't think it was possible. I understand 100% of the theory, about 60% of the regex, and I'm down to 0% when it comes to writing it on my own. Oh, well, maybe one of these days. – Azmisov

How's My WiFi?

08 December 2017 2 comments   Node, Javascript, MacOSX

This was one of those late-evening-after-the-kids-are-asleep project. Followed by some next-morning-sober-readme-fixes-and-npmjs-paperwork.

It's a little Node script that will open with puppeteer, and record, using document.querySelector('#speed-value') what my current Internet speed is according to that app. It currently only works on OSX but it should be easy to fix for someone handy on Linux or Windows.

You can either run it just once and get a readout. That's basically as useful as opening in a new browser tab.
The other way is to run it in a loop howsmywifi --loop and sit and watch as it tries to figure out what your Internet speed is after multiple measurements.


That's it!

The whole point of this was for me to get an understanding of what my Internet speed is and if I'm being screwed by Comcast. The measurements are very erratic and they might sporadically depend on channel noise on the WiFi or just packet crowding when other devices is overcrowding the pipes with heavy downloads such as video chatting or watching movies or whatever.

I've seen 98 Mbps with my iPhone on this network. Not so much today.

And Screenshots!

As a bonus, it will take a screenshot (if you pass the --screenshots flag) of the page each time it has successfully measured. Not sure what to do with this. If you have ideas, let me know.

How to create-react-app with Docker

17 November 2017 10 comments   Docker, ReactJS, Javascript, Web development, Linux

Why would you want to use Docker to do React app work? Isn't Docker for server-side stuff like Python and Golang etc? No, all the benefits of Docker apply to JavaScript client-side work too.

So there are three main things you want to do with create-react-app; dev server, running tests and creating build artifacts. Let's look at all three but using Docker.

Create-react-app first

If you haven't already, install create-react-app globally:

▶ yarn global add create-react-app

And, once installed, create a new project:

▶ create-react-app docker-create-react-app
...lots of output...

▶ cd docker-create-react-app
▶ ls    node_modules package.json public       src          yarn.lock

We won't need the node_modules here in the project directory. Instead, when building the image we're going let node_modules stay inside the image. So you can go ahead and... rm -fr node_modules.

Create the Dockerfile

Let's just dive in. This Dockerfile is the minimum:

FROM node:8

ADD yarn.lock /yarn.lock
ADD package.json /package.json

ENV NODE_PATH=/node_modules
ENV PATH=$PATH:/node_modules/.bin
RUN yarn

ADD . /app

EXPOSE 35729

ENTRYPOINT ["/bin/bash", "/app/"]
CMD ["start"]

A couple of things to notice here.
First of all we're basing this on the official Node v8 repository on Docker Hub. That gives you a Node and Yarn by default.

Note how the NODE_PATH environment variable puts the node_modules in the root of the container. That's so that it doesn't get added in "here" (i.e. the current working directory). If you didn't do this, the node_modules directory would be part of the mounted volume which not only slows down Docker (since there are so many files) it also isn't necessary to see those files.

Note how the ENTRYPOINT points to That's a file we need to create too, alongside the Dockerfile file.

#!/usr/bin/env bash
set -eo pipefail

case $1 in
    # The '| cat' is to trick Node that this is an non-TTY terminal
    # then react-scripts won't clear the console.
    yarn start | cat
    yarn build
    yarn test $@
    exec "$@"

Lastly, as a point of convenience, note that the default CMD is "start". That's so that when you simply run the container the default thing it does is to run yarn start.

Build container

Now let's build it:

▶ docker image build -t react:app .

The -t react:app is up to you. It doesn't matter so much what it is unless you're going to upload your container the a registry. Then you probably want the repository to be something unique.

Let's check that the build is there:

▶ docker image ls react:app
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
react               app                 3ee5c7596f57        13 minutes ago      996MB

996MB! The base Node image is about ~700MB and the node_modules directory (for a clean new create-react-app) is ~160MB (at the time of writing). What the remaining difference is, I'm not sure. But it's empty calories and easy to lose. When you blow away the built image (docker image rmi react:app) your hard drive gets all that back and no actual code is lost.

