Using the JavaScript Prototype Pattern

This is the 2nd post in a series on techniques, strategies and patterns for writing JavaScript code. In my previous post I introduced what I call “function spaghetti code” and explained some of the problems it introduces. I also talked about the impact of global variables and how closures add a much needed solution. In this post I’ll introduce the Prototype Pattern and show how it relies on built-in functionality in the JavaScript language.

The Prototype Pattern can be broken out into two main sections including a constructor section and a prototype section. Prototyping allows functions and properties to be associated with objects. However, instead of each object instance getting a copy of all functions/properties each time an object is created, only one set of functions/properties exists across all objects resulting in less memory consumption. In other words, functions and properties are defined once per prototype rather than once per object.

As a quick review, I showed the following code in a previous post. The code simply lists all functions directly with no encapsulation and defines several global variables. While the code works fine this way, I’ll examine how we can restructure it to follow the Prototype Pattern.

window.onload = function () {
    eqCtl = document.getElementById('eq');
    currNumberCtl = document.getElementById('currNumber');
};

var eqCtl,
    currNumberCtl,
    operator,
    operatorSet = false,
    equalsPressed = false,
    lastNumber = null;

function add(x,y) {
    return x + y;
}

function subtract(x, y) {
    return x - y;
}

function multiply(x, y) {
    return x * y;
}

function divide(x, y) {
    if (y == 0) {
        alert("Can't divide by 0");
        return 0;
    }
    return x / y;
}
     
function setVal(val) {
    currNumberCtl.innerHTML = val;
}
        
function setEquation(val) {
    eqCtl.innerHTML = val;
}
        
function clearNumbers() {
    lastNumber = null;
    equalsPressed = operatorSet = false;
    setVal('0');
    setEquation('');
}

function setOperator(newOperator) {
    if (newOperator == '=') {
        equalsPressed = true;
        calculate();
        setEquation('');
        return;
    }
            
    //Handle case where = was pressed
    //followed by an operator (+, -, *, /)
    if (!equalsPressed) calculate();
    equalsPressed = false;
    operator = newOperator;
    operatorSet = true;
    lastNumber = parseFloat(currNumberCtl.innerHTML);
    var eqText = (eqCtl.innerHTML == '') ? 
        lastNumber + ' ' + operator + ' ' : 
        eqCtl.innerHTML + ' ' + operator + ' ';
    setEquation(eqText);
}

function numberClick(e) {
    var button = (e.target) ? e.target : e.srcElement;
    if (operatorSet == true || currNumberCtl.innerHTML == '0') {
        setVal('');
        operatorSet = false;            
    }
    setVal(currNumberCtl.innerHTML + button.innerHTML);
    setEquation(eqCtl.innerHTML + button.innerHTML);
}

function calculate() {
    if (!operator || lastNumber == null) return;
    var currNumber = parseFloat(currNumberCtl.innerHTML),
        newVal = 0;
    //eval() would've made this a whole lot simpler
    //but didn't want to use it in favor of a more
    //"robust" set of methods to demo patterns
    switch (operator) {
        case '+':
            newVal = add(lastNumber, currNumber);
            break;
        case '-':
            newVal = subtract(lastNumber, currNumber);
            break;
        case '*':
            newVal = multiply(lastNumber, currNumber);
            break;
        case '/':
            newVal = divide(lastNumber, currNumber);
            break;
    }
    setVal(newVal);
    lastNumber = newVal;
}

To start using the Prototype Pattern you need to first create a constructor as shown next. The constructor can accept one or more parameters and define any variables that the object needs. Note that the variables are scoped to the object rather than to the global scope.

var Calculator = function (tb, eq) {
    this.eqCtl = document.getElementById(eq);
    this.currNumberCtl = document.getElementById(tb);
    this.operator = null;
    this.operatorSet = false;
    this.equalsPressed = false;
    this.lastNumber = null;
};

Once the constructor is defined a prototype can be created using the prototype keyword. Although you can create a prototype for each function, it’s more convenient (and less typing) to take advantage of JSON-style syntax where the function name represents the JSON property name and the value represents the function. An example of defining two functions in a prototype is shown next. Notice that each function is separated with a comma just as you would separate properties defined in a normal JSON object. It's officially called a JavaScript object literal, but if you're familiar with JSON it uses the same standard syntax (thanks to Craig Stuntz for clarifying that):

Calculator.prototype = {

    add: function (x, y) {
        return x + y;
    },

    subtract: function (x, y) {
        return x - y;
    }
}

The original function-based calculator code shown earlier can be refactored to follow the Prototype Pattern as shown shown next. A prototype is created for the Calculator object and functions/properties are defined within the prototype using JavaScript object literal syntax.

