Scientific calculator

This is an online javascript scientific calculator, designed and coded by Yogesh Agarwala.
View it here: https://yogeshiitm.github.io/scientific-calculator/

Some Features

1. The input of this calculator can be a multiple function expression, just like a physical scientific calculator.

2. The back button in the calculator is designed to work smartly, e.g. if the back button is pressed after doing a calculation, then it will act as a clear button and will delete the entire string from the input text area, but at other times it will delete only the last character from the input expression.

3. The calculator intelligently interprets a user's inputs even if proper parenthesis or multiplication signs are not used, instead of displaying an error, for example:

   15√(3+5!7π)sin(60°)^ln(log(5)e)7! ---> 15× √(3+5!×7×π) × sin(60°)ln(log(5)×e) × 7!
   3sin(60)4 ---> 3 × sin(60) × 4
   3sin(60×4 ---> 3 × sin(60×4)
   4²3²√(5+6 ---> 4²×3²×√(5+6).
   4²3²√(5)6 ---> 4²×3²×√(5) × 6
   4%3 ---> 4% × 3 ---> 0.04 × 3
   4^(3)πe ---> (4^3) × π × e
   4^(3πe) ---> 4^(3×π×e)
   3!5 ---> 3! × 5

4. It also checks if all the parentheses are balanced in the input expression (using Stack data structure), and then displays an error accordingly.

   abs(-5)) ---> Error displayed: closing bracket has been used before an opening bracket.
   (abs(-5) ---> Error displayed: brackets are not balanced.

5. The calculator can take input not only through mouse clicks but also from keyboard button press.

Near future plan

1. Option to switch between degree and radian: currently by default angle has to be entered in degree.
2. Inverse trigonometric functions are yet to be added.

Note: I have tried to ensure that there is no mistake in the calculations done by the app but still if you find one, please do let me know.