Execute hook

[matthewp]

A hook just prior to execution would be extremely helpful in scenarios where you need to alter the universe for a particular module. Consider something like this:

var execute = loader.execute;
loader.execute = function(key){
  var logger = loader.lookup("logger");

  var log = logger.log;
  logger.log = function(msg){
    console.log(key + ":" + msg);
  };

  var mod = execute.apply(this, arguments);
  logger.log = log;
  return mod;
};

In this example a logger module is redefined so that it prints out the key of the module using it.

This won't work if used async but it's still quite useful.

[MajorBreakfast]

System builder's behavior to not execute modules can be implemented by setting this hook to a NOP. What you're proposing is a generalized version instead of an on/off switch. Seems useful.
https://github.com/systemjs/builder/blob/a5d76f63e4362936363de4439274a61d116f486f/lib/builder.js#L69

The instantiate hook is very similar, though. Are both needed?

[matthewp]

Preventing executing for a trace is another use-case, yes.

Instantiate might be able to serve this use case. If you could return an execute function and this applies to dynamic and declarative modules, that would work. The spec is not filled out on instantiate though: http://whatwg.github.io/loader/#browser-instantiate

[caridy]

@matthewp I haven't look at this problem yet (I will), but at the first glance, there is a problem with your example, modifying the result of lookup is not allowed. Maybe you should simplify it to modify some runtime values instead of dealing with the module's dependencies.

[matthewp]

Ah, yes you're right. I'll try to come up with a better example.

[caridy]

@matthewp is this still valid? do you have a better example? Otherwise let's close this.

[matthewp]

I do not for the time being, let's close this and I'll reopen in the future if a good scenario comes up.