Before we run it, lets go inside and see what was created:

▶ docker container run -it react:app bash
root@996e708a30c4:/app# ls
Dockerfile  package.json  public  src  yarn.lock
root@996e708a30c4:/app# du -sh /node_modules/
148M    /node_modules/
root@996e708a30c4:/app# sw-precache
Total precache size is about 355 kB for 14 resources.
service-worker.js has been generated with the service worker contents.

The last command (sw-precache) was just to show that executables in /node_modules/.bin are indeed on the $PATH and can be run.

Run container

Now to run it:

▶ docker container run -it -p 3000:3000 react:app
yarn run v1.3.2
$ react-scripts start
Starting the development server...

Compiled successfully!

You can now view docker-create-react-app in the browser.

  Local:            http://localhost:3000/
  On Your Network:

Note that the development build is not optimized.
To create a production build, use yarn build.

Default app running

Pretty good. Open http://localhost:3000 in your browser and you should see the default create-react-app app.

Next step; Warm reloading

create-react-app does not support hot reloading of components. But it does support web page reloading. As soon as a local file is changed, it sends a signal to the browser (using WebSockets) to tell it to... document.location.reload().

To make this work, we need to do two things:
1) Mount the current working directory into the Docker container
2) Expose the WebSocket port

The WebSocket thing is set up by exposing port 35729 to the host (-p 35729:35729).

Below is an example running this with a volume mount and both ports exposed.

▶ docker container run -it -p 3000:3000 -p 35729:35729 -v $(pwd):/app react:app
yarn run v1.3.2
$ react-scripts start
Starting the development server...

Compiled successfully!

You can now view docker-create-react-app in the browser.

  Local:            http://localhost:3000/
  On Your Network:

Note that the development build is not optimized.
To create a production build, use yarn build.

Compiled successfully!
Compiled with warnings.

  Line 7:  'neverused' is assigned a value but never used  no-unused-vars

Search for the keywords to learn more about each warning.
To ignore, add // eslint-disable-next-line to the line before.

Failed to compile.

Module not found: Can't resolve './Apps.css' in '/app/src'

In the about example output. First I make a harmless save in the src/App.js file just to see that the dev server notices and that my browser reloads when I did that. That's where it says

Compiled successfully!

Secondly, I make an edit that triggers a warning. That's where it says:

Compiled with warnings.

  Line 7:  'neverused' is assigned a value but never used  no-unused-vars

Search for the keywords to learn more about each warning.
To ignore, add // eslint-disable-next-line to the line before.

And lastly I make an edit by messing with the import line

Failed to compile.

Module not found: Can't resolve './Apps.css' in '/app/src'

This is great! Isn't create-react-app wonderful?

Build build :)

There are many things you can do with the code you're building. Let's pretend that the intention is to build a single-page-app and then take the static assets (including the index.html) and upload them to a public CDN or something. To do that we need to generate the build directory.

The trick here is to run this with a volume mount so that when it creates /app/build (from the perspective) of the container, that directory effectively becomes visible in the host.

▶ docker container run -it -v $(pwd):/app react:app build
yarn run v1.3.2
$ react-scripts build
Creating an optimized production build...
Compiled successfully.

File sizes after gzip:

  35.59 KB  build/static/js/main.591fd843.js
  299 B     build/static/css/main.c17080f1.css

The project was built assuming it is hosted at the server root.
To override this, specify the homepage in your package.json.
For example, add this to build it for GitHub Pages:

  "homepage" : "",

The build folder is ready to be deployed.
You may serve it with a static server:

  yarn global add serve
  serve -s build

Done in 5.95s.

