Project: Calculator

Foundations Course


You made it! By now you should have a really firm grasp on the fundamentals of JavaScript. Of course there’s plenty more to learn, but you should be able to create quite a bit at this point. Our final project is going to combine everything you’ve learned so far: you’re going to make an on-screen calculator using JavaScript, HTML, and CSS.

As usual with these things, there are elements of this project that are not going to be trivially easy for you, but if you’ve been following the course so far, you definitely have everything you need to finish it. We’re going to walk you through the various steps you can take, but again, how you actually implement them is up to you!


Before you get started with this calculator project, we need to cover a word of warning. As you look into how to evaluate complex mathematical statements in JavaScript, you will likely come across the tantalizing eval() function. However, this function can be very dangerous and MDN does a good job documenting why you should never use eval! You’ll need to build your own functions to evaluate expressions as part of this calculator project. On the same note, when researching how to calculate expressions for this project, you may encounter solutions that suggest that you return a new Function() that evaluates a string. Similarly to eval(), this should not be used due to potential pitfalls of evaluating insecure data. Besides, where’s the fun in solutions that do all the work for you? Let’s get to it!


Don’t forget to commit early & often! You can reference the Commit Messages lesson here!

Here are some use cases (abilities your project needs to have):

  1. Your calculator is going to contain functions for all of the basic math operators you typically find on calculators, so start by creating functions for the following items and testing them in your browser’s console.
    • add
    • subtract
    • multiply
    • divide
  2. A calculator operation will consist of a number, an operator, and another number. For example, 3 + 5. Create three variables for each of the parts of a calculator operation. Create a variable for the first number, the operator, and the second number. You’ll use these variables to update your display later.
  3. Create a new function operate that takes an operator and 2 numbers and then calls one of the above functions on the numbers.
  4. Create a basic HTML calculator with buttons for each digit, each of the above functions and an “Equals” key.
    • Do not worry about wiring up the JS just yet.
    • There should also be a display for the calculator. Go ahead and fill it with some dummy numbers so it looks correct.
    • Add a “clear” button.
  5. Create the functions that populate the display when you click the number buttons. You should be storing the ‘display value’ in a variable somewhere for use in the next step.
  6. Make the calculator work! You’ll need to store the first number and second number that are input into the calculator, utilize the operator that the user selects, and then operate() on the two numbers when the user presses the “=” key.
    • You should already have the code that can populate the display, so once operate() has been called, update the display with the ‘solution’ to the operation.
    • This is the hardest part of the project. You need to figure out how to store all the values and call the operate function with them. Don’t feel bad if it takes you a while to figure out the logic.
  7. Gotchas: watch out for and fix these bugs if they show up in your code:
    • Your calculator should not evaluate more than a single pair of numbers at a time. Example: you press a number button (12), followed by an operator button (+), a second number button (7), and finally a second operator button (-). Your calculator should then do the following: first, evaluate the first pair of numbers (12 + 7), second, display the result of that calculation (19), and finally, use that result (19) as the first number in your new calculation, along with the next operator (-). An example of the behavior we’re looking for can be seen in this student’s calculator live preview.
    • You should round answers with long decimals so that they don’t overflow the screen.
    • Pressing = before entering all of the numbers or an operator could cause problems!
    • Pressing “clear” should wipe out any existing data. Make sure the user is really starting fresh after pressing “clear”
    • Display a snarky error message if the user tries to divide by 0… and don’t let it crash your calculator!

Extra credit

  • Users can get floating point numbers if they do the math required to get one, but they can’t type them in yet. Add a . button and let users input decimals! Make sure you don’t let them type more than one though: It is hard to do math on these numbers. (disable the decimal button if there’s already one in the display)
  • Make it look nice! This is a great project to practice your CSS skills. At least make the operations a different color from the keypad buttons.
  • Add a “backspace” button, so the user can undo if they click the wrong number.
  • Add keyboard support!

Support us!

The Odin Project is funded by the community. Join us in empowering learners around the globe by supporting The Odin Project!