Math operation in JS

Question

my code is suppose to calculate mortgage payments

var LA = 100000;
var RA=0.07;
var YA=30;

var R = ( RA / 12);
var r = (1 + R);
var Yr = (YA * 12);
var pay = (LA * Math.exp(r,Yr)*R)/(Math.pow(r,Yr)-1);

returns $224.12

which is wrong it needs to be $665.30 payment = [ LA * r^Yr * R ] / [ r ^Yr - 1]

For example:

30 year mortgage for $100,000 at 7% interest (0.07)

0.07 / 12 = 0.00583 (this is R)

30 * 12 = 360 (this is Yr)

1 + 0.00583 = 1.00583 (this is r)

payment = [ $100,000 * (1.00583)^360 * 0.00583 ] / [ (1.00583)^360 - 1 ]

Monthly Payments will be $665.30

any tips?

You cannot use square brackets in algebraic expressions. Just parentheses (round ones). — Pointy
– Pointy, Commented Oct 21, 2012 at 23:54
(LA * Math.exp(r,Yr)*R)/(Math.pow(r,Yr)-1); returns the same so i dont believe that is the issue but rather an standard that needs to be implemented — wat
– wat, Commented Oct 21, 2012 at 23:56
It is absolutely, positively true that square brackets should not be used as parts of algebraic computational expressions. They're syntactically valid because they construct arrays in JavaScript, and due to the way automatic type coercion happens you don't get errors, but they will absolutely not do what you think they do and you're asking for trouble. — Pointy
– Pointy, Commented Oct 22, 2012 at 0:14

Niet the Dark Absol · Accepted Answer · 2012-10-21 23:58:10Z

3

Use the correct function: Math.pow and not Math.exp.

Also, although square brackets will work, it's only because JavaScript is casting the arrays to strings, and then to numbers. Use parentheses instead.

answered Oct 21, 2012 at 23:58

Niet the Dark Absol

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2 Comments

Daniel Fischer Over a year ago

Is Math.exp silently ignoring the second argument?

Niet the Dark Absol Over a year ago

Pretty much, yes. It's not an error to have too many arguments - nor is it one to have too few (although that ususally causes other errors)

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