feat: Add hook support.

src/HookRunner.js

@@ -0,0 +1,213 @@
+const UNKNOWN_HOOK_NAME = 'unknown hook';
+const BEFORE_EVALUATION_STAGE_NAME = 'beforeEvaluation';
+const AFTER_EVALUATION_STAGE_NAME = 'afterEvaluation';
+const BEFORE_IDENTIFY_STAGE_NAME = 'beforeIdentify';
+const AFTER_IDENTIFY_STAGE_NAME = 'afterIdentify';
+
+/**
+ * Safely executes a hook stage function, logging any errors.
+ * @param {{ error: (message: string) => void } | undefined} logger The logger instance.
+ * @param {string} method The name of the hook stage being executed (e.g., 'beforeEvaluation').
+ * @param {string} hookName The name of the hook.
+ * @param {() => any} stage The function representing the hook stage to execute.
+ * @param {any} def The default value to return if the stage function throws an error.
+ * @returns {any} The result of the stage function, or the default value if an error occurred.
+ */
+function tryExecuteStage(logger, method, hookName, stage, def) {
+  try {
+    return stage();
+  } catch (err) {
+    logger?.error(`An error was encountered in "${method}" of the "${hookName}" hook: ${err}`);
+    return def;
+  }
+}
+
+/**
+ * Safely gets the name of a hook from its metadata.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {{ getMetadata: () => { name?: string } }} hook The hook instance.
+ * @returns {string} The name of the hook, or 'unknown hook' if unable to retrieve it.
+ */
+function getHookName(logger, hook) {
+  try {
+    return hook.getMetadata().name || UNKNOWN_HOOK_NAME;
+  } catch {
+    logger.error(`Exception thrown getting metadata for hook. Unable to get hook name.`);
+    return UNKNOWN_HOOK_NAME;
+  }
+}
+
+/**
+ * Executes the 'beforeEvaluation' stage for all registered hooks.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {Array<{ beforeEvaluation?: (hookContext: object, data: object) => object }>} hooks The array of hook instances.
+ * @param {{ flagKey: string, context: object, defaultValue: any }} hookContext The context for the evaluation series.
+ * @returns {Array<object>} An array containing the data returned by each hook's 'beforeEvaluation' stage.
+ */
+function executeBeforeEvaluation(logger, hooks, hookContext) {
+  return hooks.map(hook =>
+    tryExecuteStage(
+      logger,
+      BEFORE_EVALUATION_STAGE_NAME,
+      getHookName(logger, hook),
+      () => hook?.beforeEvaluation?.(hookContext, {}) ?? {},
+      {}
+    )
+  );
+}
+
+/**
+ * Executes the 'afterEvaluation' stage for all registered hooks in reverse order.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {Array<{ afterEvaluation?: (hookContext: object, data: object, result: object) => object }>} hooks The array of hook instances.
+ * @param {{ flagKey: string, context: object, defaultValue: any }} hookContext The context for the evaluation series.
+ * @param {Array<object>} updatedData The data collected from the 'beforeEvaluation' stages.
+ * @param {{ value: any, variationIndex?: number, reason?: object }} result The result of the flag evaluation.
+ * @returns {void}
+ */
+function executeAfterEvaluation(logger, hooks, hookContext, updatedData, result) {
+  // This iterates in reverse, versus reversing a shallow copy of the hooks,
+  // for efficiency.
+  for (let hookIndex = hooks.length - 1; hookIndex >= 0; hookIndex -= 1) {
+    const hook = hooks[hookIndex];
+    const data = updatedData[hookIndex];
+    tryExecuteStage(
+      logger,
+      AFTER_EVALUATION_STAGE_NAME,
+      getHookName(logger, hook),
+      () => hook?.afterEvaluation?.(hookContext, data, result) ?? {},
+      {}
+    );
+  }
+}
+
+/**
+ * Executes the 'beforeIdentify' stage for all registered hooks.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {Array<{ beforeIdentify?: (hookContext: object, data: object) => object }>} hooks The array of hook instances.
+ * @param {{ context: object, timeout?: number }} hookContext The context for the identify series.
+ * @returns {Array<object>} An array containing the data returned by each hook's 'beforeIdentify' stage.
