wip: warn users of deprecated keys in app configurations

Signed-off-by: Colton Padden <colton.padden@fastmail.com>

packages/config/src/reader.ts

@@ -83,9 +83,27 @@ export class ConfigReader implements Config {
     // Merge together all configs into a single config with recursive fallback
     // readers, giving the first config object in the array the lowest priority.
     return configs.reduce<ConfigReader>(
-      (previousReader, { data, context, filteredKeys }) => {
+      (previousReader, { data, context, filteredKeys, deprecatedKeys }) => {
         const reader = new ConfigReader(data, context, previousReader);
         reader.filteredKeys = filteredKeys;
+
+        // TODO(cmpadden) introduce `deprecatedKeys` and `notifiedDeprecatedKeys` & use logger
+        if (deprecatedKeys) {
+          for (const { key, description } of deprecatedKeys) {
+            if (description) {
+              // eslint-disable-next-line no-console
+              console.warn(
+                `The configuration key '${key}' is deprecated and may be removed soon. (reason: ${description})`,
+              );
+            } else {
+              // eslint-disable-next-line no-console
+              console.warn(
+                `The configuration key '${key}' is deprecated and may be removed soon.`,
+              );
+            }
+          }
+        }
+
         return reader;
       },
       undefined!,

packages/config/src/types.ts

@@ -36,6 +36,12 @@ export type AppConfig = {
    * This can be used to warn the user if they try to read any of these keys.
    */
   filteredKeys?: string[];
+  /**
+   * A list of deprecated keys that were found in the  configuration when it was loaded.
+   *
+   * This can be used to warn the user if they are using deprecated properties.
+   */
+  deprecatedKeys?: { key: string; description: string }[];
 };
 
 /**