今天來做一個計算機。

如畫面所示，是一個簡易的計算機，有加、減、乘、除、清除 的功能

設變數

const numberButtons = document.querySelectorAll("[data-number]");
const operationButtons = document.querySelectorAll("[data-operation]");
const equalsButton = document.querySelector("[data-equals]");
const deleteButton = document.querySelector("[data-delete]");
const allClearButton = document.querySelector("[data-all-clear]");
const previousOperandTextElement = document.querySelector("[data-previous-operand]");
const currentOperandTextElement = document.querySelector("[data-current-operand]");

const calculator = new Calculator(previousOperandTextElement,
    currentOperandTextElement);

類別(Class)

下面我們做了一個 Calculator 的 class 和 constructor

類別 (Class) 是先定義好物件的整體結構藍圖(blue print)，然後再用這個類別定義，用來產生相同結構的多個的物件，類別在定義時不會直接產生出物件，要經過實體化的過程 (new) ，才會產生真正的物件實體。

函式建構式（function constructor）就是普通的 function，只是我們可以透過這個 function 來建立物件。

在 function 前面加上 new，它會把函式中 this 這個關鍵字建立成一個新的物件，如果沒有在該函式的最後指定回傳出其它物件的話，它就會自動回傳這個新的物件給你。

class Calculator {
    constructor(previousOperandTextElement, currentOperandTextElement){
        this.previousOperandTextElement = previousOperandTextElement;
        this.currentOperandTextElement = currentOperandTextElement;
        this.readyToReset = false;
        this.clear();
    }
    
    // 清除數字
    clear() {
        this.currentOperand = "";
        this.previousOperand = "";
        this.operation = undefined;

    }
    // 刪除數字
    delete() {
        this.currentOperand = this.currentOperand.toString().slice(0, -1);
    }
    
    // 計算機上的數字判斷
    
    appendNumber(number) {
        if (number === "." && this.currentOperand.includes(".")) return
        this.currentOperand = this.currentOperand.toString() + number.toString();
    }
    
    // 計算符號判斷
    
    chooseOperation(operation) {
        if (this.currentOperand === "") return
        if (this.currentOperand !== "" && this.previousOperand !== "") this.compute();
        this.operation = operation;
        this.previousOperand = this.currentOperand;
        this.currentOperand = "";
    }

    // 計算判斷
    
    compute() {
        let computation
        const prev = parseFloat(this.previousOperand);
        const current = parseFloat(this.currentOperand);
        if (isNaN(prev) || isNaN(current)) return;
        switch (this.operation)  {

        case "+":
            computation = prev + current;
        break;

        case "-":
            computation = prev - current;
        break;

        case "*":
            computation = prev * current;
        break;

        case "÷":
            computation = prev / current;
        break;

    default:
        return;
        }
        this.readyToReset = true;
        this.currentOperand = computation;
        this.operation = undefined;
        this.previousOperand = "";
    }
    
    // 計算結果顯示
    
    updateDisplay() {
        this.currentOperandTextElement.innerText = this.currentOperand;
        if (this.operation != null) {
            this.previousOperandTextElement.innerText =
            `${this.previousOperand} ${this.operation}`
        } else {
            this.previousOperandTextElement.innerText = "";
        }

    }
}

監聽事件

allClearButton.addEventListener("click", () => {
    calculator.clear();
    calculator.updateDisplay();
})

numberButtons.forEach(button => {
    button.addEventListener("click", () => {

        if(calculator.previousOperand === "" &&
        calculator.currentOperand !== "" &&
    calculator.readyToReset) {
            calculator.currentOperand = "";
            calculator.readyToReset = false;
        }
        calculator.appendNumber(button.innerText)
        calculator.updateDisplay();
    })
})

operationButtons.forEach(button => {
    button.addEventListener("click", () => {
        calculator.chooseOperation(button.innerText)
        calculator.updateDisplay();
    })
})

equalsButton.addEventListener("click", button => {
    calculator.compute();
    calculator.updateDisplay();
})

deleteButton.addEventListener("click", button => {
    calculator.delete();
    calculator.updateDisplay();
})

建立 class 的方法，讓思考的思路變得清楚，明天再來看看，其他應用的例子。明天見