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.

4 files changed, 32 insertions, 3 deletions

diff --git a/cli/js/repl.ts b/cli/js/repl.ts
index 79273ed33..f38324d11 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 606a40516..f892b9a87 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 31f539a27..847f3435f 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 0004d055d..fdf1fa872 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;