Get Programming with JavaScript

Get Programming with JavaScript - Listing 4.12

Listing 4.12 - Using functions to add and display tax

var sale1;
var sale2;
var sale3;
var sale;
var calculateTax;
var displaySale;

sale1 = { price: 140, taxRate: 15 };
sale2 = { price: 40, taxRate: 10 };
sale3 = { price: 120, taxRate: 20 };

calculateTax = function () {
    sale.tax = sale.price * sale.taxRate / 100;
    sale.total = sale.price + sale.tax;
};

displaySale = function () {
    console.log("price = $" + sale.price);
    console.log("tax @ " + sale.taxRate + "% = $" + sale.tax);
    console.log("total cost = $" + sale.total);
};

sale = sale1;
calculateTax();
displaySale();

sale = sale2;
calculateTax();
displaySale();

sale = sale3;
calculateTax();
displaySale();

Further Adventures

Listing 4.12 - Using functions to add and display tax - Tasks 1 & 2

Add a fourth sale object.
Update the code so that tax is calculated and the new sale is displayed.

var sale1;
var sale2;
var sale3;
var sale4;    // Declare a variable
var sale;
var calculateTax;
var displaySale;

sale1 = { price: 140, taxRate: 15 };
sale2 = { price: 40, taxRate: 10 };
sale3 = { price: 120, taxRate: 20 };
sale4 = { price: 90, taxRate: 10 };    // Create an object and assign it

calculateTax = function () {
    sale.tax = sale.price * sale.taxRate / 100;
    sale.total = sale.price + sale.tax;
};

displaySale = function () {
    console.log("price = $" + sale.price);
    console.log("tax @ " + sale.taxRate + "% = $" + sale.tax);
    console.log("total cost = $" + sale.total);
};

sale = sale1;
calculateTax();
displaySale();

sale = sale2;
calculateTax();
displaySale();

sale = sale3;
calculateTax();
displaySale();

// Calculate tax and display for sale4
sale = sale4;
calculateTax();
displaySale();

Listing 4.12 - Using functions to add and display tax - Task 3

change the code so that you don't have to call both functions for every sale

var sale1;
var sale2;
var sale3;
ar sale4;
var sale;
var calculateTax;
var displaySale;

sale1 = { price: 140, taxRate: 15 };
sale2 = { price: 40, taxRate: 10 };
sale3 = { price: 120, taxRate: 20 };
sale4 = { price: 90, taxRate: 10 };

calculateTax = function () {
    sale.tax = sale.price * sale.taxRate / 100;
    sale.total = sale.price + sale.tax;
};

displaySale = function () {
    calculateTax();    // Call one function from another
    console.log("price = $" + sale.price);
    console.log("tax @ " + sale.taxRate + "% = $" + sale.tax);
    console.log("total cost = $" + sale.total);
};

// Now there's no need to call both functions for each sale
sale = sale1;
displaySale();

sale = sale2;
displaySale();

sale = sale3;
displaySale();

sale = sale4;
displaySale();

This solution calls one function from another. The tax needs to be calculated before the sale can be displayed, so this is a reasonable approach.

If you wanted to keep the calculation separate from the display, you could define a third function to call the other two, as shown below.

var sale1;
var sale;
var calculateTax;
var displaySale;
var calcAndDisplay;    // Declare a variable

sale1 = { price: 140, taxRate: 15 };

calculateTax = function () {
    sale.tax = sale.price * sale.taxRate / 100;
    sale.total = sale.price + sale.tax;
};

displaySale = function () {
    console.log("price = $" + sale.price);
    console.log("tax @ " + sale.taxRate + "% = $" + sale.tax);
    console.log("total cost = $" + sale.total);
};

// Define a new function to call the other two
calcAndDisplay = function () {
    calculateTax();
    displaySale();
};

sale = sale1;
calcAndDisplay();    // Call the new function