matrix.js

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

let cw = window.innerWidth;
let ch = window.innerHeight;

window.onresize = () => {
	canvas.width = window.innerWidth;
	canvas.height = window.innerHeight;
	cw = window.innerWidth;
	ch = window.innerHeight;
	maxColumns = cw/fontSize;
};

let charArr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '1', '2', '3', '4', '5', '6', '7', '8', '9', '€', '$', '£', 'µ', '§'];
let maxCharCount = 100;
let fallingCharArr = [];
let fontSize = 15;
let maxColumns = cw/fontSize;

canvas.width = cw;
canvas.height = ch;

let frames = 0;

class FallingChar {

	constructor(x, y) {
		this.x = x;
		this.y = y;
	}

	draw(ctx) {
		this.value = charArr[Math.floor(Math.random() * (charArr.length - 1))].toUpperCase();
		this.speed = Math.random() * fontSize * 3 /4 + fontSize * 3 /4;

		ctx.fillStyle = "rgba(0, 255, 0)";
		ctx.font = fontSize + "px Courier";
		ctx.fillText(this.value, this.x, this.y);
		this.y += this.speed;

		if (this.y > ch) {
			this.y = Math.random() * ch/2 - 50;
			this.x = Math.floor(Math.random() * maxColumns) * fontSize;
			this.speed = - Math.random() * fontSize * 3 /4 + fontSize * 3 /4;
		}
	}
}

let update = () => {
	if (fallingCharArr.length < maxCharCount) {
		let fallingChar = new FallingChar(Math.floor(Math.random() * maxColumns) * fontSize, Math.random() * ch/2 - 50);
		fallingCharArr.push(fallingChar);
	}

	ctx.fillStyle = "rgba(0, 0, 0, 0.05)";
	ctx.fillRect(0, 0, cw, ch);

	for (let i = 0; i < fallingCharArr.length && frames % 2 == 0; i++){
		fallingCharArr[i].draw(ctx);
	}

	requestAnimationFrame(update);
	frames++;
}

update();