Cloudflare Workers - Build time constants with Wrangler

Replacing global identifiers with build-time constants has a few uses - inserting a Git hash or release number for using with Sentry SDKs or eliding away code that is only meant for development & staging environments.

Wrangler, the Cloudflare Workers command-line tool, exposes a few esbuild APIs that we can interact with. The two we’re interested in for the sake of this scenario are define and minify - let’s take a look.

define

The define feature allows us to replace a global identifier with a constant expression - let’s say replacing DEV with true or ENVIRONMENT with “staging”.

Let’s try replacing ENV with a string value of “dev”:

name = "test-worker"
main = "src/index.ts"
compatibility_date = "2023-01-11"

[define]
ENV = '"prod"'

[env.dev.define]
ENV = '"dev"'

The value of a key under the define table is the literal substitution - meaning, if you wish to substitute ENV with a string value of “dev” then you would write ENV = ‘“dev”’ - as otherwise it would be replaced with a variable called dev which would of course be undefined.

declare const ENV: string

export default <ExportedHandler> {
  async fetch() {
    if (ENV === "dev") {
      console.log("i'm dev")
    }

    if (ENV === "staging") {
      console.log("i'm staging")
    }

    return new Response();
  },
};

// src/index.ts
var src_default = {
  async fetch() {
    if (true) {
      console.log("i'm dev");
    }
    if (false) {
      console.log("i'm staging");
    }
    return new Response();
  }
};
export {
  src_default as default
};
//# sourceMappingURL=index.js.map

This is great - but the if (false) branch is dead code since that condition will never be true. Can we get rid of it?

minify

Of course we can! If we add —minify to our Wrangler dry-run publish, we’ll see that the if (false) branch has been elided away.

var e={async fetch(){return console.log("i'm dev"),new Response}};export{e as default};
//# sourceMappingURL=index.js.map

This can also be added to your Wrangler.toml on a per-environment basis, such as minifying production code but not development, with the minify = true|false property.