diff --git a/src/operator.test.ts b/src/operator.test.ts
new file mode 100644
index 0000000..13879da
--- /dev/null
+++ b/src/operator.test.ts
@@ -0,0 +1,21 @@
+import { isStandardFunction, standardFunctions } from './operators';
+
+describe('Operators tests', () => {
+  describe('isStandardFunction', () => {
+    it('should return true for standard functions', () => {
+      expect(Object.keys(standardFunctions).every(isStandardFunction)).toBeTruthy();
+    });
+    it('should return false for non standard function', () => {
+      const nonStandardFunctions = [
+        'toString',
+        'valueOf',
+        'toLocaleString',
+        'hasOwnProperty',
+        'isPrototypeOf',
+        'propertyIsEnumerable',
+        'constructor',
+      ];
+      expect(Object.keys(nonStandardFunctions).every(isStandardFunction)).toBeFalsy();
+    });
+  });
+});
diff --git a/src/operators.ts b/src/operators.ts
index 31fde54..bcb3b3c 100644
--- a/src/operators.ts
+++ b/src/operators.ts
@@ -179,3 +179,7 @@ export const standardFunctions = {
    }`,
   keys: 'function keys(obj) { return Object.keys(obj); }',
 };
+
+export function isStandardFunction(name: string): boolean {
+  return Object.prototype.hasOwnProperty.call(standardFunctions, name);
+}
diff --git a/src/translator.ts b/src/translator.ts
index 5842b99..cb6dc35 100644
--- a/src/translator.ts
+++ b/src/translator.ts
@@ -7,7 +7,7 @@ import {
   VARS_PREFIX,
 } from './constants';
 import { JsonTemplateTranslatorError } from './errors';
-import { binaryOperators, standardFunctions } from './operators';
+import { binaryOperators, isStandardFunction, standardFunctions } from './operators';
 import {
   ArrayExpression,
   AssignmentExpression,
@@ -514,7 +514,7 @@ export class JsonTemplateTranslator {
     }
     const functionArgsStr = this.translateSpreadableExpressions(expr.args, result, code);
     const functionName = this.getFunctionName(expr, result);
-    if (expr.id && standardFunctions[expr.id]) {
+    if (expr.id && isStandardFunction(expr.id)) {
       this.standardFunctions[expr.id] = standardFunctions[expr.id];
       code.push(`if(${functionName} && typeof ${functionName} === 'function'){`);
       code.push(result, '=', functionName, '(', functionArgsStr, ');');
diff --git a/test/scenarios/functions/data.ts b/test/scenarios/functions/data.ts
index e582341..556b19e 100644
--- a/test/scenarios/functions/data.ts
+++ b/test/scenarios/functions/data.ts
@@ -15,7 +15,7 @@ export const data: Scenario[] = [
   },
   {
     templatePath: 'js_date_function.jt',
-    output: [2022, 8, 19],
+    output: ['2022', 8, 19],
   },
   {
     templatePath: 'new_operator.jt',
diff --git a/test/scenarios/functions/js_date_function.jt b/test/scenarios/functions/js_date_function.jt
index 2f1b301..93b2e75 100644
--- a/test/scenarios/functions/js_date_function.jt
+++ b/test/scenarios/functions/js_date_function.jt
@@ -1,2 +1,2 @@
 const date = new Date('2022-08-19');
-[date.getFullYear(), date.getMonth() + 1, date.getDate()];
+[date.getFullYear().toString(), date.getMonth() + 1, date.getDate()];