HOWTOs for Ruby on Rails and JavaScript

Efficiently add an event listener to many elements

When you need to add a event listener to hundreds of elements, this might slow down the browser.

An alternative is to register an event listener at the root of the DOM tree (document). Then wait for events to bubble up and check whether the triggering element (event.target) matches the selector before you run your callback.

This technique is called event delegation.

Performance considerations

Because you only register a single listener, registering is ...

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Capybara: Find the innermost DOM element that contains a given string

Let's say you want to find the element with the text hello in the following DOM tree:

<html>
  <body>
    <article>
      <strong>hello</strong>
      <strong>world</strong>
    </article>
  </body>
</html>

You might think of XPath's contain() function:

page.find(:xpath, ".//*[contains(text(), 'hello')")

Unfortunately that returns a lot more elements than you expect:

[ <html>...<html>,
  <body>...</body>,
  <article>...</article>,
  <strong>hello</strong> ]

What you need to do instead is to *find all...

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Guide to localizing a Rails application

Localizing a non-trivial application can be a huge undertaking. This card will give you an overview over the many components that are affected.

When you are asked to give an estimate for the effort involved, go through the list below and check which points are covered by your requirements. Work with a developer who has done a full-app localization before and assign an hour estimate to each of these points.

Static text

Static strings and template text in app must be translated: Screens, mailer templates, PDF templates, helpe...

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Managing vendor libraries with the Rails asset pipeline

The benefit of the Rails asset pipeline is that it compiles your stylesheets and javascripts to a single file, respectively. However, the consequences are startling if you don't understand them. Among others, the raw asset pipeline requires you to have all your asset libraries in the same folder, which quickly becomes confusing as your set of assets grows. To overcome this, we have two different solutions.

Custom solution

We are using a custom workaround to keep library files apart in their own directories. To avoid b...

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Capybara: Find an element that contains a string

There is no CSS selector for matching elements that contains a given string ¹. Luckily, Capybara offers the :text option to go along with your selector:

page.find('div', text: 'Expected content')

You can also pass a regular expression!

page.find('div', text: /Expected contents?/i)

Note that if your CSS selector is as generic as div, you might get a lot more results than you expect. E.g. a <div class="container"> that surrounds your entire layout will probably also contain that text (in a descendant) and ...

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Detecting when fonts are loaded via JavaScript

Webfonts are not always available when your JavaScript runs on first page load. Since fonts may affect element sizes, you may want to know when fonts have been loaded to trigger some kind of recalculation.

Vanilla JavaScript / Modern DOM API

In modern browsers (all but IE and legacy Edge) you can use document.fonts. Use load to request a font and receive a Promise that will be resolved once the font is available. Example:

document.fonts.load('1rem "Open S...

patrickmarabeas.github.io

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Check whether an element is visible or hidden with Javascript

jQuery

You can say:

$(element).is(':visible')

and

$(element).is(':hidden')

jQuery considers an element to be visible if it consumes space in the document. For most purposes, this is exactly what you want.

Native DOM API

Emulate jQuery's implementation :

element.offsetWidth > 0 && element.offsetHeight > 0;

jQuery > 3

Query 3 slightly modifies the meaning of :visible (and therefore of :hidden).

Emulate jQuery'...

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Lazy-loading images

Note

This card does not reflect the current state of lazy loading technologies. The native lazy attribute could be used, which is supported by all major browsers since 2022.

Since images are magnitudes larger in file size than text (HTML, CSS, Javascript) is, loading the images of a large web page takes a significant amount of the total load time. When your internet connection is good, this is usually not an issue. However, users with limited bandwidth (i.e. on mobile) need to mine their data budget...

github.com

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Playing audio in a browser

If you want to play music or sounds from a browser, your choice is to use either Flash or the new <audio> tag in HTML5. Each method has issues, but depending on your requirements you might not care about all of them.

Flash

Works in all desktop browsers, even Internet Explorer. Does not work on iPads or iPhones.
Requires you to embed a Flash component into your page which will later play the audio for you.
Can play MP3s or Wave files. Cannot play OGG Vorbis audio.
Cannot reliably seek to a given position when playing VBR-enco...

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Unobtrusive JavaScript helper to progressively enhance HTML

The attached compiler() function below applies JavaScript behavior to matching HTML elements as they enter the DOM.

This works like an Unpoly compiler for apps that don't use Unpoly, Custom Elements or any other mechanism that pairs JavaScript with HTML elements.

The compiler() function is also a lightweight replacement for our legacy [$.unobtrusive()](https://makandracards.com/makandra/4-unobtrusiv...

compiler.js

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Some tips for upgrading Bootstrap from 3 to 4

Recently I made an upgrade from Bootstrap 3 to Bootstrap 4 in a bigger project. Here are some tips how to plan and perform such an upgrade. The effort will scale with the size of the project and its structure. If your stylesheets already follow strict rules, it may take less time to adapt them to the new version.

Preparation

There are several gems and libraries that works well with bootstrap or provide at least stylesheets/plugins to easily integrate the bootstrap theme. But very often they only work with specific version or are no long...

getbootstrap.com

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Webpacker: Side effects of using window.* within the ProvidePlugin

Some older Node modules rely on window.jQuery to be present. One suggested solution is to use this config in the app/config/webpack/environment.js:

const { environment } = require('@rails/webpacker')
const webpack = require('webpack')

environment.plugins.prepend(
  'Provide',
  new webpack.ProvidePlugin({
    $: 'jquery',
    jQuery: 'jquery',
    'window.jQuery': 'jquery'
  })
)

module.exports = environment

This will work. But a side effect is that the fo...