Now, on the host:

▶ tree build
├── asset-manifest.json
├── favicon.ico
├── index.html
├── manifest.json
├── service-worker.js
└── static
    ├── css
    │   ├── main.c17080f1.css
    │   └──
    ├── js
    │   ├── main.591fd843.js
    │   └──
    └── media
        └── logo.5d5d9eef.svg

4 directories, 10 files

The contents of that file you can now upload to a CDN some public Nginx server that points to this as the root directory.

Running tests

This one is so easy and obvious now.

▶ docker container run -it -v $(pwd):/app react:app test

Note the that we're setting up a volume mount here again. Since the test runner is interactive it sits and waits for file changes and re-runs tests immediately, it's important to do the mount now.

All regular jest options work too. For example:

▶ docker container run -it -v $(pwd):/app react:app test --coverage
▶ docker container run -it -v $(pwd):/app react:app test --help

Debugging the node_modules

First of all, when I say "debugging the node_modules", in this context, I'm referring to messing with node_modules whilst running tests or running the dev server.

One way to debug the node_modules used is to enter a bash shell and literally mess with the files inside it. First, start the dev server (or start the test runner) and give the container a name:

▶ docker container run -it -p 3000:3000 -p 35729:35729 -v $(pwd):/app --name mydebugging react:app

Now, in a separate terminal start bash in the container:

▶ docker exec -it mydebugging bash

Once you're in you can install an editor and start editing files:

root@2bf8c877f788:/app# apt-get update && apt-get install jed
root@2bf8c877f788:/app# jed /node_modules/react/index.js

As soon as you make changes to any of the files, the dev server should notice and reload.

When you stop the container all your changes will be reset. So if you had to sprinkle the node_modules with console.log('WHAT THE HECK!') all of those disappear when the container is stopped.

NodeJS shell

This'll come as no surprise by now. You basically run bash and you're there:

▶ docker container run -it -v $(pwd):/app react:app bash
root@2a21e8206a1f:/app# node
> [] + 1


When I look back at all the commands above, I can definitely see how it's pretty intimidating and daunting. So many things to remember and it's got that nasty feeling where you feel like your controlling your development environment through unwieldy levers rather than your own hands.

But think of the fundamental advantages too! It's all encapsulated now. What you're working on will be based on the exact same version of everything as your teammate, your dev server and your production server are using.



In my (Mozilla Services) work, the projects I work on, I actually use docker-compose for all things. And I have a Makefile to help me remember all the various docker-compose commands (thanks Jannis & Will!). One definitely neat thing you can do with docker-compose is start multiple containers. Then you can, with one command, start a Django server and the create-react-app dev server with one command. Perhaps a blog post for another day.

Ultrafast loading of CSS

01 September 2017 0 comments   Javascript, Web development

tl;dr; The ideal web performance, with regards to CSS, is to inline the minimal CSS and lazy load the rest after load.

Two key things to understand/appreciate:

  1. The fastest performing web page is one that isn't blocked on rendering.

  2. You use some CSS framework kitchen sink because you're not a CSS guru.

How to deal with this?

Things like HTTP2 and CDNs and preload are nice because they make the network lookup for your main.88c468ef.css file as fast as possible. But what's even faster is to include the CSS with the HTML that the server responds in the first place. Why? Because when the browser downloads your HTML (e.g. GET /) as it parses the HTML document it sees that <link rel="stylesheet" href="/main.88c468ef.css"> there and decides to not render any DOM to screen until that CSS file has been downloaded and parsed. It does this because it doesn't want to have to paint the DOM (as it would look like without CSS) and then repaint the DOM again, this time with CSS rules.

Point number 2 basically boils down to the likely fact that your app depends on somecssframework.min.css like Bootstrap, Bulma or Foundation. They're large blobs of CSS for doing all sorts of types of HTML (e.g. cards, tables, navbar menus etc.). These CSS frameworks are super useful because they make your app look pretty. But they're usually big. Really big.

