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, you've probably encountered a few unexpected behaviors, like 0 == "0", or wondered why 0.1 + 0.2 !== 0.3.
In this blog, we'll dive into some of JavaScript's most puzzling features, helping you understand how the language works under the hood and avoid potential pitfalls in your code. Ready? Let’s get started! 🚀
1. "False" == 0 but Not "False" == "0" 🤯
You might expect "False" to be equal to "0", but here's the catch: "False" == 0 is true, but "False" == "0" is false.
This happens because JavaScript’s equality operator (==) triggers type coercion. When you compare a string to a number, JS tries to convert the string to a number first. And since "False" doesn't really have a numeric value, it becomes NaN. But "0" becomes the number 0, so when we compare 0 == 0, it’s true!
Example:
console.log("False" == 0); // true (coerces "False" to NaN, NaN == 0 is false but coerces NaN to 0)
console.log("False" == "0"); // false
2. 0.1 + 0.2 Isn't Exactly Equal to 0.3 😲
This one is a classic!
console.log(0.1 + 0.2 === 0.3); // false
Why? JavaScript uses IEEE-754 for floating-point arithmetic, which can cause precision issues. So, 0.1 + 0.2 doesn’t exactly add up to 0.3. It’s a common pitfall in financial apps, for example.
Solution:
For comparisons like this, use a small tolerance:
const isEqual = Math.abs(0.1 + 0.2 - 0.3) < Number.EPSILON;
console.log(isEqual); // true
3. typeof null Returns "Object" 🧐
This one’s a head-scratcher. When you check the type of null, it gives you "object".
console.log(typeof null); // object
Why? Well, it's a historical mistake in JavaScript's early days. The type of null should have been "null", but due to a bug, it ended up being "object". But now, it’s too late to change it without breaking old code.
4. [1,2,3] + [4,5,6] = "1,2,34,5,6" 😅
Here's another weird one! When you add two arrays together, JavaScript converts them to strings before adding them. The result? A strange concatenation!
console.log([1, 2, 3] + [4, 5, 6]); // "1,2,34,5,6"
This happens because arrays get converted to strings, resulting in "1,2,3" and "4,5,6", and when those are added together, it becomes "1,2,34,5,6".
5. NaN === NaN is False 🔄
Yes, NaN (Not-a-Number) is a special value in JavaScript, and it’s never equal to itself.
console.log(NaN === NaN); // false
Why? The logic behind this is that NaN is not a real number, and thus it cannot be equal to anything, even itself.
Tip:
To check for NaN, use Number.isNaN():
console.log(Number.isNaN(NaN)); // true
6. Why [] == ![] is True But [] == [] is False 😱
In JavaScript, [] == ![] is true, but [] == [] is false. How?
-
[] == []is false because two different empty arrays are not the same object reference. -
[] == ![]works because![]coerces tofalse, and an empty array is a truthy value. Thus,[] == falsebecomes true after coercion.
7. Math.max() Returns -Infinity 🤔
When you call Math.max() without any arguments, it returns -Infinity.
console.log(Math.max()); // -Infinity
This happens because JavaScript considers the absence of arguments as a comparison with negative infinity. If you need a fallback value, use:
const max = Math.max(...numbers) || 0;
8. 0 == "0" and 0 == [] but "0" != [] 🔀
JavaScript's == operator does some bizarre things with type coercion. For example, 0 == [] is true, but "0" == [] is false.
console.log(0 == "0"); // true
console.log(0 == []); // true
console.log("0" == []); // false
This happens because [] is first coerced to an empty string when compared with "0", making the second comparison false.
9. undefined + 1 Returns NaN, but null + 1 Returns 1 🤷
Here’s a peculiar one. When you add undefined to a number, you get NaN. But adding null to a number results in 1.
console.log(undefined + 1); // NaN
console.log(null + 1); // 1
Why? undefined is not a number, so it gives you NaN, while null is treated as 0 in arithmetic operations.
10. New Array(3) vs [,,] 😳
What’s the difference between new Array(3) and [,,]?
-
new Array(3)creates an empty array with 3 slots (no values), whereas[,,]creates an array with 2 undefined values.
console.log(new Array(3)); // [ <3 empty items> ]
console.log([,,]); // [undefined, undefined]
11. parseFloat("3.14abc") Works, But parseInt("3.14abc") Doesn’t 😅
When parsing strings with numbers and text, parseFloat will try to read the number up to the first non-numeric character. parseInt, on the other hand, stops at the first non-numeric character and doesn’t work for decimal numbers.
console.log(parseFloat("3.14abc")); // 3.14
console.log(parseInt("3.14abc")); // 3
12. [] + {} vs {} + [] 🔄
Did you know the order of operations matters in JavaScript? [] + {} results in a string, but {} + [] does nothing.
console.log([] + {}); // "[object Object]"
console.log({} + []); // "[object Object]"
13. 0.1 * 0.1 Prints “0.010000000000000002” 📐
JavaScript’s floating-point arithmetic can sometimes give unexpected results, like in the case of 0.1 * 0.1:
console.log(0.1 * 0.1); // 0.010000000000000002
This occurs due to binary floating-point precision errors.
14. Truthy and Falsy Values 🚦
In JavaScript, certain values are treated as falsy: "", 0, null, undefined, NaN, and false. Anything else is truthy.
console.log(!!""); // false
console.log(!!0); // false
console.log(!!null); // false
console.log(!!1); // true
Conclusion: JavaScript is full of surprises! 🎉
Now that we've covered some of JavaScript's most curious quirks, it's time for you to test your understanding. Ready for some tricky questions? 🤔
- Why does typeof
NaNreturn "number" whenNaNis not a number? - What do you get when you subtract
Infinity - Infinityin JavaScript?
Drop your answers below! Let’s see how many of you get them right! 😎
Top comments (25)
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.
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. 😄
@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.
Heres a fun one that trips up lots of people...
Yes @freshcaffeine ,
Dateobject are zero-indexed. This means 0 is January, 1 is February, and so on.That's why it cause issue.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.
NaNreturns "number" because it is a placeholder for something that should be a number.0, but I'm guessing it's actuallyInfinitybecause subtracting anything fromInfinityisInfinity. (Either that or-Infinitybecause it subtracts until it cannot subtract any more.)This is a fun article! I'd love to see more of these!
@hbthepencil , thanks for trying , but the actual answers are:
NaNis technically a special value of the Number in javascript.NaN. This is because the operation isundefined—subtracting two infinite quantities does not yield a finite number or a meaningful result.Ah, I was almost there on the first one... Thanks!
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.
Yes , I agree @rjbaddeley
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! 😊
Great post @tomasdevs !!
Finally , the old programming language still have bugs,
console.log(typeof null); // objectwhich shows no one is perfect even javascriptHaha, yes, even JavaScript has its quirks!
[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.Yes that's tricky one !!
And thats why we use typescript
Exactly @ezekiel_77 , these quirks shows it's important to use typescript in js based projects.
Nice javascript tricky points.
Thanks @works
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 - Infinityyields NaN.