7 ms·

For those wondering how you get numbers, strings and other primitives from a whole bunch of empty arrays and objects in JavaScript, here's what happens when you

For those wondering how you get numbers, strings and other primitives from a whole bunch of empty arrays and objects in JavaScript, here's what happens when you do arithmetic and other operations on arrays and objects: > [] + [] "" (An empty string) > {} + [] 0 (The number zero) > [] + {} "[object Object]" (.toString() called on a plain object). > ![] false (!{} is the same) > !![] true (not false) > +[] 0 (the number zero) > -[] -0 (negative zero) > +{} NaN (Not a number, same goes for -{}) > "" + [] "" (empty string) > "" - [] 0 (number zero, not empty string) And the one that gets more people than the previous list: > typeof [] === typeof {} true Incidentally, some of these things can be useful. For example, +(x) will always evaluate to a double (unless it is preceded by a string), while (x|0) will always evaluate to a 32bit integer. asm.js abuses (to an extent) this to have more control over types, and most JavaScript engines would now store the result of (x|0) as an integer internally instead of a double.

jaseemabid 11y ago
Does knowing any of this make you a better programmer? I'd say no. Should you be using any of this in production code? No The next programmer even if he is a really good one might not know that particular esoteric trick. I'm not trying to vote down or anything, but what is the point? Most of it look like language design warts to me. Most of them should have thrown exceptions and errored out. Now I'm loving the idea of static typing a lot more.
- xanderjanz 11y ago
  Useful for obfuscating executable JS for security reasons.
  - thomasfoster96 11y ago
    It'd have to be something that really had to be hidden, because most of these edge cases are very very slow to run.
  - berbc 11y ago
    Why? I believe you have more sophisticated ways of obfuscating code. Those tricks seem to be easy to reverse.
  - BinaryIdiot 11y ago
    Security through obscurity is not security. This would be easy to translate back into the original code anyway. This is also not efficient from a file size perspective which is cancer on the web.
- thomasfoster96 11y ago
  Most of it is warts and is completely useless. Sometimes things like knowing that an array is an abject are pretty important if you're writing production code. The bit at the end (integer and float casting) is almost essential knowledge now for anyone writing JavaScript code that does anything mildly intensive.
- Veedrac 11y ago
  The problem is the strong-typing/weak-typing distinction, not the static-typing/dynamic-typing distinction. For example, Python is about as strongly typed as C++ yet they're on opposite sides wrt. how dynamic their types are.
- Manishearth 11y ago
  > Does knowing any of this make you a better programmer? I'd say no. Yes it does, it makes sure you never do this ;P Adding arrays sounds like something I'd totally try in JS, had I not known that it causes magic unicorn sparkles to happen)
- AgentME 11y ago
  Using +x to convert things to numbers and x|0 to convert things to 32-bit integers are useful and common operations.
jroseattle 11y ago
Here's a slightly more entertaining version of the discrepancies/oddities found in javascript: https://www.destroyallsoftware.com/talks/wat https://www.destroyallsoftware.com/talks/wat
octatoan 11y ago
https://www.destroyallsoftware.com/talks/wat https://www.destroyallsoftware.com/talks/wat
esrauch 11y ago
A bunch of that is wrong or at least misleading. > {} + [] > 0 (The number zero) This is wrong, it is only true in the weird scenario that you type it into the jsconsole and don't use the result. It parses as an empty block ({}) and then an unrelated unary +[] after that. var x = {} + []; gives you a string and it is the same as var x = [] + {}. There is more logic than it seems here: Unary plus: > always gives a number (a double: NaN is a double) > on array: +a -> if len>=2: NaN else +a[0] (recursively) > on object: NaN > on string: parseNumber, or NaN if that fails > on bool: 0 for false, 1 for true > on null: 0 > on undefined: NaN binary plus (addition): > order doesn't matter except for the jsconsole pseudo-bug > if both sides are number or boolean, add them > else, toString() both sides and concat them boolean negate (not): > false for 0, false, "", null, undefined > true otherwise binary - (subtraction) > x-y coerces x and y to number and subtracts them. > Most mainstream languages that use + for string concat don't use - for some sort of string unconcat so its not too crazy. > typeof [] === typeof {} This isn't so crazy, typeof any non-primitive gives you "object". [] is pretty much just an object plus magic .length property, var x = []; x.blah = 7; console.log(x.blah) works. It doesn't work on number or bool.

jaseemabid 11y ago

Does knowing any of this make you a better programmer? I'd say no. Should you be using any of this in production code? No The next programmer even if he is a really good one might not know that particular esoteric trick. I'm not trying to vote down or anything, but what is the point? Most of it look like language design warts to me. Most of them should have thrown exceptions and errored out. Now I'm loving the idea of static typing a lot more.

xanderjanz 11y ago
Useful for obfuscating executable JS for security reasons.
- thomasfoster96 11y ago
  It'd have to be something that really had to be hidden, because most of these edge cases are very very slow to run.
- berbc 11y ago
  Why? I believe you have more sophisticated ways of obfuscating code. Those tricks seem to be easy to reverse.
- BinaryIdiot 11y ago
  Security through obscurity is not security. This would be easy to translate back into the original code anyway. This is also not efficient from a file size perspective which is cancer on the web.
thomasfoster96 11y ago
Most of it is warts and is completely useless. Sometimes things like knowing that an array is an abject are pretty important if you're writing production code. The bit at the end (integer and float casting) is almost essential knowledge now for anyone writing JavaScript code that does anything mildly intensive.
Veedrac 11y ago
The problem is the strong-typing/weak-typing distinction, not the static-typing/dynamic-typing distinction. For example, Python is about as strongly typed as C++ yet they're on opposite sides wrt. how dynamic their types are.
Manishearth 11y ago
> Does knowing any of this make you a better programmer? I'd say no. Yes it does, it makes sure you never do this ;P Adding arrays sounds like something I'd totally try in JS, had I not known that it causes magic unicorn sparkles to happen)
AgentME 11y ago
Using +x to convert things to numbers and x|0 to convert things to 32-bit integers are useful and common operations.

jroseattle 11y ago

Here's a slightly more entertaining version of the discrepancies/oddities found in javascript: https://www.destroyallsoftware.com/talks/wat https://www.destroyallsoftware.com/talks/wat

octatoan 11y ago

https://www.destroyallsoftware.com/talks/wat https://www.destroyallsoftware.com/talks/wat

esrauch 11y ago

A bunch of that is wrong or at least misleading. > {} + [] > 0 (The number zero) This is wrong, it is only true in the weird scenario that you type it into the jsconsole and don't use the result. It parses as an empty block ({}) and then an unrelated unary +[] after that. var x = {} + []; gives you a string and it is the same as var x = [] + {}. There is more logic than it seems here: Unary plus: > always gives a number (a double: NaN is a double) > on array: +a -> if len>=2: NaN else +a[0] (recursively) > on object: NaN > on string: parseNumber, or NaN if that fails > on bool: 0 for false, 1 for true > on null: 0 > on undefined: NaN binary plus (addition): > order doesn't matter except for the jsconsole pseudo-bug > if both sides are number or boolean, add them > else, toString() both sides and concat them boolean negate (not): > false for 0, false, "", null, undefined > true otherwise binary - (subtraction) > x-y coerces x and y to number and subtracts them. > Most mainstream languages that use + for string concat don't use - for some sort of string unconcat so its not too crazy. > typeof [] === typeof {} This isn't so crazy, typeof any non-primitive gives you "object". [] is pretty much just an object plus magic .length property, var x = []; x.blah = 7; console.log(x.blah) works. It doesn't work on number or bool.