Popular CSS frameworks:

Framework Size Gzipped
bootstrap.min.css 122K 18K
foundation.min.css 115K 16K
semantic.min.css 553K 93K
bulma.min.css 141K 18K

Actually the size difference isn't hugely important. What's important is that it's yet another thing that needs to be downloaded before the page can start to render. If the URL is in the user's cache, great. Even better, if it's cached by a service worker. However if you care about loading performance (judging by the fact that you're still reading), you know that a large majority of your visitors only come to your site sometimes (according to Google Analytics, 92.7% of my visitors are "new visitors"). Perhaps from a Google search. Or perhaps they visit sometimes but rarely enough that by the time they return their browser cache will have "moved on" and reset (to save disk space) what was previously cached.

CSS is a render blocking resource

With and without render blocking CSS
See Ilya Grigorik's primer on Render Blocking CSS.

It's also easy to demonstrate. Check out this Webpagetest Visual Comparison that compares two pages that are both styled with bootstrap.min.css except one of them uses a piece of JavaScript at the bottom of the page that enables the stylesheet after the page has loaded.

So if it's blocking. What to do about it? Well, make it not blocking. But how?

Solution 1

The simplest solution is to simply move any <link rel="stylesheet" href="bootstrap.min.css"> out of the <head> and put them just before the </body> tag. Here's an example of that.

It's valid HTML5 and seems to work just fine in Safari iOS. The only problem is that pesky "Flash of Unstyled Content" (aka. "FOUT") effect where the user is presented with the page very briefly without any styling, then the whole page re-renders onces the stylesheets have loaded. Chrome and iOS actually block the rendering still. So it's not like JavaScript whereby putting it late in the DOM. In other words, not really a good solution at all.

You can see in this Webpagetest that the "Start render" happens after the .css files have been loaded and parsed.

Solution 2

With JavaScript you can put in code that's definitely going to be executed after the rendering starts and, also, after the first rendering is finished (i.e. "DOM Content Loaded").

This technique is best done with loadCSS which can be done really well if you tune it. In particular the rel="preload" feature is getting more and more established. It used to only work in Chrome and Opera but will soon work in Firefox and iOS Safari too. Note, loadCSS contains a polyfill solution to the rel="preload" thing.

The basics is that you load a piece of JavaScript late which, as soon as it can, puts the <link rel="stylesheet" href="bootstrap.min.css"> into the DOM. You still have the Flash of Unstyled Content effect to confront and that's annoying.

Here's an example implementation. It uses the scripts and techniques laid out by filamentgroup's loadCSS.

It works and the rel="preload" is a bonus for Chrome and Opera users because once the JavaScript "kicks in" the network loading is quite possible already done. As seen in this Webpagetest using Chrome the .css files start downloading before the lazyloadcss.js file has even started downloading.

It's not as hot in Firefox because all the .css files downloading is delayed until after the lazyloadcss.js has loaded and executed.

Solution 3

Just inline all the CSS. Instead of <link rel="stylesheet" href="bootstrap.min.css"> you just make it inline. Like:

<style type="text/css">
 * Bootstrap v4.0.0-beta (
 * Copyright 2011-2017 The Bootstrap Authors
 * Copyright 2011-2017 Twitter, Inc.
 * Licensed under MIT (
 */@media print{*,::after,::before{text-shadow:none!important;box-shadow:none!important}......

All 123KB of it. Why not?! It has to be downloaded sooner or later anyway, might as well nip it in the bud straight away. The Flash of Unstyled Content problem goes away. So does the problem of having to load JavaScript tricks to make the CSS loading non-blocking.

The obvious and immediate caveat is that now the whole HTML document is huge! In this example page the whole HTML document is 127KB (20KB gzipped) whence the regular one is 4.1KB (1.4KB gzipped). And if your visitors, if you're so lucky, click on any other internal link that's another 127KB that has to be downloaded again.

The biggest caveat is that downloading a large HTML document is bad because no other resources (images for example) can be downloaded in parallel whilst the browser is working on rendering the page with what it's downloaded so far. If you compare this Webpagetest with the regular traditional one, you can see that it takes almost 354ms to download the HTML with all CSS inlined compared to 262ms when the CSS was linked. That's roughly 100ms wasted where the browser could start download other resources, like images.

