1. `function` gives you a new `this`, whereas `=>` closes over the `this` from the parent scope. Which to use depends on what you want. (Full disclosure: I would've had to think for a bit whether the syntax was `->` or `=>` – though I'd probably guess `=>` because `->` means something in C.)
2. `!==` is strict comparison, and `!=` tries to do incomprehensible magick to munge incomparable types together. I still know some of the rules (array to string sticks in my mind), but I doubt many people know all of them. There's almost never a good reason to use `!=`, outside `elem.value != my_integer`. (Though `NaN !== NaN`, so you need to watch out for that: since equality is defined recursively on objects I'm pretty sure this is infectious.)
3. The length of the chain is easy (though, iirc there are performance implications since it's not lazy). The hard part is the fact that map callbacks can take one, two or three arguments, so stuff like `["123", "456", "10", "1234"].map(parseInt)` will behave unexpectedly. (Something like `[123, undefined, 2, 5]`, though I might be thinking of PHP.) Not sure whether `Array.prototype.filter` has similar gotchas.
That's off the top of my head. I haven't seriously used JavaScript since the ES5 times, but JavaScript's a very… memorable… language. (I can't even remember hello world in Pascal.)
2. `!==` is strict comparison, and `!=` tries to do incomprehensible magick to munge incomparable types together. I still know some of the rules (array to string sticks in my mind), but I doubt many people know all of them. There's almost never a good reason to use `!=`, outside `elem.value != my_integer`. (Though `NaN !== NaN`, so you need to watch out for that: since equality is defined recursively on objects I'm pretty sure this is infectious.)
3. The length of the chain is easy (though, iirc there are performance implications since it's not lazy). The hard part is the fact that map callbacks can take one, two or three arguments, so stuff like `["123", "456", "10", "1234"].map(parseInt)` will behave unexpectedly. (Something like `[123, undefined, 2, 5]`, though I might be thinking of PHP.) Not sure whether `Array.prototype.filter` has similar gotchas.
That's off the top of my head. I haven't seriously used JavaScript since the ES5 times, but JavaScript's a very… memorable… language. (I can't even remember hello world in Pascal.)