fix let without assignment

src/eval.js

@@ -1015,7 +1015,12 @@ const eval_statement = (s, scope, calls, context) => {
         } 
         if(stmt.type == 'let' && s.type == 'identifier') {
           const node = {...s, result: {ok: true}}
-          return {ok, children: [...children, node], scope, calls}
+          return {
+            ok, 
+            children: [...children, node], 
+            scope: {...scope, [s.value]: undefined},
+            calls
+          }
         }
         const {
           ok: next_ok, 

test/test.js

@@ -329,6 +329,16 @@ export const tests = [
     assert_equal(i.value_explorer.result.value, {y: 2, z: 3, q: 4})
   }),
 
+  test('let variable not initialized bug', () => {
+    const code = `
+      let x
+      x /*label*/
+    `
+    const i = test_initial_state(code, code.indexOf('x /*label'))
+    assert_equal(i.value_explorer.result.ok, true)
+    assert_equal(i.value_explorer.result.value === undefined, true)
+  }),
+
   test('else if', () => {
     const code = `
       let x