Save last execution result in for REPL (#2843)

js/repl.ts

@@ -25,6 +25,7 @@ function replError(...args: unknown[]): void {
 }
 
 const helpMsg = [
+  "_       Print last execution output",
   "exit    Exit the REPL",
   "help    Print this help message"
 ].join("\n");
@@ -71,10 +72,15 @@ function isRecoverableError(e: Error): boolean {
 
 // 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.
     replLog(result);
   } else if (errInfo.isCompileError && isRecoverableError(errInfo.thrown)) {
     // Recoverable compiler error
@@ -107,6 +113,9 @@ export async function replLoop(): Promise<void> {
     exit(exitCode);
   };
 
+  // Make _ a valid property on window to avoid confusing error.
+  window._ = undefined;
+
   while (true) {
     let code = "";
     // Top level read

tools/repl_test.py

@@ -58,6 +58,7 @@ def test_exit_command(self):
     def test_help_command(self):
         out, err, code = self.input("help")
         expectedOut = '\n'.join([
+            "_       Print last execution output",
             "exit    Exit the REPL",
             "help    Print this help message",
             "",
@@ -150,6 +151,12 @@ 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):
+        out, err, code = self.input("1", "_")
+        self.assertEqual(out, '1\n1\n')
+        self.assertEqual(err, '')
+        self.assertEqual(code, 0)
+
 
 if __name__ == "__main__":
     run_tests()