Forbid non-null assertion in '.js' files (#17481)

src/compiler/checker.ts

@@ -16384,6 +16384,10 @@ namespace ts {
         }
 
         function checkNonNullAssertion(node: NonNullExpression) {
+            if (isInJavaScriptFile(node)) {
+                error(node, Diagnostics.non_null_assertions_can_only_be_used_in_a_ts_file);
+            }
+
             return getNonNullableType(checkExpression(node.expression));
         }
 

src/compiler/diagnosticMessages.json

@@ -3459,6 +3459,10 @@
         "category": "Error",
         "code": 8012
     },
+    "'non-null assertions' can only be used in a .ts file.": {
+        "category": "Error",
+        "code": 8013
+    },
     "'enum declarations' can only be used in a .ts file.": {
         "category": "Error",
         "code": 8015

tests/baselines/reference/jsFileCompilationNonNullAssertion.errors.txt

@@ -0,0 +1,8 @@
+/src/a.js(1,1): error TS8013: 'non-null assertions' can only be used in a .ts file.
+
+
+==== /src/a.js (1 errors) ====
+    0!
+    ~~
+!!! error TS8013: 'non-null assertions' can only be used in a .ts file.
+    

tests/baselines/reference/jsFileCompilationNonNullAssertion.js

@@ -0,0 +1,6 @@
+//// [a.js]
+0!
+
+
+//// [a.js]
+0;

tests/cases/compiler/jsFileCompilationNonNullAssertion.ts

@@ -0,0 +1,5 @@
+// @allowJs: true
+// @checkJs: true
+// @filename: /src/a.js
+// @out: /bin/a.js
+0!