Why Does JavaScript’s parseInt(0.0000005) Print “5”? 🤔

It's personally why I convert things into numbers using + rather than parsing. I don' have to remember or even know if it's a number or a string. My preference:

    const number = Math.floor(+someValueIWantToConvertToAnInt)

    // or often

   if(+someValue > 1) {}

Nice catch!

As you correctly pointed out, parseInt converts the number to a string. And JavaScript converts numbers to strings using scientific notation when the magnitude of the number (i.e. the number without the sign) is larger or equal than 1000000000000000000000 (21 zeroes) or smaller or equal than 0.0000001 (7 zeroes.)

I don't know what's happening in JS. It is javascript bug or something else, okay I understand the concept of parseInt(0.0000005) thanks for explaining that but why console.log(0.1 + 0.2 == 0.3) returns False.

Also I think developers must know these edge cases points of js because somehow it will used in case of payments then it really hard to debug for begineers to debug

This is another one of those issues which isn't an issue if you use the function as it's defined, and it's defined to take a string parameter. There's no reason I can think of to pass a float into it, you'd use an explicit cast or a dedicated function like Math.floor for that purpose.

At least

parseInt("0.0000005")
// => 0

but that is great catch I don't know about it ( I was never use parseInt )

That's cool, I already know about second one , what about this

console.log(null == undefined);

That would be great you’d started with the history and mentioned that parseInt was designed to parse stings to numbers for cases like “10px” -> “10”, that explains a lot.
That worth mentioning that according to IEEE 754 all other programming languages do the same math and add a link to 0.30…4.com

This is such an interesting topic—parseInt() is one of those quirks in JavaScript that always surprises people! I really liked how you explained the behavior with the radix and how it defaults to 10 in most cases. It’s definitely one of those ‘gotcha’ moments for developers. Personally, I try to stick with Number() for clarity, but there are definitely cases where parseInt() shines. Have you ever run into unexpected issues with it in real-world projects? Great work on the article!

The 0.1+0.2===0.3 isn't a JavaScript issue, that's an IEEE 754 floating point math error. This is pretty common to a number of languages including JavaScript, Python, Perl, C/C++, etc.

parseInt() is indeed handy when used correctly. Calling it with a number instead of a string would be considered a mistake.
If the intention was to convert a float into an integer, the popular quick hack is |0 as in 0.0000005|0. The more official way is to use Math.floor() obviously. parseInt() is simply the wrong method to do this.
Also note that negative values might or might not do what you'd expect. Depends on the use case.
I understand that the logic was maybe expecting a string and suddenly a number comes along at runtime and it was "working most of the time". The solution from Mike Talbot is indeed the best if you need to counter this issue. You can of course still combine it with the |0 hack

+0.0000005|0