BigDecimal arithmetic in Ruby

Ruby comes with a class BigDecimal which you can use for arbitrary precision arithmetic. You should use BigDecimal instead of Float whenever you care about rounding errors, e.g. whenever you are dealing with money.

You should remember these two rules when working with BigDecimal values:

  • When you add or multiply a BigDecimal with another BigDecimal, the …

JavaScript: Testing the type of a value

Checking if a JavaScript value is of a given type can be very confusing:

  • There are two operators typeof and instanceof which work very differently.
  • JavaScript has some primitive types, like string literals, that are not objects (as opposed to Ruby, where every value is an object).
  • Some values are sometimes are primitive value (e.g. "foo") and sometimes an object (new String("foo")) and each form requires different checks
  • There are three different types for null (null, undefined and NaN) and each has different rules f…

Capybara: Easiest way to get the parent of an element

If you already selected an element and want to get its parent, you can call find(:xpath, '..') on it.
To get the parents parent, call find(:xpath, '../..').


Find a link which contains a twitter icon and check that it links to the correct page:

<a href="">
  <i class="icon is-twitter"></i>
link = page.find("a").find(:xpath, '..')
link[:href].should == ""

There is a good overview on xpath syntax on [w3schools](…


Running Rspec examples by name, or: Running a single shared example

When an Rspec example fails, I usually investigate by running that example again using rspec <file:line>. However, this does not work with shared examples, since Rspec doesn't know in which context the shared example should be run.

But there is a different way: You can run the shared example using the -e, --example option. It takes a string value and runs all scenarios containing that substring in their full description.

This allows you to run a single uniquely named example, all examples with
similar names, all the examples in a u…


How to: Use git bisect to find bugs and regressions

Git allows you to do a binary search across commits to hunt down the commit that introduced a bug.

Given you are currently on your branch's HEAD that is not working as expected, an example workflow could be:

git bisect start # Start bisecting
git bisect bad # Tag the revision you are currently on (HEAD) as bad. You could also pass a commit's SHA1 like below:
git bisect good abcdef12345678 # Give the SHA1 of any commit that was working as it should
# shorthand:
git bisect start HEAD abcdef12345678

Git will fetch a commi…


Be careful to use correct HTTP status codes for maintenance pages

When your public-facing application has a longer downtime for server maintenance or long migrations, it's nice to setup a maintenance page to inform your users.

When delivering the maintenance page, be very careful to send the correct HTTP status code. Sending the wrong status code might get you kicked out of Google, or undo years of SEO work.

Here are some ways to shoot yourself in the foot durign maintenance:

  • If all your routes send a "200 OK" with a HTML body "We're b…

Selenium: How to close another tab (popup)

If you open a pop-up window [1] in your Selenium tests and you want to close it, you can do this:

# Find our target window
handle = page.driver.find_window("My window title")

# Close it

# Have the Selenium driver point to another window
last_handle = page.driver.browser.window_handles.last

Mind these:

  • find_window returns a window handle, which is something like `"{485fa8bd-fa99-…

Git: How to configure git to push only your current branch

You can change which branches will be pushed when saying git push. Our recommendation is to set it to current.

From the git-config documentation:


Defines the action git push should take if no refspec is given on the command line, no refspec is configured in the remote, and no refspec is implied by any of the options given on the command line. Possible values are:

  • nothing - do not push anything.
  • matching - push all matching branches. All bra…

Fix Slack call overlay on Awesome WM

If you use awesome and make a Slack call, you'll constantly have an overlay window pop up taking over half of your screen.

To fix this, add the following rule to your awful.rules.rules section in your rc.lua:

awful.rules.rules = {
  -- ...
  { rule = { name = "Slack Call Minipanel" }, properties = { floating = true, ontop = true } },
Auto-destruct in 37 days

Updated: Clicking issues with chromedriver

Another issue appeared as consequence of the chrome driver way of clicking.
Updated the card:

  • Another element which is now located at the position of the desired element receives the click.

Capybara steps to match stuff within any selector

These steps are now part of Spreewald.

Since Capybara 0.4.1 a within scope will only look at the first element that matches. We find this behavior to be impractical, but it is by design.

In order to perform a test or action in all matching elements, do not use within but prefer the attached "inside any" Cucumber steps like these:

When I follow "Foo" inside any "table"
Then I should see "Bar" inside any "li"

MySQL will not use indexes if you query the wrong data type

When MySQL refuses to use your index, you can do a number of things wrong. One of them might be conditions with improper data types.

An example

For example, let's assume you have a users table with an email field (varchar) which is indexed.

MySQL will use the index when your query is well-formed:

mysql> EXPLAIN SELECT * FROM users WHERE email = '';
| id | select_type | table | type  | poss...

Ruby: How to make an object that works with multiple assignment

Ruby allows multiple assignment:

a, b, c = o

In order to prove multiple values from a single object, Ruby calls #to_a on the object:

o ='O')
def o.to_a

a, b, c = o # Implicit call to #to_a here
a # => 1
b # => 2
c # => 3

Hence, if you want your class to support multiple assignment, make sure to define a #to_a method.


Async control flow in JavaScript: Promises, Microtasks, async/await

Slides for Henning's talk on Sep 21st 2017.

Understanding sync vs. async control flow

Talking to synchronous (or "blocking") API

print('script start')
html = http_get('/foo')
print('script end')

Script outputs 'script start', (long delay), '<html>...</html>', 'script end'.

Talking to asynchronous (or "evented") API

print('script start')
http_get('foo', done: function(html) {
print('script end')

Script outputs `'script st…

Spreewald: Content-Disposition not set when testing a download's filename


  • You are not using javascript tests
  • The file is served from a public folder (not via controller)

Problem description

If you deliver files from a public folder it might be that the Content-Disposition header is not set. That's why the following spreewald step might raise an error:

Then I should get a download with filename "..."
expected: /filename="some.pdf"$/
     got: nil (using =~) (RSpec::Expectations::ExpectationNotMetError)


One solution…


Fixing flaky integration tests

This card shows basic techniques for fixing a flaky integration test suite that sometimes passes and sometimes fails. "Integration test" is a test script that remote-controls a web browser with tools like Selenium WebDriver.

Although the examples in this card use Cucumber and Selenium, the techniques are applicable to all languages and testing tools.

Why tests are flaky

Your tests probably look like this:

When I click on A
When I click on B
When I click on C
Then I should see effects of C

A test like this works fine most of t…


Be very careful with 301 redirects

Browsers support two kinds of redirects:

  • 301 moved permanently
  • 302 moved temporarily

Be very careful with the 301 type. Most browsers seem to cache these redirects forever, unless you set different Cache-Control headers. If you don't have any cache control headers, you can never change them without forcing users to empty their cache.

The only fix is to keep redirecting the user to the correct page, so if you had

/page-1 --301--> /page-2

but you want

/page-1 --301-> /page-3

your only fix is to change it to

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