Support conditional compilation

On codeplex this was a popular feature request:

https://typescript.codeplex.com/workitem/111 https://typescript.codeplex.com/workitem/1926

Personally I think preprocessor directives like #if, #elif, #else #endif with possibility to specify symbol to compiler would be very useful. And a way to mark function (or ambient function declaration) as Conditional (something like ConditionalAattribute in C#) would be great improvement too. I have lot’s of console.log like function calls that decrease performance of my application and ability to easily remove all calls to this function would be great.

[NoelAbrahams]

Some use-cases:

• Variable defintion

// Production sources and keys        
   var foo = {
        root: "https://yyy.blob.core.foobar.net/",
        googlePlusKey: { id: "888888", key: "GIzdfBy" },
        facebookKey: { id: "444444444" }
     };

// Development sources and keys        
#if(DEBUG)
   var foo = {
        root: "https://xxx.blob.core.foobar.net/",
        googlePlusKey: { id: "458588", key: "BIzdfGy" },
        facebookKey: { id: "123219585123132" }
     };
#endif

• Import statement

#if(DEBUG)
       import foo = require('debug');
#else
       import foo = require('release');
#endif

function doFoo(){
    foo.someMethod();
}

• Class definition

#if(DEBUG)
       class Foo { doFoo(){ console.log('debug'); }  }
#else
       class Foo { doFoo(){ console.log('release'); }  }
#endif

var foo = new Foo();

[oliverjanik]

Here’s our use case. This feature would greatly simplify our lives.

We have a common code-base targeting the browsers and Cordova hybrid apps. Parts of our code base only deal with one or the other.

We would love if we could leave certain parts out for different builds. They also pull in different dependencies. So when building for the web we have to pull in Cordova dependencies otherwise TS complains about missing types.