5 ms·

It fails to return nothing here: e.onclick = function() { false; } // returns nothing Translates to: e.onclick = -> false // returns false

by fooyc 15y ago

skeptical 15y ago
Just out of curiosity, how does one returns nothing in coffeescript?
- endtwist 15y ago
  fn = -> return
rgbrgb 15y ago
What's the purpose of the 'false;' statement? I believe CoffeeScript always returns the last line of a function.
- sid0 15y ago
  The purpose of the false statement is irrelevant. I'm generally not a big fan of perfection in programming, but compilers are one instance it's worth aiming for.
- fooyc 15y ago
  An example. This could be any statement evaluating to false. Coffee script returns the result of the last statement. Javascript don't. So, failing to take this into account produces coffeescript that do not have the same behavior than the original javascript. This is particularly important for iterators that return false for stopping iteration. Or for DOM event handlers that return false for stopping the propagation of the event.
  - rgbrgb 15y ago
    While I totally agree with sid0 that it would be nice for 'academic' reasons if the compiler translated js->cs perfectly, I guess I'm having trouble understanding where this would actually come up as a problem. In my opinion, js functions should always return a value (even if it's just 'this'). Maybe a real code example of where your program would fail because of this?
    - fooyc 15y ago
      > would be nice for 'academic' reasons if the compiler translated js->cs perfectly You should expect this from a compiler, actually > In my opinion, js functions should always return a value Sounds wrong > real code example of where your program would fail because of this? Simple: // That's perfectly fine; why would I return anything from this ? link.onclick = function() { doSomething(); }; // Now, translated in coffeescript: link.onclick = -> return doSomething(); // if this returns false, the "click" event is canceled
      - fooyc 15y ago
        An other good example is constructors; say you have a function like this: function MyClass(x){ this.x = x; } In Coffeescript this translates to MyClass = (x) -> @x = x `x` is returned; and if you return something from a constructor, something is returned instead of the constructed object (`new MyClass({})` returns `{}`).
    - lmkg 15y ago
      > In my opinion, js functions should always return a value (even if it's just 'this'). Yeah, well, you know, that's just, like, your opinion, man. It's perfectly valid for JavaScript functions to not return something. Your opinion about always returning something (which I agree with, btw) is only a personal stylistic preference, and from the code I've seen in the wild, it's not really that widespread. As fooyc points out, the behavior of onClick events in the DOM changes depending on what return value is received. As I understand it, when an onClick executes a script, the truthiness of the value it returns determines whether the event continues to bubble up the DOM after the onClick has executed. At a guess, 75% of the onClick handler functions I've seen in the wild do not return a value; whether they realize it or not, all of those handlers implicitly rely on the appropriate truthiness of not returning a value for the rest of the page to behave correctly. Suddenly giving those functions a return value, especially one the developer didn't intentionally choose and didn't expect to have any effect, could cause quite a bit of unexpected behavior. This also seems like an easy issue to address. No return statement in the JavaScript? Add an empty return in the CoffeeScript.
walrus 15y ago
I don't understand why this is being downvoted. The compiler's behavior is definitely wrong here, as you showed with your example. This issue could even come up in practice. Suppose you have a checkbox with the id 'cb' in your DOM. Consider the following script: var list = [true, false, true, true, false]; document.getElementById('cb').onclick = function() {list.pop();}; When the checkbox is clicked, it should remove the last item in the list, then put a checkmark into the box. Run this through the Js2coffee compiler and you will get list = [ true, false, true, true, false ] document.getElementById("cb").onclick = -> list.pop() Run this back through the CoffeeScript compiler to get: (modified slightly for brevity) var list = [true, false, true, true, false]; document.getElementById("cb").onclick = function() {return list.pop();}; Notice that it now returns the result of list.pop(). Since the last item in the list is false, this will prevent the default browser behavior, so the checkbox will not be checked when it is clicked the first time.

