[EC-473] Add feature flags to common code (#3324)

libs/common/src/misc/flags.ts

@@ -0,0 +1,61 @@
+// required to avoid linting errors when there are no flags
+/* eslint-disable @typescript-eslint/ban-types */
+export type SharedFlags = {};
+
+// required to avoid linting errors when there are no flags
+/* eslint-disable @typescript-eslint/ban-types */
+export type SharedDevFlags = {};
+
+function getFlags<T>(envFlags: string | T): T {
+  if (typeof envFlags === "string") {
+    return JSON.parse(envFlags) as T;
+  } else {
+    return envFlags as T;
+  }
+}
+
+/**
+ * Gets the value of a feature flag from environment.
+ * All flags default to "on" (true).
+ * Only use for shared code in `libs`, otherwise use the client-specific function.
+ * @param flag The name of the feature flag to check
+ * @returns The value of the flag
+ */
+export function flagEnabled<Flags extends SharedFlags>(flag: keyof Flags): boolean {
+  const flags = getFlags<Flags>(process.env.FLAGS);
+  return flags[flag] == null || !!flags[flag];
+}
+
+/**
+ * Gets the value of a dev flag from environment.
+ * Will always return false unless in development.
+ * Only use for shared code in `libs`, otherwise use the client-specific function.
+ * @param flag The name of the dev flag to check
+ * @returns The value of the flag
+ */
+export function devFlagEnabled<DevFlags extends SharedDevFlags>(flag: keyof DevFlags): boolean {
+  if (process.env.ENV !== "development") {
+    return false;
+  }
+
+  const devFlags = getFlags<DevFlags>(process.env.DEV_FLAGS);
+  return devFlags[flag] == null || !!devFlags[flag];
+}
+
+/**
+ * Gets the value of a dev flag from environment.
+ * Will always return false unless in development.
+ * @param flag The name of the dev flag to check
+ * @returns The value of the flag
+ * @throws Error if the flag is not enabled
+ */
+export function devFlagValue<DevFlags extends SharedDevFlags>(
+  flag: keyof DevFlags
+): DevFlags[keyof DevFlags] {
+  if (!devFlagEnabled(flag)) {
+    throw new Error(`This method should not be called, it is protected by a disabled dev flag.`);
+  }
+
+  const devFlags = getFlags<DevFlags>(process.env.DEV_FLAGS);
+  return devFlags[flag];
+}