fix standalone block statement scoping (#75)

Author: yyc

src/__tests__/__snapshots__/block_scoping.ts.snap

@@ -38,6 +38,13 @@ Object {
 }
 `;
 
+exports[`standalone block statements 1`] = `
+Object {
+  "status": "finished",
+  "value": true,
+}
+`;
+
 exports[`while loops use block scoping instead of function scoping 1`] = `
 Object {
   "status": "finished",

src/__tests__/block_scoping.ts

@@ -2,6 +2,27 @@ import { mockContext } from "../mocks/context";
 import { parseError, runInContext } from "../index";
 import { Finished } from "../types";
 
+// This is bad practice. Don't do this!
+test("standalone block statements", () => {
+  const code = `
+    function test(){
+      const x = true;
+      {
+          const x = false;
+      }
+      return x;
+    }
+    test();
+  `;
+  const context = mockContext();
+  const promise = runInContext(code, context, { scheduler: "preemptive" });
+  return promise.then(obj => {
+    expect(obj).toMatchSnapshot();
+    expect(obj.status).toBe("finished");
+    expect((obj as Finished).value).toBe(true);
+  });
+});
+
 // This is bad practice. Don't do this!
 test("const uses block scoping instead of function scoping", () => {
   const code = `

src/interpreter.ts

@@ -425,18 +425,11 @@ export const evaluators: { [nodeType: string]: Evaluator<es.Node> } = {
       handleError(context, error)
       return undefined
     }
-
-    // Create a new frame (block scoping)
-    const frame = createBlockFrame(context, "ifBlockFrame")
-    pushFrame(context, frame)
-
     if (test) {
       const result = yield* evaluate(node.consequent, context)
-      popFrame(context)
       return result
     } else if (node.alternate) {
       const result = yield* evaluate(node.alternate, context)
-      popFrame(context)
       return result
     } else {
       return undefined
@@ -468,13 +461,7 @@ export const evaluators: { [nodeType: string]: Evaluator<es.Node> } = {
       !(value instanceof BreakValue) &&
       !(value instanceof TailCallReturnValue)
     ) {
-      // Create a new frame (block scoping)
-      const frame = createBlockFrame(context, "whileBlockFrame")
-      pushFrame(context, frame)
-
       value = yield* evaluate(node.body, context)
-
-      popFrame(context)
     }
     if (value instanceof BreakValue) {
       return undefined
@@ -496,6 +483,11 @@ export const evaluators: { [nodeType: string]: Evaluator<es.Node> } = {
   },
   BlockStatement: function*(node: es.BlockStatement, context: Context) {
     let result: Value
+
+    // Create a new frame (block scoping)
+    const frame = createBlockFrame(context, "blockFrame")
+    pushFrame(context, frame)
+
     for (const statement of node.body) {
       result = yield* evaluate(statement, context)
       if (
@@ -506,6 +498,7 @@ export const evaluators: { [nodeType: string]: Evaluator<es.Node> } = {
         break
       }
     }
+    popFrame(context)
     return result
   },
   Program: function*(node: es.BlockStatement, context: Context) {