Using Custom Methods with Custom Objects

Question

I began working with custom objects along with custom methods in JavaScript. For practice I wrote a small script to compare two interest rates. At first I thought the program worked, although just to be sure, used document.write(John_Interest_Value), and received a NaN alert. Am I using custom methods correctly? Can anyone identify the problem? Thanks.

function SimpleInterest (principal,time1,rate) {
    this.principal = principal;
    this.time1 = time1;
    this.rate = rate;
    this.interest = calculate_interest;
}

function calculate_interest(principal,time1,rate) {
    var si;
    var si = (principal*rate*time1)/100;
    return si;
 }

var tom_interest = new SimpleInterest(1000,3,.08); 
var john_interest = new SimpleInterest(2000,3,.04);

var Tom_interest_value = tom_interest.interest(); 
var John_interest_value = john_interest.interest();

window.alert (John_interest_value);
if (Tom_interest_value > John_interest_value) {
    document.write("Tom's interest earned is more than that of John's");
} else if (Tom_interest_value < John_interest_value){
    document.write("John's interest earned is more than that of Tom's");
} else {
    document.write("Tom and John earn equal interest");
}

You almost have the function part right with calculate_interest, although you don't need the first line var si; (you only need to initialize it once). What are you trying to accomplish with the SimpleInterest function, though? — aw04
– aw04, Commented Jun 20, 2014 at 2:14

Gabs00 · Accepted Answer · 2014-06-20 02:16:38Z

1

undefined*undefined equals NaN

Given the way you chose to call the below:

var Tom_interest_value = tom_interest.interest(); 
var John_interest_value = john_interest.interest();

The variables inside of the function calculate_interest are undefined. So, as you have written it you would need to do

var Tom_interest_value = tom_interest.interest(tom_interest.principal, tom_interest.time1, tom_interest.rate);

As you can see, typing that is tedious. you want to remove the parameters from calculate_interest and instead reference the objects variables like below

function calculate_interest() {
    var si = (this.principal*this.rate*this.time1)/100;
    return si;
 }

answered Jun 20, 2014 at 2:16

Gabs00

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