Added ability to add specific node guard to the black list of nodes.

This nodes wont obfuscated.

test/functional-tests/javascript-obfuscator/JavaScriptObfuscator.spec.ts

@@ -439,6 +439,40 @@ describe('JavaScriptObfuscator', () => {
             });
         });
 
+        describe('`\'use strict\';` operator', () => {
+            const useStrictOperatorRegExp: RegExp = /'use *strict';/;
+            const stringArrayLatinRegExp: RegExp = /var _0x(\w){4} *= *\['abc'\];/;
+            const stringArrayCallRegExp: RegExp = /var *test *= *_0x(\w){4}\('0x0'\);$/;
+
+            let obfuscatedCode: string;
+
+            beforeEach(() => {
+                const code: string = readFileAsString(__dirname + '/fixtures/use-strict-operator.js');
+                const obfuscationResult: IObfuscationResult = JavaScriptObfuscator.obfuscate(
+                    code,
+                    {
+                        ...NO_CUSTOM_NODES_PRESET,
+                        stringArray: true,
+                        stringArrayThreshold: 1
+                    }
+                );
+
+                obfuscatedCode = obfuscationResult.getObfuscatedCode();
+            });
+
+            it('match #1: shouldn\'t obfuscate `use strict` operator', () => {
+                assert.match(obfuscatedCode, useStrictOperatorRegExp);
+            });
+
+            it('match #2: should return correct obfuscated code', () => {
+                assert.match(obfuscatedCode, stringArrayLatinRegExp);
+            });
+
+            it('match #3: should return correct obfuscated code', () => {
+                assert.match(obfuscatedCode, stringArrayCallRegExp);
+            });
+        });
+
         describe('mangle', () => {
             const regExp: RegExp = /var *a *= *0x1/;
 

test/functional-tests/javascript-obfuscator/fixtures/use-strict-operator.js

@@ -0,0 +1,3 @@
+'use strict';
+
+var test = 'abc';