Fix REPL '_' assignment, support '_error'

js/repl.ts

@@ -25,7 +25,8 @@ function replError(...args: unknown[]): void {
 }
 
 const helpMsg = [
-  "_       Print last execution output",
+  "_       Get last evaluation result",
+  "_error  Get last thrown error",
   "exit    Exit the REPL",
   "help    Print this help message"
 ].join("\n");
@@ -70,22 +71,25 @@ function isRecoverableError(e: Error): boolean {
   return recoverableErrorMessages.includes(e.message);
 }
 
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type Value = any;
+
+let lastEvalResult: Value = undefined;
+let lastThrownError: Value = undefined;
+
 // Evaluate code.
 // Returns true if code is consumed (no error/irrecoverable error).
-// Also attempts setting window._ to last valid execute output.
 // Returns false if error is recoverable
 function evaluate(code: string): boolean {
   const [result, errInfo] = core.evalContext(code);
   if (!errInfo) {
-    // Try setting `window._` to the returned result
-    try {
-      window._ = result;
-    } catch {} // Silently fail on error.
+    lastEvalResult = result;
     replLog(result);
   } else if (errInfo.isCompileError && isRecoverableError(errInfo.thrown)) {
     // Recoverable compiler error
     return false; // don't consume code.
   } else {
+    lastThrownError = errInfo.thrown;
     if (errInfo.isNativeError) {
       const formattedError = formatError(
         core.errorToJSON(errInfo.thrown as Error)
@@ -113,8 +117,35 @@ export async function replLoop(): Promise<void> {
     exit(exitCode);
   };
 
-  // Make _ a valid property on window to avoid confusing error.
-  window._ = undefined;
+  // Configure window._ to give the last evaluation result.
+  Object.defineProperty(window, "_", {
+    configurable: true,
+    get: (): Value => lastEvalResult,
+    set: (value: Value): Value => {
+      Object.defineProperty(window, "_", {
+        value: value,
+        writable: true,
+        enumerable: true,
+        configurable: true
+      });
+      console.log("Last evaluation result is no longer saved to _.");
+    }
+  });
+
+  // Configure window._error to give the last thrown error.
+  Object.defineProperty(window, "_error", {
+    configurable: true,
+    get: (): Value => lastThrownError,
+    set: (value: Value): Value => {
+      Object.defineProperty(window, "_error", {
+        value: value,
+        writable: true,
+        enumerable: true,
+        configurable: true
+      });
+      console.log("Last thrown error is no longer saved to _error.");
+    }
+  });
 
   while (true) {
     let code = "";

tools/repl_test.py

@@ -58,7 +58,8 @@ def test_exit_command(self):
     def test_help_command(self):
         out, err, code = self.input("help")
         expectedOut = '\n'.join([
-            "_       Print last execution output",
+            "_       Get last evaluation result",
+            "_error  Get last thrown error",
             "exit    Exit the REPL",
             "help    Print this help message",
             "",
@@ -151,12 +152,32 @@ def test_missing_deno_dir(self):
         self.assertTrue(err.startswith("Unable to save REPL history:"))
         self.assertEqual(code, 0)
 
-    def test_save_last_output(self):
+    def test_save_last_eval(self):
         out, err, code = self.input("1", "_")
         self.assertEqual(out, '1\n1\n')
         self.assertEqual(err, '')
         self.assertEqual(code, 0)
 
+    def test_save_last_thrown(self):
+        out, err, code = self.input("throw 1", "_error")
+        self.assertEqual(out, '1\n')
+        self.assertEqual(err, 'Thrown: 1\n')
+        self.assertEqual(code, 0)
+
+    def test_assign_underscore(self):
+        out, err, code = self.input("_ = 1", "2", "_")
+        self.assertEqual(
+            out, 'Last evaluation result is no longer saved to _.\n1\n2\n1\n')
+        self.assertEqual(err, '')
+        self.assertEqual(code, 0)
+
+    def test_assign_underscore_error(self):
+        out, err, code = self.input("_error = 1", "throw 2", "_error")
+        self.assertEqual(
+            out, 'Last thrown error is no longer saved to _error.\n1\n1\n')
+        self.assertEqual(err, 'Thrown: 2\n')
+        self.assertEqual(code, 0)
+
 
 if __name__ == "__main__":
     run_tests()