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Think You Know JavaScript? These 15 Quirks Will Change Your Mind! 💥

Jagroop Singh on November 13, 2024

JavaScript is a flexible, quirky language that can sometimes leave you scratching your head. If you've spent any amount of time working with JS, yo...

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Ben Sinclair • Nov 13

99.1% of these issues people have with JavaScript are caused by trying to compare or manipulate things of different types.

74.8% of these things are common to other programming languages.

I feel like most of the things people find weird about JS are because they're trying to do something that if you stop and think about it for more than a second, well, it doesn't make sense.

Lisa Resnick • Nov 18

This is pretty accurate, but one of the main reasons JavaScript is so popular is its "flexible and forgiving nature." People often mix and match different types because the language allows it. For

example:

console.log(1 + "1"); // "11"
console.log(1 - "1"); // 0

This kind of behavior can confuse new developers. But if you approach JavaScript from the perspective of strongly typed languages (like Java or C#), it might seem "weird."

The truth is, if developers understand type conversions and implicit coercion, JavaScript doesn't feel so strange anymore. And as you said, "if you stop and think about it," most of the problems do go away — that's the golden rule of programming in any language. 😄

Jagroop Singh • Nov 13

@moopet ,
I agree !!
It’s true that many JavaScript quirks stem from type coercion and comparison issues, which can often be avoided and not widely used.

Andy Robinson • Nov 14

Heres a fun one that trips up lots of people...

 const christmasDay = new Date(2024, 12, 25);

 const christmasDayString = christmasDay.toLocaleDateString("en-GB", {
   weekday: "long",
   year: "numeric",
   month: "long",
   day: "numeric"
 });
// result: Saturday 25 January 2025

Jagroop Singh • Nov 15

Yes @freshcaffeine ,
Date object are zero-indexed. This means 0 is January, 1 is February, and so on.That's why it cause issue.

Andy Robinson • Nov 15 • Edited

yes, but only month is zero-indexed. But not day and year.. which increased confusion more - I've seen some interesting bugs because of this.

HB_the_Pencil • Nov 14

I'm gonna guess... typeof NaN returns "number" because it is a placeholder for something that should be a number.
It ought to be 0, but I'm guessing it's actually Infinity because subtracting anything from Infinity is Infinity. (Either that or -Infinity because it subtracts until it cannot subtract any more.)

This is a fun article! I'd love to see more of these!

Jagroop Singh • Nov 15

@hbthepencil , thanks for trying , but the actual answers are:

NaN is technically a special value of the Number in javascript.
When you subtract Infinity from Infinity in JavaScript, you get NaN. This is because the operation is undefined—subtracting two infinite quantities does not yield a finite number or a meaningful result.

HB_the_Pencil • Nov 15

Ah, I was almost there on the first one... Thanks!

Robert Baddeley • Nov 14

The 0.1 + 0.2 and the 0.1*0.1 is not really JavaScript but because floating point math is not precise when it comes down to storing the bits in memory. Many other languages have the same issue. Less a quark of JavaScript and more of a quark of computer science.

Jagroop Singh • Nov 15

Yes , I agree @rjbaddeley

Tomas Stveracek • Nov 15

Great article! For those interested, I've also written a similar piece diving into JavaScript quirks: JavaScript Quirks: What You Need to Know. Feel free to check it out! 😊

Jagroop Singh • Nov 15

Great post @tomasdevs !!

john • Nov 13

Finally , the old programming language still have bugs, console.log(typeof null); // object which shows no one is perfect even javascript

Jagroop Singh • Nov 13

Haha, yes, even JavaScript has its quirks!

caga • Nov 13

[1,2,3] + [4,5,6] = "1,2,34,5,6" is asked in my recent interview and I fails to answer it correctly, and after checking the solution I just byheart this and will never forget that.

Jagroop Singh • Nov 13

Yes that's tricky one !!

Ezekiel • Nov 15

And thats why we use typescript

Jagroop Singh • Nov 15

Exactly @ezekiel_77 , these quirks shows it's important to use typescript in js based projects.

Web • Nov 13

Nice javascript tricky points.

Jagroop Singh • Nov 13

Thanks @works

Alex Lohr • Nov 18

About #5, you can also use Object.is(NaN, NaN). About #11, parseInt does exactly what is should and returns the integer value of the number. The rest is IEEE754 and type coercion wackyness - and after a few decades of writing JS, those were no longer surprising.

About your tricky questions: NaN is of type number, because it is specified like this in IEEE754 (not only in JS) and Infinity - Infinity yields NaN.

Alireza Balouch • Nov 15

I was just wondering in what situation you have to compare and empty array with an empty object and what other languages can actually do that? Many of those examples are just bad coding or misunderstood how JS works.

Wizard • Nov 15

For more informative knowledge, it's good article, bit deep down inside, we all know this doesn't matter