common.html

<script>
function SinCosTable(resolution) {
	const table = new Float32Array(resolution);
	for (let i = 0; i < resolution; i++) {
		table[i] = Math.cos(i * Math.PI / 2 / resolution);
	}

	const pi = Math.PI;
	return {
		cos: function (n) {
			let sign = 1;
			if (n < 0) n = -n;
			n /= pi;
			n = n - ~~(n/2)*2;
			if (n > 1) n = 2 - n;
			if (n > 0.5) {
				n = 1 - n;
				sign = -1;
			}
			const i = ~~(n * 2 * resolution);
			return table[i] * sign;
		},

		sin: function (n) {
			return this.cos( n - pi / 2);
		},
	};
}
  
const sincos= SinCosTable(100000);
</script>

Math.js

let a = 0;
for (let i = 0; i < 10000; i++) {
    a += Math.sin(i);
    a += Math.cos(i);
}

Table.js

let a = 0;
for (let i = 0; i < 10000; i++) {
    a += sincos.sin(i);
    a += sincos.cos(i);
}

Table size: 4 * resolution bytes (eg. 4 * 10000 = 40kB)