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bower-rails can rewrite your relative asset paths

The asset pipeline changes the paths of CSS files during precompilation. This opens a world of pain when CSS files reference images (like jQuery UI) or fonts (like webfont kits from Font Squirrel), since all those url(images/icon.png) will now point to a broken path.

In the past we have been using the vendor/asset-libs folder ...

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An intro to Javascript promises

Promises are the new way™ to express "Do this, and once you're done, do that". In contrast to callbacks, promises are easily chainable. From the readme of Q, an early implementer of the pattern:

The callback approach is called an “inversion of control”. A function that accepts a callback instead of a return value is saying, “Don’t call me, I’ll call you.”. Promises un-invert the inversion, cleanly separating the input arguments from control flow arguments. This simplifies the use and creation of APIs, p...

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Capybara: evaluate_script might freeze your browser

Capybara gives you two different methods for executing Javascript:

page.evaluate_script("$('input').focus()")
page.execute_script("$('input').focus()")

While you can use both, the first line (with evaluate_script) might freeze your browser window for 10 seconds.

The reason is that evaluate_script will always return a result. The return value will be converted back to Ruby objects, which in case of complex objects (e.g. a jQuery collection) is very expensive.

Because of this we recommend to only use evaluate_script whe...

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Manually uploading files via AJAX

To upload a file via AJAX (e.g. from an <input type='file'>) you need to wrap your params in a FormData object.

You can initialize a FormData using the contents of a form:

var form = document.querySelector('form.my-form') // Find the <form> element
var formData = new FormData(form); // Wrap form contents

Or you can construct it manually, param by param:

var fileInput = document.querySelector('form input[type=file]');
var attachment = fileInput.files[0];

var f...

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Vortrag: Content Security Policy: Eine Einführung

Grundidee

CSP hat zum Ziel einen Browser-seitigen Mechanismus zu schaffen um einige Angriffe auf Webseiten zu verhindern, hauptsächlich XSS-Angriffe.

Einschub: Was ist XSS?

XSS = Cross Site Scripting. Passiert wenn ein User ungefiltertes HTML in die Webseite einfügen kann.

<div class="comment">
  Danke für den interessanten Beitrag! <script>alert('you have been hacked')</script>
</div>

Rails löst das Problem weitgehend, aber

Programmierfehler weiter möglich
manchmal Sicherheitslücken in Gems oder Rails

Lösungsid...

csp-error.png

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Testing drag&drop with Selenium

When using jQueryUI's Sortable plugin (either directly or via Angular's ui.sortable), you might struggle testing your nice drag&drop GUI since Selenium webdriver does not support native dragging events.

But jQueryUI uses jquery.simulate for their testing, so why shouldn't you? There is even an extension to it that makes testing drag & drop quite easy.

Here is what you need:

jquery.simulate.js
[`jquery.simula...

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A solid and unobtrusive plugin for form field placeholders

jquery-placeholder is a simple jQuery plugin that enables form placeholders in browsers that do not support them natively, i.e. IE < 10.

Properties

Works in IE6.
Automatically checks whether the browser natively supports the HTML5 placeholder attribute for input and textarea elements. If this is the case, the plugin won’t do anything. If @placeholder is only supported for input elements, the plugin will leave those alone and apply to textareas exclusively. (This is the case for Safari 4, Opera 11.00, and possibly other browsers.)
...

github.com

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Date or datetime picker for touch devices

jQuery UI's date picker and date time picker doesn't work on touch interfaces.

Solution 1: Use Mobiscroll

Another way is to detect touch devices and for those devices use the Date and DateTime picker from Mobiscroll instead:

if (isTouchDevice()) {
  $('.date_picker').scroller();
} else...

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Bug in Chrome 56+ prevents filling in fields with slashes using selenium-webdriver/Capybara

There seems to be a nasty bug in Chrome 56 when testing with Selenium and Capybara: Slashes are not written to input fields with fill_in. A workaround is to use javascript / jquery to change the contents of an input field.

Use the following code or add the attached file to your features/support/-directory to overwrite fill_in.

module ChromedriverWorkarounds

  def fill_in(locator, options = {})
    text = options[:with].to_s
    if Capybara.current_driver == :selenium && text.include?('/')
      # There is a nasty Bug in Chrome ...

chromedriver.rb

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How to quickly inspect an Angular scope in your webkit browser

Current webkit browsers like Chrome and Safari have a special variable in their consoles that refers to the selected DOM node in the elements panel. This lets us easily inspect Angular scopes.

Right click in the page and click "Inspect" to open the Dev Tools
Select the element you're interested in from the elements panel
Focus the console (in Chrome, hit ESC)

Get the scope object and store it

s=$($0).scope()

// That is:
element = $0 // Store element
$element = $(element) // Wrap with j...

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Disable built-in dragging of text and images

Most browsers have built-in drag and drop support for different page elements like text and images. While this may be useful in most situations, it may become annoying in others. If you e.g. want to allow the user to scroll/move horizontally within a container by grabbing an item and moving the mouse, you will notice that nothing will move and you'll instead start dragging that element.

To disable this, add the following CSS to your content:

-webkit-user-drag: none
user-drag: none

-webkit-user-drag is only fully supported in ...

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Flexible overflow handling with CSS and JavaScript

You can use text-overflow to truncate a text using CSS but it does not fit fancy requirements.

Here is a hack for the special case where you want to truncate one of two strings in one line that can both vary in length, while fully keeping one of them. See this example screenshot where we never want to show an ellipsis for the distance:

![Flexible overflow with optional ellipsis](https://makandracards.com/makandra/5885-a-flexible-overflow-ellipsis/at...

flexible-overflow.png

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