Solution 4

Solution 3 was kinda good because it avoided the Flash of Unstyled Content and it avoided all extra resource loading. However, we can do better.

Instead of inlining all CSS, how about we take out exactly only the CSS we need out of bootstrap.min.css and just inline that. Then, after the page has loaded, we can download the rest of bootstrap.min.css and that way it's ready with all the other selectors and stuff it needs as the page probably changes and morphs depending on interactive JavaScript which is stuff that can and will happen after the initial load.

But how do you know exactly which CSS you need for that initial load? Really, you don't. You have two options:

  1. Manually inspected what DOM elements you have in your initial HTML and start slowly plucking that out of the Bootstrap CSS file.

  2. Automate the inspection of what DOM elements you have in your initial HTML.

Before we dig deeper into the how to automate the inspection let's look at what it'd look like: This page and when Webpagetested. What's cool here is that the DOM is ready in 265ms (it was 262ms when there was no linked CSS).

Notice that there's no Flash of Unstyled Content. No external dependencies. It's basically an inline <style> block with exactly the selectors that are needed and nothing more. The HTML is larger, at 13KB (3.3KB gzipped), but remember it was 4.1KB when we started and the solution where we inlined everything was 127KB.

The immediate problem with this is that we're missing some nice CSS for things that haven't been needed yet. For example, there might be some JavaScript that changes the DOM based on something the user does with the page. For example, clicking on something that adds more elements to the DOM. Or, equally likely, after the the DOM has loaded, an XHR query is made to download some data and display it in a way that needs CSS selectors that weren't included in the minimal set.

By the way, this very blog post builds on this solution. If you're on your desktop browser you can view source and see that there's only inline style blocks.

Solution 5

This builds on Solution 4. The HTML contains the minimal CSS needed for that first render and as soon as possible we additionally download the whole bootstrap.min.css so that it's available if/when the DOM mutates and needs the full CSS not in the minimal CSS.

Basically, let's take Solution 2 (JavaScript lazy loads in the CSS) + Solution 4 (the minimal CSS inlined). Here is one such solution

And there we have it! The ideal solution. The only thing remaining is to verify that it actually makes a difference.

The Webpagetest Final Showdown

We have 5 solutions. Each one different from the next. Let's compare them against each other.

Here it is in its full glory

Visual comparison on
(image if you can't open the Webpagetest page right now)

What we notice:

  1. The regular do-nothing solution is 50% slower than the best solution. 3.2 seconds verus 2.2 sceonds.
  2. Putting the <link rel="stylesheet" ...> tags at the bottom of the document doesn't work in Chrome and doesn't do anything good.
  3. Lazy loading the CSS with JavaScript (with no initial CSS) displays content very early but the repaint means it takes unnecessarily longer to load the whole thing.
  4. The ideal solution (Solution 5) loads as fast, visually, as Solution 4 but has the advantage that all CSS is there, eventually.
  5. Inlining all CSS (Solution 3) is only 23% slower than the ideal solution (Solution 5). But, it's much easier to implement. Seriously consider this if your tooling is limited.


One humbling thing to notice is that the difference isn't actually that huge. In this particular example we managed to go from 3.2 seconds to 2.2 seconds (using a 3G connection). The example playground used in this experiment is very far from a real site. Most possibly, a real site is a lot more complex and full of lots more potential bottlenecks that slows things down. For example, instead of obsessing over the CSS payload, perhaps you can make a bigger impact by simply dropping some excessive JavaScript plugins that might not necessarily be needed. Or you can focus on your 2.5MB total of big images.

However, a key ingredient to web performance is to leverge the loading time the best possible way. If you get the CSS un-blocking rendering right, your users' browsers can spend more time, sooner, on other resources such as images and XHR.

React lifecycle hooks must-have

13 August 2017 0 comments   ReactJS, Javascript, Web development

I don't know who made this flowchart originally, but whoever you are: Thank you!

