Use of "has" to check in-code classes

Thanks Peter!  You are right - self works in my case. Forgot to try it. 

It depends on the scope that the symbol (class/function/variable/constant) is declared in. Typically users put symbols into the global scope ($), so the has check would look like this:

if ($ has :symbol) {
    // symbol is declared at global scope
}

If the symbol is inside a module, you'd reference that module, just like we do with Toybox in the example:

if (UserModule has :symbol) {
    // UserModule.symbol exists
}

You could also combine these and write:

if ($.UserModule has :symbol) {
    // UserModule.symbol exists
}

As mentioned by peterdedecker, you can also use instance variables on the left side of the has check. Keeping in mind that self is a keyword that represents the current object:

if (self has :symbol) {
    // self.symbol exists
}

THX. @travis.vitek 

    //Curious about the differect result  
  function getTemperature() {
    var b = $ has :getTemperatureData;//The function is excluded
    if (b) {
      return getTemperatureData();//Undefined symbol ':getTemperatureData' detected
    } else {
      return "N";
    }
  }
  
    function getTemperature() {//It works 
    if ($ has :getTemperatureData) {//The function is excluded
      return getTemperatureData();
    } else {
      return "N";
    }
  }

I don’t often work on the compiler, but my assumption is that the compiler only considers cases with constant expressions when determining that code is unreachable. This is more in Marianne.ConnectIQ’s area. Perhaps she can step in.

I think the big thing right now is that if you build with optimizations on, the has check would become a constant, and then the branch elimination optimization would remove the condition check entirely in the former case.

I’d like to think that you capture the result of the has expression in a const, we’d be able to do the branch elimination in that case too, but my guess is that we do not.

Travis.ConnectIQ said:

I think the big thing right now is that if you build with optimizations on,

I didn't turn on the optimization. Perhaps it is an automatical one in branch prediction.

Yes. I created a "const b = $ has :getBodyBatteryInfo;" in current class. And used  "b" as the condition, it worked. 

Travis.ConnectIQ said:

I’d like to think that you capture the result of the has expression in a const, we’d be able to do the branch elimination in that case too, but my guess is that we do not.

i.e. Your compiler optimization won't work for context actually cuz it won't know if the variable will change.

Well. "const" is not friendly to memory. I added 5 "const" but to find the memory increase by 0.2kB. So I am going to use "has" as before.

li2niu🏃🏻 said:

i.e. Your compiler optimization won't work for context actually cuz it won't know if the variable will change.

The result of the expression $ has :getTemperatureData() cannot change at runtime. Either the global function is defined or it is not.

li2niu🏃🏻 said:

I added 5 "const" but to find the memory increase by 0.2kB

I'd expect the optimization pass to be able to eliminate const values entirely, but maybe I'm expecting too much.