Add 'finance.js'

finance.js

@@ -0,0 +1,25 @@
+/**
+ * Calculate the # of periods to break-even on an initial investment,
+ * given some array of cash flows for as many periods.
+ *
+ * Returns -1 if never break even.
+ */
+function payback_period(initial, flows = []) {
+    let balance = -initial;
+    let periods = 0;
+
+    for ( const flow of flows ) {
+        console.log({flow, balance, periods})
+        if ( balance + flow > 0 ) {
+            periods += (Math.abs(balance)) / flow;
+            return periods;
+        } else {
+            balance += flow;
+            periods += 1;
+        }
+    }
+
+    return -1;
+}
+
+module.exports = exports = { payback_period }