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JavaScript

Build a JavaScript Calculator

Build a working calculator in the browser using HTML, CSS, and JavaScript. Learn how user interactions become application behavior by building the project step by step.

  • Beginner
  • About 2 weeks
  • 6 milestones
  • 31 steps
JavaScriptHTMLCSSHTML/CSS
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What you will have builtA working JavaScript calculator that accepts user input, performs basic arithmetic operations, and responds correctly to real user interactions.Start this journey

What this asks of you

Every figure below is read from the journey itself, not written about it.

BeginnerNo prior experience needed
About 2 weeksAn estimate, not a deadline
6 milestonesEach one claims a capability
31 stepsShort, and every one is built

What you will be able to do

Every milestone claims one engineering capability. You reach it by building something, not by finishing a chapter.

Understand the CalculatorDescribe the behavior of a calculator as an interactive application, including its inputs, state, operations, and output.Before writing the calculator, the learner needs a clear picture of what they are building and how a user's actions become application behavior. This establishes the model they will use when they begin building the interface and JavaScript logic in the following milestones.
4 steps
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  • Explore the Calculator

    Identify what the calculator should do from a user's point of view before building it.

  • Describe the Calculator Behaviour

    Describe how the calculator should respond to a user's actions before implementing that behaviour in JavaScript.

  • Identify What the Calculator Remembers

    Identify the information the calculator needs to keep track of while the user performs a calculation.

  • Define the Calculator Rules

    Define the rules the calculator must follow to produce the expected result for each supported operation.

Build the Calculator InterfaceBuild a browser-based calculator interface with a display and controls that represent the calculator's required interactions.The learner has already defined what the calculator should do and what information it needs. Now they need a concrete interface through which a user can perform those actions before JavaScript behaviour is connected.
4 steps
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  • Create the Calculator Structure

    Create the HTML structure for the calculator display and controls.

  • Style the Calculator

    Use CSS to turn the calculator's HTML structure into a clear and usable visual interface.

  • Break the Calculator into Parts

    Identify the main parts of a calculator interface and the role each part plays.

  • Make the Interface Usable

    Improve the calculator interface so that its controls are clear, readable, and easy for a user to interact with.

Connect JavaScript to the InterfaceUse JavaScript to find calculator controls and respond to user interactions with the page.The learner has already built and styled the calculator interface, so now they have a concrete interface to connect JavaScript to. This creates the foundation for making the calculator respond to user actions before introducing the calculation logic itself.
7 steps
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  • Select and Read the Buttons

    Use JavaScript to find calculator buttons and read the information they provide.

  • Respond to Button Clicks

    Use a click event listener to detect when a calculator button is pressed.

  • Update the Calculator Display

    Use JavaScript to change the calculator display when a number button is clicked.

  • Clear the Display

    Use JavaScript to reset the calculator display when the Clear button is clicked.

  • Enter Multiple Digits

    Build a multi-digit number by adding each clicked number to the calculator display.

  • Handle the Decimal Point

    Prevent invalid decimal input while allowing the calculator to build numbers containing a decimal point.

  • Prepare for Operator Input

    Separate the current number from the operator input so the calculator can prepare for a second number.

Perform CalculationsUse the stored numbers and operator to perform an arithmetic calculation and produce a numeric result.The learner can now collect calculator input, preserve the first number, and remember the selected operator, so they have everything needed to introduce the calculation itself.
8 steps
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  • Convert Input to Numbers

    Convert the calculator's displayed text into a number that JavaScript can use in arithmetic.

  • Perform Addition

    Use two numeric values to perform addition and display the result in the calculator.

  • Calculate from the Calculator State

    Use the stored first number, selected operator, and current display value to calculate and display the result.

  • Handle the Other Operators

    Extend the calculator's calculation logic to perform subtraction, multiplication, and division using the selected operator.

  • Handle Division by Zero

    Prevent the calculator from performing division when the second number is zero.

  • Start a New Calculation

    Make the calculator begin a new number when the learner enters a digit immediately after a completed calculation.

  • Continue After a Result

    Allow the calculator to use a completed result as the starting value for the next calculation.

  • Reset the Calculator

    Add a clear function that returns the calculator to its initial state.

Handle Real User BehaviourHandle common user inputs and calculator situations without producing broken or confusing behaviour.The calculator can now perform its core operations, so the next step is to make those operations behave reliably when users interact with the calculator in less ideal ways.
4 steps
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  • Handle Invalid Input

    Prevent invalid values from causing the calculator to produce a broken or confusing result.

  • Prevent Duplicate Operators

    Prevent the calculator from accepting another operator when an operator has already been selected for the current calculation.

  • Test and Fix Calculator Behaviour

    Use simple debugging techniques to find and fix a problem in the calculator's behaviour.

  • Test the Complete Calculator

    Test the calculator across normal and unusual interactions and fix any remaining problems you find.

Finish, Test & ShipFinish, test, and present a working JavaScript calculator that behaves reliably for normal and invalid user interactions.The learner has built the calculator's interface, connected it to JavaScript, implemented its calculation logic, and handled common user behaviour. Now they need to bring those pieces together into a finished project that can be tested and shared.
4 steps
Show the 4 steps
  • Review the Complete Calculator

    Review the calculator's structure, behaviour, and code to identify anything that still needs to be completed or improved.

  • Polish the Calculator Interface

    Improve the calculator's visual appearance and usability using CSS without changing its functionality.

  • Prepare the Project

    Organize and clean the calculator project so that another person can understand and run it.

  • Present the Finished Calculator

    Demonstrate the completed calculator and explain the main features and JavaScript behaviour you built.

How the work is verified

  1. ReadThe context you need for this step, and why it matters. Never more than that.
  2. BuildYou produce something real. The work is yours, in your own repository.
  3. SubmitYou send it, with a note on what you chose and what you were unsure about.
  4. ReviewA senior engineer reads it and responds. They may ask for changes — that is the normal path through engineering review, not a failure.
  5. VerifiedThe capability is confirmed, it goes on your record, and the next step opens.

How you want to work

You choose when you start. Both cover the same journey and the same definition of done.

Mentor guidedReviewed by a person

A senior engineer is assigned to you. They read every submission, respond in writing, and decide when a capability is verified. Expect to be asked for changes.

Self pacedVerified by you

You work through the same journey and verify your own work against the same definition of done. Nothing is submitted to anyone, and nothing waits on anyone.

What you end up with

Counted from this journey, not described in general.

A working JavaScript calculator that accepts user input, performs basic arithmetic operations, and responds correctly to real user interactions.Built by you, in your own repository.
6 verified capabilitiesEach one confirmed against its definition of done, with the name of whoever confirmed it. There is no certificate. The work is the credential.

Ready to start Build a JavaScript Calculator?

You will choose how you want to work on the next screen. Nothing starts until you confirm it. You can run several journeys at once, and each one is mentor-guided or self-paced on its own.

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