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DJ

Div John

Published in

October 20, 2026

· Updated: October 21, 2026

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Most early-stage founders keep their runway in a spreadsheet nobody trusts. One cell is hard-coded, another formula was dragged one row too far, and the "what if churn doubles?" question means copying the whole sheet. A small, pure JavaScript module is easier to test, easier to version, and easy to rerun with different assumptions.

In this tutorial we'll build a runway calculator in plain ES modules, with no dependencies. It runs in Node 18+ or in a browser.…