4 ms·

Both of these examples are well-known (and not unexpected) behaviours. I assume you already know why it behaves like that. If not, I can explain it. > [1, 2, 3

Both of these examples are well-known (and not unexpected) behaviours. I assume you already know why it behaves like that. If not, I can explain it. > [1, 2, 3] + [4, 5, 6] // -> "1,2,34,5,6" What would you expect instead? > [,,,].length // -> 3 Is there any use case where you would want to deal with sparse arrays?

realo 5y ago
[1,2,3] + [4,5,6] An addition operation over two numerical vectors of length 3. I would expect the result to match its inputs and provide a numerical vector of length 3. Thus: [5, 7, 9]
- fenomas 5y ago
  The issue there is that arrays aren't first class types in JS, they're just objects with numeric property names. So if applying an operator to arrays spread the operation across all the elements that way, it would imply that the same should happen generally for all object properties, with whatever weird implications that would entail. a.foo = 1 b.foo = 'there' a + b // { foo: '1there' } ?
- meowface 5y ago
  What language has this behavior by default?
  - DangitBobby 5y ago
    Fortran :)
e3bc54b2 5y ago
> [1, 2, 3] + [4, 5, 6] // -> "1,2,34,5,6" > What would you expect instead? If I let my first instinct speak: [1,2,3,4,5,6] And then if I think a little more, then maybe: [5,7,9] //with obvious caveats In no way do I expect what GP actually provided.
- FrontAid 5y ago
  I understand where you are coming from. But the addition operator simply does not have any special handling of arrays. The specification [1] clearly defines what it should be used for: "The addition operator either performs string concatenation or numeric addition." As JavaScript is weakly typed, it is the programmer's responsibility to use proper value types with these operators. That limitation (or advantage?) is also well-known and applies to all weakly-typed languages. [1] https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-addition-operator-plus https://tc39.es/ecma262/multipage/ecmascript-language-expres...
  - kristiandupont 5y ago
    It seems that by "expected", you mean "expected, by anyone who read the spec" which I don't think is a fair use of that word. Obviously, most JS developers have not and will not read the spec. I am very happy with JS and TS and I think the coercion rules are easily worked around with linter rules and policies, but they are definitely weird and I think the language would be better if it simply threw exceptions instead. But then, such an issue shoud not be surprising for a language that was designed in 10 days.
    - FrontAid 5y ago
      No, I meant "expected by anyone who learned the language". Knowing the addition operator including its limitations is quite basic. I'm not saying you need to be able to solve all this "JavaScript is weird" puzzles as they are mostly non-sense. But you definetely have to know what you can us `+` for. If someone does not like the ECMAScript specification, that is fine. But at least use a proper unofficial documentation like MDN.
  - bryanrasmussen 5y ago
    well I guess the question is then not just what would you expect instead but at what familiarity with the language should one be asking people what they expect of it? If you ask experts it is because you want to get an actual correct answer, but if you ask neophytes it is because you want to get an answer that might be obvious even if not correct.
kilburn 5y ago
> Is there any use case where you would want to deal with sparse arrays? Not really. Now explain why [,,,].map((e,i) => i) is [,,,] instead of [1,2,3] please ;)
- fenomas 5y ago
  (assuming you're really asking) It's because JS has a notion of array elements being "empty", and the map operation skips empty elements. Basically "empty" means the element has never had a value assigned to it, but its index is less than the array's length property. Array(4) // [empty × 4] a=[]; a.length=4; a // [empty × 4] Array(4).map(n => n) // [empty × 4] [,,1,,].map(n => n) // [empty × 2, 1, empty] My go-to way of avoiding this annoyance is "Array.from(Array(N))": Array.from(Array(4)).map((n,i) => i) // [0, 1, 2, 3] Alternately there's a recent "fill" method, that assigns all elements (including empty ones) to a given value: Array(4).fill(1) // [1, 1, 1, 1]

realo 5y ago

[1,2,3] + [4,5,6] An addition operation over two numerical vectors of length 3. I would expect the result to match its inputs and provide a numerical vector of length 3. Thus: [5, 7, 9]

fenomas 5y ago
The issue there is that arrays aren't first class types in JS, they're just objects with numeric property names. So if applying an operator to arrays spread the operation across all the elements that way, it would imply that the same should happen generally for all object properties, with whatever weird implications that would entail. a.foo = 1 b.foo = 'there' a + b // { foo: '1there' } ?
meowface 5y ago
What language has this behavior by default?
- DangitBobby 5y ago
  Fortran :)

e3bc54b2 5y ago

> [1, 2, 3] + [4, 5, 6] // -> "1,2,34,5,6" > What would you expect instead? If I let my first instinct speak: [1,2,3,4,5,6] And then if I think a little more, then maybe: [5,7,9] //with obvious caveats In no way do I expect what GP actually provided.

FrontAid 5y ago
I understand where you are coming from. But the addition operator simply does not have any special handling of arrays. The specification [1] clearly defines what it should be used for: "The addition operator either performs string concatenation or numeric addition." As JavaScript is weakly typed, it is the programmer's responsibility to use proper value types with these operators. That limitation (or advantage?) is also well-known and applies to all weakly-typed languages. [1] https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-addition-operator-plus https://tc39.es/ecma262/multipage/ecmascript-language-expres...
- kristiandupont 5y ago
  It seems that by "expected", you mean "expected, by anyone who read the spec" which I don't think is a fair use of that word. Obviously, most JS developers have not and will not read the spec. I am very happy with JS and TS and I think the coercion rules are easily worked around with linter rules and policies, but they are definitely weird and I think the language would be better if it simply threw exceptions instead. But then, such an issue shoud not be surprising for a language that was designed in 10 days.
  - FrontAid 5y ago
    No, I meant "expected by anyone who learned the language". Knowing the addition operator including its limitations is quite basic. I'm not saying you need to be able to solve all this "JavaScript is weird" puzzles as they are mostly non-sense. But you definetely have to know what you can us `+` for. If someone does not like the ECMAScript specification, that is fine. But at least use a proper unofficial documentation like MDN.
- bryanrasmussen 5y ago
  well I guess the question is then not just what would you expect instead but at what familiarity with the language should one be asking people what they expect of it? If you ask experts it is because you want to get an actual correct answer, but if you ask neophytes it is because you want to get an answer that might be obvious even if not correct.

kilburn 5y ago

> Is there any use case where you would want to deal with sparse arrays? Not really. Now explain why [,,,].map((e,i) => i) is [,,,] instead of [1,2,3] please ;)

fenomas 5y ago
(assuming you're really asking) It's because JS has a notion of array elements being "empty", and the map operation skips empty elements. Basically "empty" means the element has never had a value assigned to it, but its index is less than the array's length property. Array(4) // [empty × 4] a=[]; a.length=4; a // [empty × 4] Array(4).map(n => n) // [empty × 4] [,,1,,].map(n => n) // [empty × 2, 1, empty] My go-to way of avoiding this annoyance is "Array.from(Array(N))": Array.from(Array(4)).map((n,i) => i) // [0, 1, 2, 3] Alternately there's a recent "fill" method, that assigns all elements (including empty ones) to a given value: Array(4).fill(1) // [1, 1, 1, 1]