It fails to return nothing here: e.onclick = function() { false; } // returns nothing Translates to: e.onclick = -> false // returns false

by fooyc 15y ago

It fails to return nothing here: e.onclick = function() { false; } // returns nothing Translates to: e.onclick = -> false // returns false

skeptical 15y ago

Just out of curiosity, how does one returns nothing in coffeescript?

endtwist 15y ago
fn = -> return

rgbrgb 15y ago

What's the purpose of the 'false;' statement? I believe CoffeeScript always returns the last line of a function.

sid0 15y ago
The purpose of the false statement is irrelevant. I'm generally not a big fan of perfection in programming, but compilers are one instance it's worth aiming for.
fooyc 15y ago
An example. This could be any statement evaluating to false. Coffee script returns the result of the last statement. Javascript don't. So, failing to take this into account produces coffeescript that do not have the same behavior than the original javascript. This is particularly important for iterators that return false for stopping iteration. Or for DOM event handlers that return false for stopping the propagation of the event.
- rgbrgb 15y ago
  While I totally agree with sid0 that it would be nice for 'academic' reasons if the compiler translated js->cs perfectly, I guess I'm having trouble understanding where this would actually come up as a problem. In my opinion, js functions should always return a value (even if it's just 'this'). Maybe a real code example of where your program would fail because of this?
  - fooyc 15y ago
    > would be nice for 'academic' reasons if the compiler translated js->cs perfectly You should expect this from a compiler, actually > In my opinion, js functions should always return a value Sounds wrong > real code example of where your program would fail because of this? Simple: // That's perfectly fine; why would I return anything from this ? link.onclick = function() { doSomething(); }; // Now, translated in coffeescript: link.onclick = -> return doSomething(); // if this returns false, the "click" event is canceled
    - fooyc 15y ago
      An other good example is constructors; say you have a function like this: function MyClass(x){ this.x = x; } In Coffeescript this translates to MyClass = (x) -> @x = x `x` is returned; and if you return something from a constructor, something is returned instead of the constructed object (`new MyClass({})` returns `{}`).
  - lmkg 15y ago
    > In my opinion, js functions should always return a value (even if it's just 'this'). Yeah, well, you know, that's just, like, your opinion, man. It's perfectly valid for JavaScript functions to not return something. Your opinion about always returning something (which I agree with, btw) is only a personal stylistic preference, and from the code I've seen in the wild, it's not really that widespread. As fooyc points out, the behavior of onClick events in the DOM changes depending on what return value is received. As I understand it, when an onClick executes a script, the truthiness of the value it returns determines whether the event continues to bubble up the DOM after the onClick has executed. At a guess, 75% of the onClick handler functions I've seen in the wild do not return a value; whether they realize it or not, all of those handlers implicitly rely on the appropriate truthiness of not returning a value for the rest of the page to behave correctly. Suddenly giving those functions a return value, especially one the developer didn't intentionally choose and didn't expect to have any effect, could cause quite a bit of unexpected behavior. This also seems like an easy issue to address. No return statement in the JavaScript? Add an empty return in the CoffeeScript.

walrus 15y ago

I don't understand why this is being downvoted. The compiler's behavior is definitely wrong here, as you showed with your example. This issue could even come up in practice. Suppose you have a checkbox with the id 'cb' in your DOM. Consider the following script: var list = [true, false, true, true, false]; document.getElementById('cb').onclick = function() {list.pop();}; When the checkbox is clicked, it should remove the last item in the list, then put a checkmark into the box. Run this through the Js2coffee compiler and you will get list = [ true, false, true, true, false ] document.getElementById("cb").onclick = -> list.pop() Run this back through the CoffeeScript compiler to get: (modified slightly for brevity) var list = [true, false, true, true, false]; document.getElementById("cb").onclick = function() {return list.pop();}; Notice that it now returns the result of list.pop(). Since the last item in the list is false, this will prevent the default browser behavior, so the checkbox will not be checked when it is clicked the first time.