Can You Find the Bug in This Code?

Honestly -- then they'll never be convinced. I haven't hit an ASI bug in the last decade.

So, sorry but you couldn't possibly be more wrong. The great majority of the other commenters here disagree with you as well. You got lucky if you have in fact never had an ASI bug as you claim, but in this thread there is at least one link to a famous bug that has been caused by it and mention of others. The fact that you anecdotally gambled and "won" doesn't hold a lot of weight.

There is one much less popular style guide which actually made headlines for recommending against semi-colons and they got roundly criticized for it by many, many famous JS programmers including the likes of Dan Abramov from Facebook. The author of this style guide tried to weasel more popularity out of it by calling his guide "Standard" since he knows for a fact that the vast majority of JS coders use a de facto style that includes semi-colons. That's exactly the type of behavior that I expect from young, inexperienced coders who make poor decisions based on vanity. I see it all day long from junior programmers. Luckily for everyone in my company, it's my job to set them straight.

I started JS in the mid 90's. I still use semi-colons. To me it's like using a period to end a sentence. It's a pause. It defines the end of a statement.

I feel that too much emphasis is placed on the desire to remove curlys and semi colons from JS. It's part of the language. There's always coffeescript or those other flavours that will suit you.

I do agree that good use cases are few and far between.

I don't really write IIFEs since let and const. I'm sure the outputs from babel/typescript/etc use them, but I don't really need to write them myself, now that we have proper block scoping.

And if I want to declare an array and immediately iterate over it, I just put it in a variable first. That gives it a name, too, which tends to improve readability anyways.

I work with Javascript / Typescript projects on both sides of the argument, and I have to say not using semicolons never seems to cause any problems. Especially not with auto formatting / linters around.

But that's not always the solution, you still need to know when ASI kicks in and when it doesn't.

"just end lines with ;" isn't true in multi-line arrays, in multi-line objects, and a ton of other situations where a semi at the end of the line is a syntax error. And while i'm sure you know that and didn't actually mean "every" line, it just goes to show that you already have the knowledge of when to add them and when not to. But that knowledge is learned, and it isn't a simple rule by any means.

For example, the following code is probably not going to do what many beginners think it will do:

    return
      [
        1,
        2,
        3,
      ];

Semicolons at the end of lines or not, you still need to understand when they are inserted and when they aren't. The real solution is to use a linter which will check these cases for you. I don't care if you use them or don't, just make sure something else enforces it.

Note: If you're often writing code like this, you may be trying to be too clever.

Clever short-hands are discouraged, in favor of clear and readable expressions, whenever possible.

Instead of this:

    ;[1, 2, 3].forEach(bar)

    var nums = [1, 2, 3]
    nums.forEach(bar)

Or if you're serious, switch to BuckleScript and reduce all your side-effecting expressions down to unit. (Fully eliminates unhandled promise bugs, for one thing.)

    void function () {
      window.alert('ok')
    }()

Or better yet, stop using IIFEs. Unless you need IE11 support and can't transpile your code there is absolutely no need.

    {
      let x = 42
    }
    console.log(typeof x) // undefined