Maclaurin/Taylor Series Calculation of Sin and Cos in JavaScript

sin_cos_functions.js

// round a number (x) to nearest 10 digits
const rounded = (x) => {
	return parseFloat(x.toFixed(10));
}

// get the factorial of a number (x)
// factorial(x) is the product of every number from 1 to x inclusive
const factorial = (x) => {
	let n = 1; // n is the result
	// multiply n by every number from 1 to x inclusive
	for(let i = 2; i <= x; i++) {
		n *= i;
	}
	return n;
}

/* get the result of the cos and sin formulas
   where the functions are sin(x radians) or cos(x radians),
   n is the start value (x for sin, 1 for cos), and i_start
   is the exponent and factorial base in the first term */
const computeSeries = (x, n, i_start) => {
	const iterations = 20; // iterations is twice the amount of terms to use
	let multiplier = 1;
	let i = i_start;
	while(i < i_start + iterations) {
		multiplier *= -1; // alternates between addition and subtraction each iteration
		const next_term = (x**i) / factorial(i); // each term is (x^i) / i!
		n += multiplier * next_term // add or subtract from final result
		i += 2 // i increases by 2 each term
	}
	return n
}

// get sin of x radians
const sin = (x) => {
	return rounded(computeSeries(x, x, 3));
}
// get cos of x radians
const cos = (x) => {
	return rounded(computeSeries(x, 1, 2));
}
// get sin of x degrees
const sinDeg = (x) => {
	return sin(x * Math.PI / 180);
}
// get cos of x degrees
const cosDeg = (x) => {
	return cos(x * Math.PI / 180);
}

// test the functions
console.log(sin(Math.PI / 6)); // 0.5
console.log(sinDeg(45)); // 0.7071
console.log(sinDeg(52)); // 0.78801

console.log(cos(Math.PI / 3)); // 0.5
console.log(cosDeg(45)); // 0.7071
console.log(cosDeg(52)); // 0.615661