HTML and CSS Reference
In-Depth Information
Creating the monthlyPmt() User-Defined Function
The monthlyPmt()
function is a user-defined function that calculates the monthly payment amount. The
JavaScript code for the monthlyPmt() function is shown in Table 10-18.
Table 10-18 Code for monthlyPmt() User-Defined Function
Line
Code
67
function monthlyPmt(loanAmt,loanRate,loanYears) {
68
var interestRate = loanRate/1200
69
var Pmts = loanYears * 12
70
var Amnt = loanAmt * (interestRate / (1 - (1 /
Math.pow(1+interestRate,Pmts))))
return Amnt.toFixed(2)
71
}
72
Math Object
The Math object cannot
be used to create other
objects. Most of the
properties of the Math
object return preset
values. Other properties
of the Math object really
are methods and act as
functions.
Line 67 declares the monthlyPmt() function. Line 68 determines the monthly
interest rate percentage by dividing the annual rate by 1200. The result is assigned to the
interestRate (interest rate) variable. Line 69 determines the number of monthly payments
on the loan, by multiplying the number of years in the loan by 12. The resulting value is
assigned to the Pmts variable.
Line 70 is the formula for calculating a monthly payment based on the amount of
the loan, the monthly interest percentage, and the number of monthly payments. The
mathematical representation of the formula is:
loan amount * (monthly interest rate / (1 - (1 /(1 + monthly
interest rate)
number of payments
)))
JavaScript, however, does not use typical programming language symbols
to represent exponentiation in code. Instead, to calculate the expression
(1 + monthly interest rate)
number of payments
, JavaScript uses the pow() method associated
with the Math object. Table 10-19 shows the general form of the pow() method.
Table 10-19 Math.pow() Method
General form:
Math.pow(number, exponent)
Comment:
where
number
is the value raised to the power of the
exponent
value. The pow() method accepts
variables (X,n), constants (2,3), or both (Sidelength,2).
Math.pow(2,3)
Math.pow(X,n)
Math.pow(Sidelength,2)
Examples:
The return statement in line 71 tells the function to send the results of the
expression back as a fixed decimal value with a length of two. The Number object's
toFixed() method returns a value set to a specific decimal length, as shown in Table 10-20.
Table 10-20 Number.toFixed() Method
General form:
Number.toFixed(digits)
Comment:
where
digits
is the exact number of digits after the decimal point. The number is rounded or
padded with zeros if necessary.
Examples:
Pmt = 234.8932
Pmt.toFixed(3)
Result: 234.893
Amt = 843.6778
Amt.toFixed(2)
Result: 843.68
