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Discussion on: Do you know these weird parts of JavaScript?

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tchaflich profile image
Thomas C. Haflich

> 0.1 + 0.2
0.30000000000000004

This is a floating point rounding error and is definitely not exclusive to JavaScript. It happens with basically any language that implements floats.

3 > 2
true

3 > 2 > 1
false

I also don't think this one is language specific. It should evaluate the same nearly anywhere that follows a left to right execution order for same-precedence operators.

A lot of the other stuff is solid "weird language spec" though. I especially like this one:

> ['1', '7', '11'].map(parseInt)
(3) [1, NaN, 3]

Took me a moment to remember what it was doing there and figure out the trick. The fact that there are two separate bits of knowledge required is especially neat.

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lqj profile image
QJ Li Author

yes, you are right about these ones, I am including them to just showcase some 'weird' / interesting parts in JS. some of them are not actually weird, it is weird only when comparing with other languages.

sorry my eye catching title is kind of misleading

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shreyasminocha profile image
Shreyas Minocha

I found that one interesting too. Took me some time.

I noticed that

['1', '7', '11'].map((n) => parseInt(n, 10))

works as expected. parseint is misinterpreting the second parameter somehow?

Then I saw docs for Array.prototype.map.

var new_array = arr.map(function callback(currentValue[, index[, array]]) {
    // Return element for new_array
}[, thisArg])

If the callback accepts a second parameter, it passes the index of the element to it. parseint does accept two parameters, but of course, it doesn't expect an index in the third one.

[
    parseint('1', 0),
    parseint('7', 1),
    parseint('11', 2)
]

And sure enough, the output confirms my theory!

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lqj profile image
QJ Li Author

you are close, if you take a look at the api doc, you will fully understand it.

developer.mozilla.org/en-US/docs/W...

parseInt(string, radix);
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shreyasminocha profile image
Shreyas Minocha

Yep! I'm aware. Pretty cool.