feat: calculate emissions from page speed results

Author: jonoliver

package-lock.json

@@ -22,7 +22,7 @@
     "node": ">=14.0.0"
   },
   "dependencies": {
+    "@tgwf/co2": "^0.16.9",
     "archiver": "^5.3.1"
-  },
-  "devDependencies": {}
-}
+  }
+}

package.json

@@ -0,0 +1,21 @@
+import { co2 } from "@tgwf/co2";
+
+/**
+ * Calculates the carbon emissions for a web page based on the results from Google's PageSpeed Insights (Lighthouse).
+ *
+ * @param {Object} pageSpeedResults - The PageSpeed Insights (Lighthouse) result object.
+ * @returns {Object} An object containing:
+ *   @property {number} emissionsOneByte - Estimated emissions CO2e (in grams) from the OneByte model.
+ *   @property {number} emissionsSWD - Estimated emissions CO2e (in grams) from the Sustainable Web Design model.
+ * TODO: This function currently returns emissions for both the OneByte and SWD models. Not sure which one we ultimately want to use.
+ */
+export const calculateEmissionsFromPageSpeedResults = (pageSpeedResults) => {
+  const totalByteWeight = pageSpeedResults.lighthouseResult.audits['total-byte-weight'].numericValue;
+
+  // Sustainable Web Design (SWD) model
+  // https://developers.thegreenwebfoundation.org/co2js/models/#using-the-sustainable-web-design-model-default-v0110
+  const swd = new co2({ model: "swd", version: 4 });
+  const emissionsPerByte = swd.perByte(totalByteWeight);
+
+  return { bytesTransferred: totalByteWeight, totalCO2: emissionsPerByte };
+}