Swizec Teller - a geek with a hatswizec.com

    Words that scare developers

    Have you ever had a tweet go so viral it changed your whole view of software engineering?

    Swizec Teller published ServerlessHandbook.dev avatarSwizec Teller published ServerlessHandbook.dev@Swizec
    Scar a developer with 1 word, I'll start


    That was a typo, it's supposed to say "scare".

    We had fixed yet another timezone bug in our codebase ... an API call that removed UTC offset from startDate, but not endDate. Made 3 hours of data unfindable πŸ™ƒ

    Small obvious fix in the end, but it took 2 engineers over an hour to find and verify.

    fetch(..., {
    startDate: datefns.parse(
    new Date()
    endDate: datefns.addDays(
    new Date(formValues.startDate),
    // πŸ‘‡
    const startDateWithoutTime = parse(
    new Date()
    fetch(..., {
    startDate: startDateWithoutTime,
    endDate: datefns.addDays(

    new Date('2021-06-05') gives you a timestamp set to midnight in the user's timezone. 5pm UTC in San Francisco, 3pm UTC in New York, etc. And when the server expects midnight UTC, you got a problem.

    We did that for startDate but not for endDate.



    I cannot tell you how much of my life I've lost to timezones, my friend. From coding to traveling across oceans and international calls.

    Timezones are a mess.

    And they're getting better! Europe wanted to end daylight savings in fall of 2021.

    For countries that choose to end it ... on the winter or the summer time ... their choice ... making the CET timezone any of 3 possible current times ...

    Wait that's not better at all!

    If you have not been scarred by timezones my friend, consider yourself lucky. It's coming.

    And before you say "Abolish timezones, it's the right thing to do, remember that China does that. Everyone is on Beijing time.

    You can't have a 9am meeting between Beijing and Xinjiang because Xinjiang's sunrise is 2 hours later and they start work at 11am. "This is understood".

    Sounds a lot like timezones πŸ€”


    You know what they say: A beginner looks at the problem, sees that regex can solve it, now they have 2 problems.

    Regular expressions are great. Little bits of inscrutable logic that parse strings. Versatile as heck.

    Quick! What does this do?

    const valid = formValue.match(/[a-z]+@[a-z]+\.[a-z]+/i)

    It's a bad way to validate email inputs.

    Checks that you have at least 1 letter, then an @, then more letters, followed by a dot and even more letters.

    The correct regex for email validation looks like this:


    Yeah I don't get it either. StackOverflow says that regex internally translates to this state machine:

    Email regex state machine
    Email regex state machine

    Oh and it doesn't work. Comments point to several bugs where this regex fails to match the spec.


    PS: if you pass a very long string this regex will hang your server


    CORS is a safety feature on modern browsers. Stands for Cross-Origin-Resource-Sharing.

    And it's a mess. Kind of.

    The rules are simple: Your browser can load any resource that might contain executable code which the responding server has whitelisted for the domain it's being loaded from.

    If you're on swizec.com and make a request to api.swizec.com, the API server has to explicitly say "Requests from swizec.com are okay". Otherwise the browser pretends the result is invisible.

    But if you load an image from cdn.swizec.com, it works without special headers. Images aren't executable.

    Fonts, however, are πŸ™ƒ

    State and Mutation

    State sounds harder than it is.

    const value = 1

    There. State!

    Where state gets tricky is domain modeling. And I think that's what people mean when they say "state is hard".

    Domain modeling is a cruel mistress

    ~ Me when we found yet another exception to the rules

    How do you take the natural world with its fuzzy edges and weird exceptions and put it inside a computer? Computers need certainty.

    When does a bunch of pebbles become a pile? When does a brown bear become a polar bear?

    Brown bears and polar bears are creating a new prizzly bear species. They can mate.

    A prizzly bear
    A prizzly bear

    Nature is complex. It doesn't care about your classifications and your rules.

    Mutation ... mutation is bad when state is shared. Because shared state is wrong state.

    Unicode and charsets and localization

    I have been lucky enough to avoid this whole mess.

    Here's a smol taste instead, paste this in your console:

    '🏳' + '\u200d' + '🌈'


    `\u200d' is a zero-width-joiner character. Combines emojis. See how you make an emoji family:


    Try it. I promise it's fun :)

    What scary words have in common

    All these words are powerful tools that are not meant to be safe.

    It's a sword. They're not meant to be safe.
    It's a sword. They're not meant to be safe.

    And that's an important lesson.

    Cheers, ~Swizec

    PS: it was 3 hours of unfindable data because we have people on the East and West coast. UTC+4 and UTC+7. If we had a person in every UTC+N, we could find it all πŸ™ƒ

    Did you enjoy this article?

    Published on June 7th, 2021 in

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