From 2668637e9bad75bef016e7f8a5f481b3c6221891 Mon Sep 17 00:00:00 2001
From: Kitson Kelly <me@kitsonkelly.com>
Date: Fri, 29 May 2020 20:24:06 +1000
Subject: [PATCH] fix: REPL evaluates in strict mode (#5565)

Since everything that Deno loads is treated as an ES Module,
it means that all code is treated as "use strict" except for
when using the REPL. This PR changes that so code in the
REPL is also always evaluated with "use strict". There are
also a couple other places where we load code as scripts
which should also use "use strict" just in case.
---
 cli/js/repl.ts                   | 13 ++++++++++---
 cli/tests/integration_tests.rs   | 18 ++++++++++++++++++
 deno_typescript/compiler_main.js |  2 ++
 deno_typescript/system_loader.js |  2 ++
 4 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/cli/js/repl.ts b/cli/js/repl.ts
index 79273ed338..f38324d118 100644
--- a/cli/js/repl.ts
+++ b/cli/js/repl.ts
@@ -49,11 +49,18 @@ let lastThrownError: Value = undefined;
 // Returns true if code is consumed (no error/irrecoverable error).
 // Returns false if error is recoverable
 function evaluate(code: string): boolean {
-  const [result, errInfo] = core.evalContext(code);
+  // each evalContext is a separate function body, and we want strict mode to
+  // work, so we should ensure that the code starts with "use strict"
+  const [result, errInfo] = core.evalContext(`"use strict";\n\n${code}`);
   if (!errInfo) {
-    lastEvalResult = result;
+    // when a function is eval'ed with just "use strict" sometimes the result
+    // is "use strict" which should be discarded
+    lastEvalResult =
+      typeof result === "string" && result === "use strict"
+        ? undefined
+        : result;
     if (!isCloseCalled()) {
-      replLog(result);
+      replLog(lastEvalResult);
     }
   } else if (errInfo.isCompileError && isRecoverableError(errInfo.thrown)) {
     // Recoverable compiler error
diff --git a/cli/tests/integration_tests.rs b/cli/tests/integration_tests.rs
index 606a405169..f892b9a87d 100644
--- a/cli/tests/integration_tests.rs
+++ b/cli/tests/integration_tests.rs
@@ -693,6 +693,24 @@ fn repl_test_eof() {
   assert!(err.is_empty());
 }
 
+#[test]
+fn repl_test_strict() {
+  let (_, err) = util::run_and_collect_output(
+    true,
+    "repl",
+    Some(vec![
+      "let a = {};",
+      "Object.preventExtensions(a);",
+      "a.c = 1;",
+    ]),
+    None,
+    false,
+  );
+  assert!(err.contains(
+    "Uncaught TypeError: Cannot add property c, object is not extensible"
+  ));
+}
+
 const REPL_MSG: &str = "exit using ctrl+d or close()\n";
 
 #[test]
diff --git a/deno_typescript/compiler_main.js b/deno_typescript/compiler_main.js
index 31f539a275..847f3435f6 100644
--- a/deno_typescript/compiler_main.js
+++ b/deno_typescript/compiler_main.js
@@ -5,6 +5,8 @@
 // understood by the TypeScript language service, so it allows type safety
 // checking in VSCode.
 
+"use strict";
+
 const ASSETS = "$asset$";
 
 /**
diff --git a/deno_typescript/system_loader.js b/deno_typescript/system_loader.js
index 0004d055d6..fdf1fa8723 100644
--- a/deno_typescript/system_loader.js
+++ b/deno_typescript/system_loader.js
@@ -2,6 +2,8 @@
 
 // This is a specialised implementation of a System module loader.
 
+"use strict";
+
 // @ts-nocheck
 /* eslint-disable */
 let System, __instantiateAsync, __instantiate;