Calculator.prototype = {

    add: function (x, y) {
        return x + y;
    },

    subtract: function (x, y) {
        return x - y;
    },

    multiply: function (x, y) {
        return x * y;
    },

    divide: function (x, y) {
        if (y == 0) {
            alert("Can't divide by 0");
        }
        return x / y;
    },
            
    setVal: function(val) {
        this.currNumberCtl.innerHTML = val;
    },
        
    setEquation: function (val) {
        this.eqCtl.innerHTML = val;
    },

    clearNumbers: function () {
        this.lastNumber = null;
        this.equalsPressed = this.operatorSet = false;
        this.setVal('0');
        this.setEquation('');
    },

    setOperator: function (newOperator) {
        if (newOperator == '=') {
            this.equalsPressed = true;
            this.calculate();
            this.setEquation('');
            return;
        }

        //Handle case where = was pressed
        //followed by an operator (+, -, *, /)
        if (!this.equalsPressed) this.calculate();
        this.equalsPressed = false;
        this.operator = newOperator;
        this.operatorSet = true;
        this.lastNumber = parseFloat(this.currNumberCtl.innerHTML);
        var eqText = (this.eqCtl.innerHTML == '') ?
            this.lastNumber + ' ' + this.operator + ' ' :
            this.eqCtl.innerHTML + ' ' + this.operator + ' ';
        this.setEquation(eqText);
    },

    numberClick: function () {
        var button = (event.target) ? event.target : event.srcElement;
        if (this.operatorSet == true || this.currNumberCtl.innerHTML == '0') {
            this.setVal('');
            this.operatorSet = false;
        }
        this.setVal(this.currNumberCtl.innerHTML + button.innerHTML);
        this.setEquation(this.eqCtl.innerHTML + button.innerHTML);
    },

    calculate: function () {
        if (!this.operator || this.lastNumber == null) return;
        var displayedNumber = parseFloat(this.currNumberCtl.innerHTML)
            newVal = 0;
        //eval() would've made this a whole lot simpler
        //but didn't want to use it in favor of a more
        //"robust" set of methods to demo patterns
        switch (this.operator) {
            case '+':
                newVal = this.add(this.lastNumber, displayedNumber);
                break;
            case '-':
                newVal = this.subtract(this.lastNumber, displayedNumber);
                break;
            case '*':
                newVal = this.multiply(this.lastNumber, displayedNumber);
                break;
            case '/':
                newVal = this.divide(this.lastNumber, displayedNumber);
                break;
        }
        this.setVal(newVal);
        this.lastNumber = newVal;
    }
};

To use the Calculator object, create a new instance and pass the names of the HTML container objects into the constructor (the container objects identify the IDs of controls used to display calculations – see the HTML code below):

var calc = null;
window.onload = function () {
    calc = new Calculator('currNumber', 'eq');
};

The HTML used to render the calculator can reference the calc object created when the page loads. The following code demonstrates how this can be done to handle events as different div elements are clicked. I normally prefer to wire-up events to event handlers using jQuery in “real-world” applications to keep the HTML clean but wanted to focus on JavaScript patterns and avoid introducing additional libraries for this post. I’m not a big fan of defining onclick and other event handlers directly on HTML elements but let it slide for this post.

<div class="Calculator">
    <div class="CalculatorHead">
        <div id="eq" class="Equation"></div>
        <div id="currNumber" class="CurrentNumber">0</div>
    </div>
    <div>
        <div class="Button" onclick="calc.numberClick(event);">7</div>
        <div class="Button" onclick="calc.numberClick(event);">8</div>
        <div class="Button" onclick="calc.numberClick(event);">9</div>   
        <div class="Button" onclick="calc.setOperator('/');">/</div>        
        <div class="Button rowspan2" onclick="calc.clearNumbers();">C</div>
    </div>
    <div>
        <div class="Button" onclick="calc.numberClick(event);">4</div>
        <div class="Button" onclick="calc.numberClick(event);">5</div>
        <div class="Button" onclick="calc.numberClick(event);">6</div> 
        <div class="Button" onclick="calc.setOperator('*');">*</div>           
    </div>
    <div>
        <div class="Button" onclick="calc.numberClick(event);">1</div>
        <div class="Button" onclick="calc.numberClick(event);">2</div>
        <div class="Button" onclick="calc.numberClick(event);">3</div>
        <div class="Button" onclick="calc.setOperator('-')">-</div>            
        <div class="Button rowspan2" onclick="calc.setOperator('=');">=</div>
    </div>
    <div>
        <div class="Button colspan2" onclick="calc.numberClick(event);">0</div>
        <div class="Button" onclick="calc.numberClick();">.</div>
        <div class="Button" onclick="calc.setOperator('+');">+</div>            
    </div>
</div>

The Prototype Pattern provides a nice way to structure JavaScript code but there are several other roads you can travel if desired. In this next post I’ll talk about the Revealing Module Pattern and explain how it can be used.

Demos of all the patterns covered in this series can be downloaded below.

Download Code

Pluralsight Course - Structuring JavaScript Code in HTML5 Applications