Project Euler problems (numbering) : follow naming convention.

Project-Euler/Problem002.js

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+// https://projecteuler.net/problem=2
+const SQ5 = 5 ** 0.5 // Square root of 5
+const PHI = (1 + SQ5) / 2 // definition of PHI
+
+// theoretically it should take O(1) constant amount of time as long
+// arithmetic calculations are considered to be in constant amount of time
+export const EvenFibonacci = (limit) => {
+  const highestIndex = Math.floor(Math.log(limit * SQ5) / Math.log(PHI))
+  const n = Math.floor(highestIndex / 3)
+  return ((PHI ** (3 * n + 3) - 1) / (PHI ** 3 - 1) -
+    ((1 - PHI) ** (3 * n + 3) - 1) / ((1 - PHI) ** 3 - 1)) / SQ5
+}