Fix microsoft#10758 Add compiler option to parse in strict mode

 * add unit test to ensure "use strict" is not added twice
 * fix code

Author: slawomir

src/compiler/factory.ts

@@ -2235,6 +2235,36 @@ namespace ts {
         return statementOffset;
     }
 
+    /**
+     * Ensures "use strict" directive is added
+     * 
+     * @param node source file
+     */
+    export function ensureUseStrict(node: SourceFile): SourceFile {
+        let foundUseStrict = false;
+        let statementOffset = 0;
+        const numStatements = node.statements.length;
+        while (statementOffset < numStatements) {
+            const statement = node.statements[statementOffset];
+            if (isPrologueDirective(statement)) {
+                if (isUseStrictPrologue(statement as ExpressionStatement)) {
+                    foundUseStrict = true;
+                }
+            }
+            else {
+                break;
+            }
+            statementOffset++;
+        }
+        if (!foundUseStrict) {
+            const statements: Statement[] = [];
+            statements.push(startOnNewLine(createStatement(createLiteral("use strict"))));
+            // add "use strict" as the first statement
+            return updateSourceFileNode(node, statements.concat(node.statements));
+        }
+        return node;
+    }
+
     /**
      * Wraps the operand to a BinaryExpression in parentheses if they are needed to preserve the intended
      * order of operations.

src/compiler/transformers/ts.ts

@@ -438,7 +438,7 @@ namespace ts {
 
             // ensure "use strict"" is emitted in all scenarios in alwaysStrict mode
             if (compilerOptions.alwaysStrict) {
-                node = emitUseStrict(node);
+                node = ensureUseStrict(node);
             }
 
             // If the source file requires any helpers and is an external module, and
@@ -477,13 +477,6 @@ namespace ts {
             return node;
         }
 
-        function emitUseStrict(node: SourceFile): SourceFile {
-            const statements: Statement[] = [];
-            statements.push(startOnNewLine(createStatement(createLiteral("use strict"))));
-            // add "use strict" as the first statement
-            return updateSourceFileNode(node, statements.concat(node.statements));
-        }
-
         /**
          * Tests whether we should emit a __decorate call for a class declaration.
          */

tests/baselines/reference/alwaysStrictAlreadyUseStrict.js

@@ -0,0 +1,11 @@
+//// [alwaysStrictAlreadyUseStrict.ts]
+"use strict"
+function f() {
+    var a = [];
+}
+
+//// [alwaysStrictAlreadyUseStrict.js]
+"use strict";
+function f() {
+    var a = [];
+}

tests/baselines/reference/alwaysStrictAlreadyUseStrict.symbols

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/alwaysStrictAlreadyUseStrict.ts ===
+"use strict"
+function f() {
+>f : Symbol(f, Decl(alwaysStrictAlreadyUseStrict.ts, 0, 12))
+
+    var a = [];
+>a : Symbol(a, Decl(alwaysStrictAlreadyUseStrict.ts, 2, 7))
+}

tests/baselines/reference/alwaysStrictAlreadyUseStrict.types

@@ -0,0 +1,11 @@
+=== tests/cases/compiler/alwaysStrictAlreadyUseStrict.ts ===
+"use strict"
+>"use strict" : "use strict"
+
+function f() {
+>f : () => void
+
+    var a = [];
+>a : any[]
+>[] : undefined[]
+}

tests/cases/compiler/alwaysStrictAlreadyUseStrict.ts

@@ -0,0 +1,5 @@
+// @alwaysStrict: true
+"use strict"
+function f() {
+    var a = [];
+}