repl: fix repl crashing on variable declarations without init

PR-URL: #59032
Fixes: #59029
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Matthew Aitken <maitken033380023@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: dario-piotrowicz

lib/repl.js

@@ -1735,6 +1735,10 @@ function findExpressionCompleteTarget(code) {
   // what we can potentially complete on, so let's re-run the function's logic on that
   if (lastBodyStatement.type === 'VariableDeclaration') {
     const lastDeclarationInit = lastBodyStatement.declarations.at(-1).init;
+    if (!lastDeclarationInit) {
+      // If there is no initialization we can simply return
+      return null;
+    }
     const lastDeclarationInitCode = code.slice(lastDeclarationInit.start, lastDeclarationInit.end);
     return findExpressionCompleteTarget(lastDeclarationInitCode);
   }

test/parallel/test-repl-tab-complete.js

@@ -50,6 +50,12 @@ function prepareREPL() {
 }
 
 describe('REPL tab completion (core functionality)', () => {
+  it('does not break with variable declarations without an initialization', () => {
+    const { replServer } = prepareREPL();
+    replServer.complete('let a', getNoResultsFunction());
+    replServer.close();
+  });
+
   it('does not break in an object literal', () => {
     const { replServer, input } = prepareREPL();