+ */
+function executeBeforeIdentify(logger, hooks, hookContext) {
+  return hooks.map(hook =>
+    tryExecuteStage(
+      logger,
+      BEFORE_IDENTIFY_STAGE_NAME,
+      getHookName(logger, hook),
+      () => hook?.beforeIdentify?.(hookContext, {}) ?? {},
+      {}
+    )
+  );
+}
+
+/**
+ * Executes the 'afterIdentify' stage for all registered hooks in reverse order.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {Array<{ afterIdentify?: (hookContext: object, data: object, result: object) => object }>} hooks The array of hook instances.
+ * @param {{ context: object, timeout?: number }} hookContext The context for the identify series.
+ * @param {Array<object>} updatedData The data collected from the 'beforeIdentify' stages.
+ * @param {{ status: string }} result The result of the identify operation.
+ * @returns {void}
+ */
+function executeAfterIdentify(logger, hooks, hookContext, updatedData, result) {
+  // This iterates in reverse, versus reversing a shallow copy of the hooks,
+  // for efficiency.
+  for (let hookIndex = hooks.length - 1; hookIndex >= 0; hookIndex -= 1) {
+    const hook = hooks[hookIndex];
+    const data = updatedData[hookIndex];
+    tryExecuteStage(
+      logger,
+      AFTER_IDENTIFY_STAGE_NAME,
+      getHookName(logger, hook),
+      () => hook?.afterIdentify?.(hookContext, data, result) ?? {},
+      {}
+    );
+  }
+}
+
+/**
+ * Factory function to create a HookRunner instance.
+ * Manages the execution of hooks for flag evaluations and identify operations.
+ * @param {{ error: (message: string) => void }} logger The logger instance.
+ * @param {Array<object> | undefined} initialHooks An optional array of hooks to initialize with.
+ * @returns {{
+ *  withEvaluation: (key: string, context: object, defaultValue: any, method: () => { value: any, variationIndex?: number, reason?: object }) => { value: any, variationIndex?: number, reason?: object },
+ *  identify: (context: object, timeout?: number) => (result: { status: string }) => void,
+ *  addHook: (hook: object) => void
+ * }} The hook runner object with methods to manage and execute hooks.
+ */
+function createHookRunner(logger, initialHooks) {
+  // Use local variable instead of instance property
+  const hooksInternal = initialHooks ? [...initialHooks] : [];
+
+  /**
+   * Wraps a flag evaluation method with before/after hook stages.
+   * @param {string} key The flag key.
+   * @param {object} context The evaluation context.
+   * @param {any} defaultValue The default value for the flag.
+   * @param {() => { value: any, variationIndex?: number, reason?: object }} method The function that performs the actual flag evaluation.
+   * @returns {{ value: any, variationIndex?: number, reason?: object }} The result of the flag evaluation.
+   */
+  function withEvaluation(key, context, defaultValue, method) {
+    if (hooksInternal.length === 0) {
+      return method();
+    }
+    const hooks = [...hooksInternal];
+    /** @type {{ flagKey: string, context: object, defaultValue: any }} */
+    const hookContext = {
+      flagKey: key,
+      context,
+      defaultValue,
+    };
+
+    // Use the logger passed into the factory
+    const hookData = executeBeforeEvaluation(logger, hooks, hookContext);
+    const result = method();
+    executeAfterEvaluation(logger, hooks, hookContext, hookData, result);
+    return result;
+  }
+
+  /**
+   * Wraps the identify operation with before/after hook stages.
+   * Executes the 'beforeIdentify' stage immediately and returns a function
+   * to execute the 'afterIdentify' stage later.
+   * @param {object} context The context being identified.
+   * @param {number | undefined} timeout Optional timeout for the identify operation.
+   * @returns {(result: { status: string }) => void} A function to call after the identify operation completes.
+   */
+  function identify(context, timeout) {
+    const hooks = [...hooksInternal];
+    /** @type {{ context: object, timeout?: number }} */
+    const hookContext = {
+      context,
+      timeout,
+    };
+    // Use the logger passed into the factory
+    const hookData = executeBeforeIdentify(logger, hooks, hookContext);
+    /**
+     * Executes the 'afterIdentify' hook stage.
+     * @param {{ status: string }} result The result of the identify operation.
+     */
+    return result => {
+      executeAfterIdentify(logger, hooks, hookContext, hookData, result);
+    };
+  }
+
+  /**
+   * Adds a new hook to the runner.
+   * @param {object} hook The hook instance to add.
+   * @returns {void}
+   */
+  function addHook(hook) {
+    // Mutate the internal hooks array
+    hooksInternal.push(hook);
+  }
+
+  return {
+    withEvaluation,
+    identify,
+    addHook,
+  };
+}
+
+module.exports = createHookRunner;