# Calculator Reducer ## Breakdown of the Code ## 1. **JSDoc Type Imports** ```javascript /** * @typedef {import('./types.js').CalculatorState} CalculatorState * @typedef {import('./types.js').CalculatorAction} CalculatorAction */ ``` - These lines use **JSDoc** to import types from `types.js`. - `CalculatorState` represents the shape of the calculator's state, and `CalculatorAction` represents the shape of an action dispatched to the reducer. - These types provide type checking and documentation within the IDE, even though the code is in plain JavaScript. ## 2. **Initial State** ```javascript /** * The initial state for the calculator. * @type {CalculatorState} */ const initialState = { currentValue: '0', // Current input or result previousValue: null, // Previous value before an operation operator: null, // The operator (+, -, *, /) waitingForOperand: false, // Tracks if we're waiting for the next operand }; ``` - `initialState` defines the starting values for the calculator. - `currentValue`: Represents the value currently being input or the last calculated result. It starts as `'0'`. - `previousValue`: Holds the value before an operator is pressed. Initially `null`. - `operator`: Stores the operator (`+`, `-`, `*`, `/`) currently in use. It starts as `null`. - `waitingForOperand`: A boolean flag that indicates if the calculator is waiting for the next operand after an operator is pressed. ## 3. **Reducer Function** ```javascript /** * A reducer function for the calculator state. * @param {CalculatorState} state * @param {CalculatorAction} action * @returns {CalculatorState} */ export function calculatorReducer(state = initialState, action) { ``` - This is a **reducer function** that takes the current state and an action, and returns the new state based on the type of the action. - **Default value**: If no state is provided (e.g., at initialization), it uses `initialState`. ## 4. **Handling Action Types** ```javascript switch (action.type) { case 'DIGIT': ``` - The `switch` statement handles different `action.type` values. ### 4.1 **Handling `'DIGIT'` Action** ```javascript case 'DIGIT': if (state.waitingForOperand) { return { ...state, currentValue: action.payload, waitingForOperand: false, }; } return { ...state, currentValue: state.currentValue === '0' ? action.payload : state.currentValue + action.payload, }; ``` - **When a digit is pressed** (`'DIGIT'` action): - If the calculator is waiting for the next operand (`waitingForOperand` is `true`), it replaces the `currentValue` with the digit and resets the `waitingForOperand` flag. - Otherwise, it appends the digit to the `currentValue`. If `currentValue` is `'0'`, it replaces it with the new digit. ### 4.2 **Handling `'OPERATOR'` Action** ```javascript case 'OPERATOR': if (state.operator && state.previousValue !== null) { const result = evaluate(state); return { ...state, previousValue: result, currentValue: '0', operator: action.payload, waitingForOperand: true, }; } return { ...state, previousValue: state.currentValue, operator: action.payload, waitingForOperand: true, }; ``` - **When an operator is pressed** (`'OPERATOR'` action): - If there's already a `previousValue` and an operator is set, the function evaluates the current expression and stores the result as the `previousValue`. - Otherwise, it sets the `currentValue` as the `previousValue` and assigns the operator from `action.payload`. - It also sets `waitingForOperand` to `true` to signal that the calculator is waiting for the next number to be entered. ### 4.3 **Handling `'EQUALS'` Action** ```javascript case 'EQUALS': if (state.operator && state.previousValue !== null) { const result = evaluate(state); return { ...state, currentValue: result, previousValue: null, operator: null, waitingForOperand: false, }; } return state; ``` - **When the equals button is pressed** (`'EQUALS'` action): - If an operator and a `previousValue` are set, it evaluates the current expression and updates `currentValue` with the result. It also clears the operator and `previousValue`. - If the necessary data for evaluation is not present, it simply returns the current state unchanged. ### 4.4 **Handling `'CLEAR'` Action** ```javascript case 'CLEAR': return initialState; ``` - **When the clear button is pressed** (`'CLEAR'` action): - It resets the calculator state to the `initialState`. ### 4.5 **Default Case** ```javascript default: return state; ``` - If an action type is not recognized, the reducer returns the current state without any changes. ## 5. **Evaluate Function** ```javascript function evaluate({ currentValue, previousValue, operator }) { const prev = parseFloat(previousValue); const current = parseFloat(currentValue); switch (operator) { case '+': return (prev + current).toString(); case '-': return (prev - current).toString(); case '*': return (prev * current).toString(); case '/': return current !== 0 ? (prev / current).toString() : 'Error'; default: return currentValue; } } ``` - The `evaluate` function takes `currentValue`, `previousValue`, and `operator` to perform the calculation. - **Steps**: 1. It converts the `previousValue` and `currentValue` from strings to numbers using `parseFloat`. 2. Based on the operator, it performs the corresponding arithmetic operation (`+`, `-`, `*`, `/`). 