diff --git a/src/_DataStructures_/Stack/postfix-expression-evaluation/index.js b/src/_DataStructures_/Stack/postfix-expression-evaluation/index.js
index 9db393e9..0a683aae 100644
--- a/src/_DataStructures_/Stack/postfix-expression-evaluation/index.js
+++ b/src/_DataStructures_/Stack/postfix-expression-evaluation/index.js
@@ -6,39 +6,56 @@
 
 const Stack = require('../index');
 
+const ERROR_STRING = 'Expression is not in order';
+
 function evaluatePostfixExpression(expression) {
-    let s = new Stack();
-    for (let i = 0; i < expression.length; i++) {
-        const char = expression[i];
-        if (!isNaN(char)) {
-            //if number push the char onto stack
-            s.push(Number(char));
-        } else {
-            // if char is an operator then pop two elements from stack, evaluate them accordingly based on operator.
-            //push the result to stack
-            let val1 = s.pop();
-            let val2 = s.pop();
-            switch (char) {
-                case '+':
-                    s.push(val2 + val1);
-                    break;
-                case '-':
-                    s.push(val2 - val1);
-                    break;
-                case '*':
-                    s.push(val2 * val1);
-                    break;
-                case '/':
-                    s.push(val2 / val1);
-                    break;
+  // eslint-disable-next-line no-param-reassign
+  expression = expression.trim();
+
+  if (expression.length === 0 || expression.length === 1) {
+    throw new Error(ERROR_STRING);
+  }
 
-            }
-        }
+  const s = new Stack();
+  // eslint-disable-next-line no-plusplus
+  for (let i = 0; i < expression.length; i++) {
+    const char = expression[i];
+    // eslint-disable-next-line no-restricted-globals
+    if (!isNaN(char)) {
+      // if number push the char onto stack
+      s.push(Number(char));
+    } else {
+      // if char is an operator then pop two elements from stack, evaluate them accordingly based on operator.
+      // push the result to stack
+      const val1 = s.pop();
+      const val2 = s.pop();
+      switch (char) {
+        case '+':
+          s.push(val2 + val1);
+          break;
+        case '-':
+          s.push(val2 - val1);
+          break;
+        case '*':
+          s.push(val2 * val1);
+          break;
+        case '/':
+          s.push(val2 / val1);
+          break;
+        default:
+          break;
+      }
     }
-    //pop the value of postfix expression
-    return s.pop();
+  }
+  // pop the value from stack
+  const result = s.pop();
+  if (s.isEmpty()) {
+    return result;
+  }
+  throw new Error(ERROR_STRING);
 }
 
 module.exports = {
   evaluatePostfixExpression,
+  ERROR_STRING,
 };
diff --git a/src/_DataStructures_/Stack/postfix-expression-evaluation/postfix-expression-evaluation.test.js b/src/_DataStructures_/Stack/postfix-expression-evaluation/postfix-expression-evaluation.test.js
index 47e0de42..25da462f 100644
--- a/src/_DataStructures_/Stack/postfix-expression-evaluation/postfix-expression-evaluation.test.js
+++ b/src/_DataStructures_/Stack/postfix-expression-evaluation/postfix-expression-evaluation.test.js
@@ -1,55 +1,62 @@
-const { evaluatePostfixExpression } = require('.');
+const { evaluatePostfixExpression, ERROR_STRING } = require('.');
 
-describe('Postfix expression evaluation', function () {
-  it('should be a function', function () {
+describe('Postfix expression evaluation', () => {
+  it('should be a function', () => {
     expect(typeof evaluatePostfixExpression).toEqual('function');
   });
-  
-  it('should return a number', function () {
+
+  it('should return a number', () => {
     const expression = '11+';
-    
-    expect(typeof evaluatePostfixExpression(expression)).toEqual('number')
+
+    expect(typeof evaluatePostfixExpression(expression)).toEqual('number');
   });
-  
-  it('should handle addition', function () {
+
+  it('should handle addition', () => {
     const expression = '23+';
     const expected = 5;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
-  
-  it('should handle subtraction', function () {
+
+  it('should handle subtraction', () => {
     const expression = '54-';
     const expected = 1;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
-  
-  it('should handle multiplication', function () {
+
+  it('should handle multiplication', () => {
     const expression = '34*';
     const expected = 12;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
-  
-  it('should handle division', function () {
+
+  it('should handle division', () => {
     const expression = '62/';
     const expected = 3;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
-  
-  it('should handle negative numbers', function () {
+
+  it('should handle negative numbers', () => {
     const expression = '25-';
     const expected = -3;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
-  
-  it('should handle multiple operators', function () {
+
+  it('should handle multiple operators', () => {
     const expression = '123*+';
     const expected = 7;
-    
+
     expect(evaluatePostfixExpression(expression)).toEqual(expected);
   });
+
+  describe('should throw error on invalid expressions', () => {
+    const invalidExpressions = ['12', '1', '+', '1+2', '+12'];
+    test.each(invalidExpressions)('running for %p', (expression) => {
+      expect(() => evaluatePostfixExpression(expression)).toThrow(ERROR_STRING);
+    });
+  });
 });