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

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

JavaScript’s parseInt() function is quite handy for converting strings into integers, but it can sometimes lead to surprising results. One such mystery is when you call:

parseInt(0.0000005)

And the output is 5! 😲

The Reason Behind It

Here’s the simple explanation: parseInt() doesn't just look at the number itself. It first converts the value to a string. So, when we pass 0.0000005, JavaScript automatically converts it to the string "5e-7". 🎢

Now, parseInt() starts reading the string from the left and stops at the first non-numeric character. In "5e-7", it sees 5 first, so it stops there and returns 5. It doesn’t process the scientific notation part (e-7), which is why it ignores the decimals.

Summary 📜

• parseInt() processes numbers as strings.
• It only reads up to the first non-numeric character.
• The result is the integer before the first non-digit character.

Final Trick Question! 🧩

What will console.log(0.1 + 0.2 == 0.3) return? 🤨 Try it and see if you can crack this JavaScript mystery!

Comments

[Mike Talbot ⭐]

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) {}

[Jagroop Singh]

Nice trick @miketalbot ,
Using the + simplifies conversion by implicitly handling type coercion, making the code more concise without needing explicit parsing.

[@whereisthebug]

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.)

[Jagroop Singh]

Exactly! @whereisthebug Do you know any other tricky things about JS?

[Shifa Ur Rehman]

What do you mean? I thought the whole javascript was a tricky thing 😂

[Jagroop Singh]

[john]

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

[Jagroop Singh]

@john12 ,
The behavior you're noticing isn't a bug, but a well-known quirk in JavaScript (and many other programming languages) related to how it handles floating-point arithmetic.

[Peter Vivo]

At least

parseInt("0.0000005")
// => 0

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

[Jagroop Singh]

Yes, parseInt("0.0000005") returns 0 because parseInt stops parsing at the decimal point.
So , I thinks it's good to convert numbers to string before parsing into integer.

[A. Merezhanyi]

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

[Web]

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

console.log(null == undefined);

[Jagroop Singh]

That one is easy @works ,
== means loose equality that convert both values to a common type before comparing. So it returns true

If we usr === then it returns false

[Web]

Yes correct.

[Boniface Gordian]

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!

[Jagroop Singh]

Thanks @boniface_gordian

[Mike Stemle]

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.

[Bernard Poulin]

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

[José Pablo Ramírez Vargas]

It's your own fault as a developer. The function's name clearly states it will return an integer. You cannot possible expect a floating-point value to come out correctly from parse*Int*.

[Nikunj Bhatt]

I hope you are just joking.

[José Pablo Ramírez Vargas]

Why? Is there a punch line in what I wrote? Or is it normal to expect a floating-point value from a function named parseInt, especially when there's another function called parseFloat? What's confusing here?

[Daniel Santana]

I laughed at first as well because I was thinking with JS mindset. And for anyone with a bit of experience with JS that is indeed funny.
The parseInt function is supposed to receive anything and try to return an int from it.

So yes, most of the time the input is a non int value that will return an int, that’s why Jose’s first comment sound like a sarcastic joke.

But my guess is that the name of the function is not so specific and one could understand it as a function that receives and int then parses it into something else.

But what makes it funny is the arrogant tone of blaming who wrote the code based on a wrong understanding of what the function does. 😂

It’s funny if it is a joke, but even funnier because it’s not 😆

[José Pablo Ramírez Vargas]

Ah, I see. FYI and full disclosure: I don't blame the author directly when I said "you as a programmer". "You" here refer to anybody that stumbled into this problem expecting a float out of parseInt, hehe.