Calculating the amount of money you need to set aside to reach a certain amount of wealth. Basically reversing compound interest by brute force.

reverse-compound-interest.js

// This script is terribly inefficient, but on a decent computer it takes 1 second to complete anyway. I made it on an airplane while half asleep. 

const target = 600000		// The target net worth
const roi 	 = 4			// The assumed return on investment yearly
const term 	 = 20			// Time until retirement
const start  = 0     		// Starting capital
const desire = 600000		// Desired capital

// Configuration
const verbose = false

// Function to calculate the compound result of investing with the above parameters based on a monthly addition
// The monthly addition is assumed to be a yearly addition for simplicity sake
const compound = monthly => {
	let result = start
	let input  = start
	// Loop over the years
	for (var i = term - 1; i >= 0; i--) {
		result += monthly * 12
		result *= ( 1 + ( roi / 100 ) )
		input  += monthly * 12
	}
	// Log out the results and return the resulting value
	if ( verbose ) console.log( 'Monthly ' + monthly + ' totals ' + input + ' becomes ' + result )
	return result
}


// Brute force the needed monthly investment based on an accuracy parameter
// The smaller the parameter the more accurate the amount, but the more expensive the calculation
const bruteForce = step => {
	let calculator = desire + 1
	let monthly = ( desire / term ) / 12

	while ( calculator > desire ) {
		calculator = compound( monthly )
		monthly -= step
	}
	console.log( 'Monthly investment needed: ' + monthly )
}

bruteForce( 1 )