At this point, in my React learning I think I've memorized much of this but it's taken me a lot of time and having to dig up the documentation again. (Also, not to mention the number of times I've typo'ed componentWillReciveProps and componentWillRecevieProps etc.)

Remember this; You don't need to know all of these by heart to be good at React. In fact, there's several of these that I almost never use.

React lifecycle hooks flowchart


The above link is dead. Use this blog post instead.

Web Console trick to get all URLs into your clipboard

27 April 2017 0 comments   Javascript, Web development

This isn't rocket science in the world of Web Development but I think it's so darn useful that if you've been unlucky to miss this, it's worth mentioning one more time.

Suppose you're on a site with lots of links. Like
And you want to get a list of all URLs that contain word "react" in the URL pathname.
And you want to get this into your clipboard.

Here's what you do:

  1. Open your browsers Web Console. In Firefox it's Alt+Cmd+K on OSX (or F12). In Chrome it's Alt+Cmd+J.

  2. Type in the magic: copy([...document.querySelectorAll('a')].map(a => a.href).filter(u => u.match(/react/i)))

  3. Hit Enter, go to a text editor and paste like regular.

It should look something like this:


Web Console in Firefox
The example is just that. An example. The cool thing about this is:

The limit is your imagination. You can also do things like copy([...document.querySelectorAll('a')].filter(a => a.textContent.match(/react/i)).map(a => a.href)) and you filter by the links' text.

Public Class Fields saves sooo many keystrokes in React code

14 April 2017 0 comments   ReactJS, Javascript

tl;dr; I'm not a TC39 member and I barely understand half of what those heros are working on but there is one feature they're working on that really stands out, in my view, for React coders; Public Class Fields.

The Problem?

Very common pattern in React code is that have a component that has methods that are tied to DOM events (e.g. onClick) and often these methods need acess to this. The component's this. So you can reach things like this.state or this.setState().

You might have this in your code:

class App extends Component {
  state = {counter: 0}

  constructor() {

    // Like homework or situps; something you have to do :(
    this.incrementCounter = this.incrementCounter.bind(this) 

  incrementCounter() {
    this.setState(ps => {
      return {counter: ps.counter + 1}

  render() {
    return (
          <button onClick={this.incrementCounter}>Increment</button>


If you don't bind the class method to this in the constructor, this will be undefined inside the class instance field incrementCounter. Buu!

Suppose you don't like having the word incrementCounter written in 4 placecs, you might opt for this shorthand notation where you create a new unnamed function inside the render function:

class App extends Component {
  state = {counter: 0}

  render() {
    return (
          <button onClick={() => {
            this.setState(ps => {
              return {counter: ps.counter + 1}


Sooo much shorter and kinda nice that the code can be so close in proximity to the actual onClick event definition.

But this notation has a horrible side-effect. It creates a new function on every render. If instead of a regular DOM jsx object <button> it might be a sub-component like <CoolButton/> then that sub-component would be forced to re-render every time (unless you write your own shouldComponentUpdate.

Also, this notation works when the code is small and light but it might get messy quickly if you need that functionality on other element's onClick. Or it might become mess with really deep indentation.

The Solution?

Public Class Fields.

That new feature is currently in the "Draft" stage at TC39. Aka. stage 1.

However, I discovered that you can use stage-2 in Babel to use this particular feature.

Note! I don't understand why you only have to put on your stage-2-brave socks for this feature when it's part of a definition that is stage 1.

Anyway, what it means is that you can define your field (aka method) like this instead:

class App extends Component {
  state = {counter: 0}

  incrementCounter = () => {
    this.setState(ps => {
      return {counter: ps.counter + 1}

  render() {
    return (
          <button onClick={this.incrementCounter}>Increment</button>


Now it's only mentioned by name incrementCounter twice. And no need for that manual binding in a constructor.
And since it's automatically bound, the function isn't recreated and those can easily make sub-components be pure.

So, let's always write our React methods this way from now on.

Oh, and in case you wonder. Inheritence works the same with these public class fields as the regular class instance fields.