I almost got caught by this during an interview:

const foo = ['a', 'b'];

const bar = foo.findIndex(x => x === 'a'); // 0

if (bar) { // I'm an idiot
console.log('Do something');
}

🤦‍♂️

Don’t worry too much. It’s a trap that many of us walk into.
That’s the reason why some of the good languages got rid of implicit conversions to bool.

Can you add some explanation for people who don't understand this language?

@happygimp0 in JavaScript, 0 evaluates to false. I was going to use that number to retrieve a value from the array, which would have been a bug because it's a valid index number but falsy value.

@happygimp0

bar is set to 0 due to the findIndex() returning 0 as the index.

If(bar)

is checking 0 against true.
In Javascript 0 is false.

@C0D4 but why would you check against a 0 ?

You know you are working with javascript so you should know that 0 = false but 0 is a valid index.

Javascript is know for this type of behavior.

Never assume implicit conversion is doing what you think it is doing.

Also in C/C++, 0 is both a valid array index and a boolean false. Completely normal.

what does it return when it's not present? isn't -1 the universally accepted standard for fallback index?

@Fast-Nop Maybe not that normal since I can only think of JS and C/C++.
Are there any other examples? Php maybe (not sure).

@theabbie traditionally, -1, yes. But the modern way would be to return null or something similar.

@Lensflare Not sure either. Even in C, I compare to 0, '\0', or NULL to clarify which kind of 0-ish comparison is going on: value range check, string terminator, or pointer check.

The only exception are variables intended for purely boolean usage anyway.

@Grumm you are missing the point, they forgor to check if it's not -1.

Also hecc u i will rely on implict conversions even more

There is no excuse for not doing an explicit identity comparison when your variable isn't actually a boolean.

Don't shoot yourself, always use explicit comparisons and conversions.

@Fast-Nop NULL=='\0' && NULL==0

@saintograph Yes, false is a fancy way of saying 0, no question there.

I don't understand this part: foo.findIndex(x => x === 'a')

And why do you check if bar is not 0?

@C0D4 I don't see the problem. And i still don't understand this part: foo.findIndex(x => x === 'a')

I mean, yes he gets some kind of index and does something with it when the index is not 0, but i don't understand why. What is special about index 0?

@Fast-Nop There is no false in C.

@happygimp0 I reckon I wrote that due to force of habit. Writing JavaScript means having to constantly do type checking, something which switching to Typescript mitigates to a certain degree.

@saintograph Yes. I am sorry, but i still don't understand what your mistake was. You say it has something to do with types, but i only see 2 types: The array of chars/strings (don't know enough about JavaScript to know if there is a difference) and a integer which can be 0 (of false, if you want to call it that) or non-zero.

@happygimp0 I do know that '\0', NULL and 0 are the same in terms of code, but conveying which kind of 0-comparison is going on makes the code easier to understand for the next dev or even myself after a few months. It's about maintainability.

Of course there is "false" in C, it's in <stdbool.h>.

I would respond with “what idiot calls their variables ‘foo’ and ‘bar’ - he should not be allowed to even look at the repo”.

@saintograph Ok, now i understand. You didn't want to check for the index but something else, correct?

Sorry, but your rant is a bit unclear in this regard. When i see that code i think the purpose is to check for index==0 vs something else. I think it is really bad decision to set invalid index to false. False and 0 is exactly the same in many (most?) languages.

When an index can't be found, it's assumed -1, he skipped a mind-step and left it as just the result.

You could do something like this is :

foo.includes => returns a valid bool true or false

Then if true, findIndex.

But again, learning that any array has always a zero-based index is a must.

bar > -1 will be quicker