Created this function to help determine how much to contribute (annually) to my 401(k) to retire a decamillionaire.

gistfile1.js

/**
 * Calculate the annual contributions required to reach a desired future value.
 * @param {number} P - Current principle value.
 * @param {number} FV - Desired future value.
 * @param {number} r - Rate of return.
 * @param {number} Y - Years of growth.
 * @returns {number} Annual contribution.
 */
function annualContributions(P, FV, r, Y) {
	var z = 1 + r;
	return -(P * Math.pow(z, Y) - FV) / ((Math.pow(z, Y + 1) - z) / r);
}

// To become a millionaire by retirement with 40 years to contribute
annualContributions(0, 1e6, 0.12, 40); // 1163.9514133216733