Add ENVIRONMENT to specify which environment the code will run in

[kripken]

By default we still emit code that can run in node, web, shell, etc., but with the option the user can fix only one to be emitted.

This also sets ENVIRONMENT to "web" if the user specifies an HTML output (emcc [..] -o code.html). In that case the indication is clear that the code should only run on the web. This is technically a breaking change though, as people may have tried to run the .js file outside of the web, and that may have worked in some cases. To prevent confusion, when ASSERTIONS are on we show an error if we hit a problem, and also in debug logging we mention doing HTML output setting the environment that way. The benefit to doing this is that it gets all the benefits of web-only emitting in a natural way for the main use case.

For more details on the benefits, see #6542 #1140, but basically

• When targeteting only web, we don't emit node.js support code which includes require() operations that confuse bundlers.
• We emit less unneeded code.

[jgravelle-google]

Nice overall

[jgravelle-google]

Is it profitable to move the if (ENVIRONMENT_IS_NODE) { part inside the #if? Our expectation is that if environment is set to not-X, then it's not expected to work if it is X, so we shouldn't need to guard against it. (As I understand it, if the environment is node, we have an empty if body here, so this shouldn't work anyway).

Are we able to optimize out the if with the JS Optimizer so it doesn't matter either way?

[kripken]

Moving it outside was a little messier - then there is the if-else chain to handle (where to put elses, that is, if we have if (..) { } else if (..) then we can't remove the first if until the else, we also need to remove the else).

The JS optimizer is not capable of doing this, however, closure is, so for really-optimized builds it shouldn't matter. Would be nice to find a clean way to make it happen without closure too, at low priority I'd say.

[jgravelle-google]

How about:

#if A
  if (A) {
    //...
  } else
#endif // A
#if B
  if (B) {
    //...
  } else
#endif // B
#if C
  // ...
#endif // Q
  {
    //...
  }

[kripken]

Good idea, rewritten.

[surma]

Impressed to see this! Happy with it.

Thanks for the quick turnaround.