Compound Interest Calculator — JavaScript source

/**
 * compound-interest-calculator - compound growth projection.
 *
 * Display snippet: ports the core project() math from the TypeScript lib
 * (src/lib/interest.ts). The full projection (contributions, inflation,
 * per-year rows) lives in the TypeScript/Go sources.
 *
 * Semantics: monthly simulation with the monthly rate set to the exact
 * equivalent of the stated compounding frequency — (1 + r/n)^(n/12) - 1 —
 * so the effective annual rate is preserved for every supported
 * frequency. Returns the final balance rounded to 2 decimals, or null on
 * invalid input (negative principal, non-integer years, unsupported
 * frequency, non-finite values).
 */

const VALID_COMPOUNDS = new Set([1, 2, 4, 12, 365]);

/** Round to 2 decimals, absorbing binary float noise. */
function round2(n) {
  return Math.round((n + Number.EPSILON) * 100) / 100;
}

function project(principal, ratePct, years, compoundsPerYear) {
  if (![principal, ratePct, years, compoundsPerYear].every(Number.isFinite)) return null;
  if (principal < 0 || years < 0 || !Number.isInteger(years)) return null;
  if (!VALID_COMPOUNDS.has(compoundsPerYear)) return null;

  const monthlyRate = Math.pow(1 + ratePct / 100 / compoundsPerYear, compoundsPerYear / 12) - 1;

  let balance = principal;
  for (let month = 0; month < years * 12; month++) {
    balance = balance * (1 + monthlyRate);
  }
  return round2(balance);
}

// Example:
//   project(10000, 5, 10, 12) -> 16470.09