ability to use env vars in proxies origin with `${}`

README.md

@@ -112,6 +112,25 @@ For single page web applications like Ember, it's common to back the application
 }
 ```
 
+##### Interpolating Env Var Values
+It's common to want to be able to test the frontend against various backends. The `origin` key supports environment variable substitution using `${ENV_VAR_NAME}`. For instance, if there was a staging and production Heroku app for your API, you could setup the config above like the following:
+
+```json
+{
+  "proxies": {
+    "/api/": {
+      "origin": "https://${API_APP_NAME}.herokuapp.com/"
+    }
+  }
+}
+```
+
+Then using the [config vars](https://devcenter.heroku.com/articles/config-vars), you can point the frontend app to the appropriate backend. To match the original proxy setup:
+
+```bash
+$ heroku config:set API_APP_NAME="hone-ember-todo-rails"
+```
+
 #### Custom Headers
 Using the headers key, you can set custom response headers. It uses the same operators for pathing as [Custom Routes](#custom-routes).
 

scripts/config/lib/nginx_config.rb

@@ -20,11 +20,12 @@ class NginxConfig
     json["encoding"] ||= DEFAULT[:encoding]
     json["proxies"] ||= {}
     json["proxies"].each do |loc, hash|
-      if hash["origin"][-1] != "/"
-        json["proxies"][loc].merge!("origin" => hash["origin"] + "/")
+      evaled_origin = NginxConfigUtil.interpolate(hash['origin'], ENV)
+      if evaled_origin != "/"
+        json["proxies"][loc].merge!("origin" => evaled_origin + "/")
       end
 
-      uri = URI(hash["origin"])
+      uri = URI(evaled_origin)
       json["proxies"][loc]["path"] = uri.path
       uri.path = ""
       json["proxies"][loc]["host"] = uri.to_s

scripts/config/lib/nginx_config_util.rb

@@ -44,4 +44,16 @@ module NginxConfigUtil
 
     false
   end
+
+  def self.interpolate(string, vars)
+    regex = /\${(\w*?)}/
+
+    string.scan(regex).inject(string) do |acc, capture|
+      var_name = capture.first
+      value = vars[var_name] if vars
+      acc.sub!("${#{var_name}}", value) if value
+
+      acc
+    end
+  end
 end

spec/scripts/config/lib/nginx_config_util_spec.rb

@@ -16,4 +16,70 @@ RSpec.describe NginxConfigUtil do
       end
     end
   end
+
+  describe ".interpolate" do
+    context "single instance" do
+      let(:string) { "hello ${FOO}" }
+      let(:env) do
+        {
+          "FOO" => "world"
+        }
+      end
+      let(:result) { "hello world" }
+
+      it "should interpolate" do
+        expect(NginxConfigUtil.interpolate(string, env)).to eq(result)
+      end
+    end
+
+    context "multiple instances" do
+      let(:string) { "${FOO}, ${BAR}" }
+      let(:env) do
+        {
+          "FOO" => "hello",
+          "BAR" => "world"
+        }
+      end
+      let(:result) { "hello, world" }
+
+      it "should interpolate" do
+        expect(NginxConfigUtil.interpolate(string, env)).to eq(result)
+      end
+    end
+
+    context "instance not found" do
+      let(:string) { "${FOO}" }
+      let(:env)    { {} }
+      let(:result) { "${FOO}" }
+
+      it "should interpolate" do
+        expect(NginxConfigUtil.interpolate(string, env)).to eq(result)
+      end
+    end
+
+    context "vars is nil" do
+      let(:string) { "${FOO}" }
+      let(:env)    { nil }
+      let(:result) { "${FOO}" }
+
+      it "should interpolate" do
+        expect(NginxConfigUtil.interpolate(string, env)).to eq(result)
+      end
+    end
+
+    context "complex example" do
+      let(:string) { "${FOO} ${BAR} ${BAZ} ${FOO}" }
+      let(:env) do
+        {
+          "FOO" => "foo",
+          "BAZ" => "baz"
+        }
+      end
+      let(:result) { "foo ${BAR} baz foo" }
+
+      it "should interpolate" do
+        expect(NginxConfigUtil.interpolate(string, env)).to eq(result)
+      end
+    end
+  end
 end

spec/simple_spec.rb

@@ -196,6 +196,34 @@ STATIC_JSON
         expect(response.body.chomp).to eq("api")
       end
     end
+
+    context "env var substitution" do
+      before do
+        File.open(static_json_path, "w") do |file|
+          file.puts <<STATIC_JSON
+{
+  "proxies": {
+    "/api/": {
+      "origin": "http://${PROXY_HOST}/foo"
+    }
+  }
+}
+STATIC_JSON
+        end
+      end
+
+      let(:env) do
+        {
+          "PROXY_HOST" => "#{AppRunner::HOST_IP}:#{AppRunner::HOST_PORT}"
+        }
+      end
+
+      it "should proxy requests" do
+        response = app.get("/api/bar/")
+        expect(response.code).to eq("200")
+        expect(response.body.chomp).to eq("api")
+      end
+    end
   end
 
   describe "custom headers" do