IPMT Function

IPMT.js

// Copyright (c) 2012 Sutoiku, Inc. (MIT License)
 
// Some algorithms have been ported from Apache OpenOffice:
 
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function IPMT(rate, period, periods, present, future, type) {
  // Credits: algorithm inspired by Apache OpenOffice

  // Initialize type
  var type = (typeof type === 'undefined') ? 0 : type;

  // Evaluate rate and periods (TODO: replace with secure expression evaluator)
  rate = eval(rate);
  periods = eval(periods);

  // Compute payment
  var payment = PMT(rate, periods, present, future, type);
  
  // Compute interest
  var interest;
  if (period === 1) {
    if (type === 1) {
      interest = 0;
    } else {
      interest = -present;
    }
  } else {
    if (type === 1) {
      interest = FV(rate, period - 2, payment, present, 1) - payment;
    } else {
      interest = FV(rate, period - 1, payment, present, 0);
    }
  }
  
  // Return interest
  return interest * rate;
}

Thank you very much for this - saved a lot of head banging :)