repl: avoid crash when typing incomplete import statement

lib/repl.js

@@ -247,10 +247,19 @@ fixReplRequire(module);
 const writer = (obj) => inspect(obj, writer.options);
 writer.options = { ...inspect.defaultOptions, showProxy: true };
 
-// Converts static import statement to dynamic import statement
+// Converts static import statement to dynamic import statement.
+// Returns an empty string if `codeLine` cannot be parsed as a module — this
+// happens when the user typed something that triggers V8's "Cannot use import
+// statement outside a module" but isn't a complete, parseable import statement
+// (e.g. `import` alone in the REPL).
 const toDynamicImport = (codeLine) => {
   let dynamicImportStatement = '';
-  const ast = acornParse(codeLine, { __proto__: null, sourceType: 'module', ecmaVersion: 'latest' });
+  let ast;
+  try {
+    ast = acornParse(codeLine, { __proto__: null, sourceType: 'module', ecmaVersion: 'latest' });
+  } catch {
+    return '';
+  }
   acornWalk.ancestor(ast, {
     ImportDeclaration(node) {
       const awaitDynamicImport = `await import(${JSONStringify(node.source.value)});`;
@@ -1036,8 +1045,12 @@ class REPLServer extends Interface {
             const importErrorStr = 'Cannot use import statement outside a ' +
               'module';
             if (StringPrototypeIncludes(e.message, importErrorStr)) {
-              e.message = 'Cannot use import statement inside the Node.js ' +
-                'REPL, alternatively use dynamic import: ' + toDynamicImport(ArrayPrototypeAt(this.lines, -1));
+              const dynamicImport =
+                toDynamicImport(ArrayPrototypeAt(this.lines, -1));
+              e.message = dynamicImport ?
+                'Cannot use import statement inside the Node.js REPL, ' +
+                  `alternatively use dynamic import: ${dynamicImport}` :
+                'Cannot use import statement inside the Node.js REPL.';
               e.stack = SideEffectFreeRegExpPrototypeSymbolReplace(
                 /SyntaxError:.*\n/,
                 e.stack,

test/parallel/test-repl-import-statement-no-crash.js

@@ -0,0 +1,31 @@
+'use strict';
+// Regression test for https://github.com/nodejs/node/issues/63551:
+// Typing a bare `import` keyword in the REPL used to terminate the process.
+//
+// V8 throws SyntaxError("Cannot use import statement outside a module"),
+// which the REPL enriched with a dynamic-import suggestion via
+// `toDynamicImport`. That helper called acorn without a try/catch; on
+// incomplete input acorn threw, and the exception escaped the REPL's input
+// pipeline to crash the process.
+//
+// The fix wraps the acorn call in try/catch and falls back to a plain
+// error message when the line cannot be parsed as a complete import
+// statement. This test asserts that:
+//   1. Emitting an incomplete `import` line through the REPL does not
+//      throw out of the line-handler, and
+//   2. The REPL still surfaces a SyntaxError to the user.
+
+require('../common');
+const assert = require('assert');
+const { startNewREPLServer } = require('../common/repl');
+
+const { replServer, output } = startNewREPLServer();
+
+replServer.emit('line', 'import');
+replServer.emit('line', '.exit');
+
+assert.match(output.accumulator, /SyntaxError/);
+assert.match(
+  output.accumulator,
+  /Cannot use import statement inside the Node\.js REPL\b/,
+);