Just like with most server side software we often want to make minor changes to the behaviour of the code depending on the environment it’s run in. This post highlights language and compiler features of ClojureScript making parameterized builds easy peasy.

On servers environment variables are a go-to solution to set things like a database URI. In ClojureScript we don’t have access to those. You can work around that with macros and emit code based on environment variables but this requires additional code and separate tools.

With ClojureScript 1.7.48 a new macro goog-define has been added which allows build customization at compile time using plain compiler options. Let’s walk through an example:

(ns your.app)
(goog-define api-uri "http://your.api.com")

goog-define emits code that looks something like this:

/** @define {string} */

The goog.define function from Closure’s standard library plus the JSDoc @define annotation tell the Closure compiler that your.app.api_uri is a constant that can be overridden at compile time. To do so you just need to pass the appropriate :closure-defines compiler option:

:closure-defines {'your.app/api-uri "http://your-dev.api.com"}
Prior to 1.7.48 you could annotate things with @define but without using goog.define overriding those defines is not possible when using optimizations :none effectively making them much less useful.

Under the hood

When compiling with :advanced optimizations the Closure compiler will automatically replace all occurrences of your defined constants with their respective values. If this leads to unreachable branches in your code they will be removed as dead code by the Closure compiler. Very handy to elide things like logging!

Without any optimizations (:none) goog.define makes sure the right value is used. There are two global variables it takes into account for that: CLOSURE_UNCOMPILED_DEFINES and CLOSURE_DEFINES. When you override the default value using :closure-defines the ClojureScript compiler prepends CLOSURE_UNCOMPILED_DEFINES with your overridden define to your build causing goog.define to use the value in there instead of the default value you defined in your source files.

For details see the source of goog.define.