My friends needed an interest calculator. For the simple interest calculator, the parameters are:
- The principal (P),
- Annual interest rate (r),
- Number of months (t), and
- An optional parameter to represent a periodic top-up amount.
Here’s v1 of my simple interest calculator:
/**
* Calculates simple interest.
*
* Uses the formula A = P(1 + rt).
*
* @param {Object} param Object containing parameters.
* @param {Number} param.principal Principal amount.
* @param {Number} param.interestRate Interest rate.
* @param {Number} param.numInterestDeposits Number of time periods.
*
* @return {Object} Object containing totalAmount and totalInterest.
*/
export const calcSimpleInterest = ({
principal,
interestRate,
numInterestDeposits,
}) => {
const totalInterest = principal * interestRate * numInterestDeposits;
return {
totalAmount: principal + totalInterest,
totalInterest,
};
};
And here’s v2 of it:
import nerdamer from 'nerdamer/nerdamer.core';
import Algebra from 'nerdamer/Algebra';
import Calculus from 'nerdamer/Calculus';
import Solve from 'nerdamer/Solve';
export const calcSimpleInterest = ({
principal,
interestRate,
numInterestDeposits,
}) => {
const totalAmount = solveForOneUnknownVariable(
'a=p+p*r*t',
{
p: principal.toString(),
r: interestRate.toString(),
t: numInterestDeposits.toString(),
},
'a',
principal,
);
return {
totalAmount,
totalInterest: totalAmount - principal,
};
};
const solveForOneUnknownVariable = (
equationString,
knownValues,
unknownVariable,
defaultValue,
) => {
const equation = nerdamer(equationString).evaluate(knownValues);
const solution = equation.solveFor(unknownVariable);
return Number(solution.text());
};
Both versions treat numInterestDeposits in years and have not handled the periodic top-ups. Also, the compound interest calculator has compoundRate. The compound rate is the number of times the annual interest compounds in a year. It looks like I still have a long way to go…