3904 cards

What collapsing margins are, how they work and when margins do not collapse

Imagine you have 2 HTML boxes. The first one has a margin-bottom of let's say 30px and the second one a margin-top of 20px. After rules of collapsing margins have been applied we have a margin of 30px (not 50px) between these two boxes . This is because no addition of both margins takes place but the maximum of both is applied. This behavior is called collapsing margins.

Oftentimes it is a good behavior but collapsing margins can be annoying, too. For example child el...


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...

Rails: How to use custom flash types in controllers

Rails supports alert and notice as default flash types. This allows you to use these keys as options in e.g. redirect_to and as a helper in views e.g. <%= notice %> (instead of flash[:notice]).

class SomeController < ApplicationRecord
  def create
    @user = User.create!
    redirect_to user_path(@user), notice: "#{@user} created!" 

In case you are using Bootstrap as CSS framework you might also want to support flashes like success. This can be done with the add_flash_types method.

class Applicat...

Git: Parsing large diffs as a human

I just finished migrating a project from the Asset Pipeline to Webpacker, this is what my diff to master looks like:

5.825 files changed, 44.805 insertions(+), 529.948 deletions(-)
warning: inexact rename detection was skipped due to too many files.
warning: you may want to set your diff.renameLimit variable to at least 5134 and retry the command.

There is no way me or my peer reviewer is able to parse 500k+ lines of code. Fortunately, git has ...


We now have our own memoization gem "Memoized"

We forked trusty memoizer to make two changes:

  1. Memoized methods now preserve their arity. Previously all memoized methods had an arity of -1.
  2. Memoized methods are now faster at runtime. This will only be noticable if you call a memoized methods many times in the same request.

We published our fork as a new gem named memoized.

memoized is API-compatible to memoizer, you just need to include Memoized instead of `M...


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 very fast. The trade-off is that your listener needs to check every event of t...


Auto-generating plain-text bodies for HTML e-mails in Rails apps

When building an application that sends e-mails to users, you want to avoid those e-mails from being classified as spam. Most obvious scoring issues will not be relevant to you because you are not a spammer.

However, your application must do one thing by itself: When sending HTML e-mails, you should include a plain-text body or tools like SpamAssassin will apply a significant score penalty. Here is how to do that automatically.

  1. Add premailer-rails to your Gemfile and bundle.
  2. Done! ...

Ruby: How to load a file with a known encoding

In case Ruby does not detected the expected encoding of a file automatically you can specify the known encoding manually.

Example with File.open

file = File.open('some.bin', encoding: Encoding::ASCII_8BIT)
text = file.read
text.encoding => #<Encoding:ASCII-8BIT>

Example with File.read

text = File.read('some.bin', encoding: Encoding::ASCII_8BIT)
text.encoding => #<Encoding:ASCII-8BIT>

More details about the encoding of strings in Ruby can be found [here](https://makandracards.com/makandra/474671-guide-to-string-encodi...

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Fixing memory leaks in web applications

In my experience, the most common sources of memory leaks are APIs like these:

  • addEventListener. This is the most common one. Call removeEventListener to clean it up.
  • setTimeout / setInterval. If you create a recurring timer (e.g. to run every 30 seconds), then you need to clean it up with clearTimeout or clearInterval. (setTimeout can leak if it’s used like setInterval – i.e., scheduling a new setTimeout inside of the setTimeout callback.)
  • IntersectionObserver, ResizeObserver, MutationObserver, etc. These new-ish APIs are very...

How to communicate between processes in Ruby with sockets

In Ruby you can communicate between processes with sockets. This might be helpful in tests that validate parallel executions or custom finalization logic after the garbage collector. Here is an example how such an communication will look like:

require 'socket'

# DGRAM has the advantage that it stops reading the pipe if the next messages starts. In case the message size is larger than the
# BUFFER_SIZE, you need to handle if you are reading another part of the current message or if you already reading the
# next mess...

Using the Truemail gem to validate e-mail addresses

The Truemail gem (not to be confused with truemail.io) allows validating email addresses, e.g. when users enter them into a sign-up form. It runs inside your application and does not depend on an external SaaS service.

Truemail supports different validation "layers":

  1. Regex validation: if the given address is syntactically valid
  2. DNS validation (called MX validation): if the given domain exists and can receive email
  3. SMTP validation: connects to the host received from DNS and starts a test d...

ExceptionNotification: Fix DNS lookup before plugins call external APIs

The ExceptionNotification has plugins that talk to external APIs rather then just sends emails, like microsoft teams or slack. You might encounter that no exceptions are delivered, which can be dangerous.

To prevent getting into trouble, simply add require 'resolv-replace' on top of your to your initializers/exception_notification.rb file:

require 'resolv-replace'
require 'exception_notification/rails'

ExceptionNotification.configure do |config|

This calls the Ruby DNS resolver before configuring `ExceptionNot...

Call original method when monkey patching

Ruby offers monkey patching methods in order to change the behavior of a library if there's no better way.

We can call the method we're overriding inside our monkey patch:

class Foo
  def bar(argument)
    'Hello' + argument

module FooExtensions
  def bar
    super(' in my') + ' World'

class Foo
  prepend FooExtensions # the only change to above: prepend instead of include

Foo.new.bar # => 'Hello in my...
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About Ruby's conversion method pairs

Ruby has a set of methods to convert an object to another representation. Most of them come in explicit and implicit flavor.

explicit implicit
to_a to_ary
to_h to_hash
to_s to_str
to_i to_int

There may be even more.

Don't name your methods like the implicit version (most prominently to_hash) but the like the explicit one.

Explicit conversion

Explicit conversion happens when requesting it, e.g. with the splat opera...


How to examine an unknown Ruby object

When debugging your application, you will come across objects created by some gem or framework. You don't have the source code at hand, still need to inspect this object. Here are some tools to do so:

Relevant methods

@object.methods - Object.instance_methods returns a list of methods excluding methods inherited from Object. This makes the methods list drastically more relevant. You can also try subtracting other base classes like ActiveRecord::Base.methods etc.
To further narrow it down you can also just look at public methods...

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Canceling promises

The cancelable promises proposal was withdrawn some time ago.

The new standard way is that your long-running function take a AbortSignal { signal } property. The caller can use this signal to send an abort request to your function. Upon receiving the request, your function should reject its promise with an error.

Async browser functions like fetch() reject their promises with a `new DOMException('Message here', 'Abo...

Transporting blank values in URL queries

URLs can transport key/value pairs ("parameters") using this syntax:


If the value is blank, mind these subtle differences:

URL Meaning
/path?foo= Parameters have a key foo. Its value is an empty string.
/path?foo Parameters have a key foo. Its value is null.
/path Parameters have no key foo.

Git diff: Deemphasizing code that was only moved around

In long diffs, it can become impossible to spot small changes in larger blocks of moved code. This may be either a method that was moved from the top to the bottom of a file, or a long test file that was split in many.

Fortunately, Git offers a special highlighting mode that directs the reader's attention to relevant code parts:

git diff --color-moved=dimmed-zebra

It will dim lines that were moved around without changes, and highlight changed lines. See this SO answer for an illustra...

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