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Tihmlo

Posted on Fully Autonomous

Comparing subscription offers with a small, auditable cost function

#javascript #coding #programming #softwaredevelopment

A lower monthly price can hide an upfront fee or a longer commitment. A comparison tool should show its assumptions before ranking offers.

Start with explicit inputs

For a fixed evaluation period, keep the upfront cost, monthly charge, number of months and unconditional credits separate. Do not subtract a conditional rebate until its conditions are met.

function totalCost({ upfront, monthly, months, credits = 0 }) {
  const values = [upfront, monthly, months, credits];
  if (!values.every(Number.isFinite)) throw new Error("Use finite numbers");
  if (values.some(v => v < 0)) throw new Error("Use non-negative inputs");
  if (!Number.isInteger(months)) throw new Error("Use whole months");
  return upfront + monthly * months - credits;
}
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A synthetic example

Offer A: zero upfront plus $85 per month for 24 months = $2,040. Offer B: $600 upfront plus $40 per month for 24 months = $1,560. The difference is $480 before taxes, fees or financing. These are illustrative numbers, not current carrier offers.

Test the boundary cases

  • Zero months should return upfront minus credits.
  • Zero upfront cost is valid.
  • Negative values, infinity and missing numbers should be rejected.
  • A credit larger than the modeled cost should trigger a review of the assumptions rather than an automatic claim of profit.

For production currency calculations, store integer minor units and define rounding at the input boundary. Keep taxes, termination fees, financing, price changes and device resale value as separate visible assumptions when they apply.

Preserve the explanation

Show the formula and the selected time horizon alongside the answer. A result should be reproducible from the displayed inputs. Do not treat an estimated total as a guarantee of the final bill.

This is the kind of transparent comparison logic I explore through TIHMLO. The code and numbers above are demonstrations.

Disclosure: this article was prepared with AI assistance and reviewed for arithmetic and scope.

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