3. **Divide by zero handling**: If the operation is division (`/`) and `currentValue` is `0`, it returns `'Error'` to avoid division by zero. 4. The result of the operation is returned as a string. ## Summary - The `calculatorReducer` function handles the state changes of a calculator by interpreting different actions (`'DIGIT'`, `'OPERATOR'`, `'EQUALS'`, `'CLEAR'`). - The `evaluate` helper function performs the arithmetic operations when required. - The state is updated incrementally as the user presses digits, operators, and other controls, making it suitable for a basic calculator UI. ## Breakdown of the Tests ### 1. **Imports and Initial Setup** ```javascript import { describe, it, expect } from 'vitest'; import { calculatorReducer } from './calculator.js'; // Assume this is the path to the reducer ``` - **Vitest** is being imported for testing, using `describe` to group the test cases, `it` for individual test cases, and `expect` for assertions. - The `calculatorReducer` is imported from a file called `calculator.js` for testing. - The `CalculatorState` and `CalculatorAction` types are imported using **JSDoc** for static type checking. ### 2. **Initial State** ```javascript const initialState = { currentValue: '0', previousValue: null, operator: null, waitingForOperand: false, }; ``` - This defines the `initialState`, representing the calculator's default state when no input has been provided yet. ### 3. **Test: Handling Digit Input** ```javascript it('should handle a digit input', () => { const action = { type: 'DIGIT', payload: '5' }; const newState = calculatorReducer(initialState, action); expect(newState.currentValue).toBe('5'); }); ``` - **What it does**: Tests that when the `'DIGIT'` action is dispatched with the digit `'5'`, the `currentValue` is updated to `'5'`. - **Why**: To ensure the reducer correctly updates the state with the inputted digit. ### 4. **Test: Appending Digits to Current Input** ```javascript it('should append digits to the current input', () => { const firstAction = { type: 'DIGIT', payload: '5' }; const secondAction = { type: 'DIGIT', payload: '3' }; let state = calculatorReducer(initialState, firstAction); state = calculatorReducer(state, secondAction); expect(state.currentValue).toBe('53'); }); ``` - **What it does**: Tests appending digits `'5'` and `'3'` to form `'53'`. - **Why**: To verify that digits are appended to `currentValue` correctly and are not overwritten when a new digit is added. ### 5. **Test: Handling Operator Input** ```javascript it('should handle operator input', () => { const stateWithDigit = { ...initialState, currentValue: '10' }; const action = { type: 'OPERATOR', payload: '+' }; const newState = calculatorReducer(stateWithDigit, action); expect(newState.previousValue).toBe('10'); expect(newState.operator).toBe('+'); expect(newState.waitingForOperand).toBe(true); }); ``` - **What it does**: Tests pressing an operator (`'+'`), ensuring the state updates with the current value as `previousValue`, sets the operator, and sets `waitingForOperand` to `true`. - **Why**: To confirm that when an operator is pressed, the reducer correctly prepares for the next operand. ### 6. **Test: Performing Addition** ```javascript it('should perform addition when equals is pressed', () => { const stateWithOperator = { currentValue: '5', previousValue: '10', operator: '+', waitingForOperand: false, }; const action = { type: 'EQUALS' }; const newState = calculatorReducer(stateWithOperator, action); expect(newState.currentValue).toBe('15'); }); ``` - **What it does**: Tests the `'EQUALS'` action when the operator is `'+'`, and the values are `10` and `5`. The result should be `'15'`. - **Why**: To verify that the addition is correctly performed when the equals button is pressed. ### 7. **Test: Performing Subtraction** ```javascript it('should perform subtraction when equals is pressed', () => { const stateWithOperator = { currentValue: '3', previousValue: '8', operator: '-', waitingForOperand: false, }; const action = { type: 'EQUALS' }; const newState = calculatorReducer(stateWithOperator, action); expect(newState.currentValue).toBe('5'); }); ``` - **What it does**: Tests subtraction (`'8 - 3'`) when the equals button is pressed, with the result being `'5'`. - **Why**: To confirm that subtraction works correctly with the equals action. ### 8. **Test: Performing Multiplication** ```javascript it('should perform multiplication when equals is pressed', () => { const stateWithOperator = { currentValue: '4', previousValue: '6', operator: '*', waitingForOperand: false, }; const action = { type: 'EQUALS' }; const newState = calculatorReducer(stateWithOperator, action); expect(newState.currentValue).toBe('24'); }); ``` - **What it does**: Tests multiplication (`'6 * 4'`) when the equals button is pressed, expecting the result `'24'`. - **Why**: To ensure the multiplication operation is working correctly. ### 9. **Test: Performing Division** ```javascript it('should perform division when equals is pressed', () => { const stateWithOperator = { currentValue: '2', previousValue: '10', operator: '/', waitingForOperand: false, }; const action = { type: 'EQUALS' }; const newState = calculatorReducer(stateWithOperator, action); expect(newState.currentValue).toBe('5'); }); ``` - **What it does**: Tests division (`'10 / 2'`) with the expected result of `'5'`. - **Why**: To confirm that division is handled properly. ### 10. **Test: Division by Zero** ```javascript it('should return "Error" when dividing by zero', () => { const stateWithOperator = { currentValue: '0', previousValue: '10', operator: '/', waitingForOperand: false, }; const action = { type: 'EQUALS' }; const newState = calculatorReducer(stateWithOperator, action); expect(newState.currentValue).toBe('Error'); }); ``` - **What it does**: Tests dividing by zero (`'10 / 0'`) and expects the result to be `'Error'`. - **Why**: To ensure the reducer correctly handles the edge case of division by zero, avoiding invalid arithmetic results. ### 11. **Test: Clearing the Calculator State** ```javascript it('should reset the state when "CLEAR" action is dispatched', () => { const stateWithValue = { currentValue: '25', previousValue: '5', operator: '+', waitingForOperand: true, }; const action = { type: 'CLEAR' }; const newState = calculatorReducer(stateWithValue, action); expect(newState).toEqual(initialState); }); ``` - **What it does**: Tests that the `'CLEAR'` action resets the state back to its initial values (`initialState`). - **Why**: To verify that pressing the clear button resets the entire calculator back to its default state. ## Summary - Each test case ensures that the **calculatorReducer** handles individual actions correctly. - The tests cover all possible scenarios, including handling digits, appending digits, performing operations, evaluating expressions, handling edge cases like division by zero, and resetting the state. - This ensures that the calculator works as expected in all cases and prevents regressions when changes are made to the code.