Original version from https://killedbyapixel.github.io/TinyCode/games/CrossMyHeart/

original.js

X=Y=V=U=Z=t=1;onkeydown=e=>(k=e.which)&1?X-=k-38:Y-=k-39;setInterval("for(++t,o=Z,i=50;i--;)for(b.innerText=o+='\\n',j=99;j--;Y>49?Z+=Y=1:o+=i-Y|j-X-50?q?'.':'█':q?'♥':Y=1)q=((j+t*Math.sin(A=i>>1)/9*Z>>3)*A*Z)**4%97<89",9)

simplified.js

X=Y=1; // player position
level=t=1;

onkeydown = e => {
	let k = e.which; // a deprecated but functional equivalent to e.keyCode
	// left  = 37
	// right = 39
	// up    = 38
	// down  = 40
	if (k & 1) { // up or down
		X -= k - 38;
	} else { // left or right
		Y -= k - 39
	}
};

setInterval(() => {
	++t; // increment time
	let out = level; // current level is displayed at the top
	for(let i = 50 + 1; i >= 0; i--) { // for each line of generated text
		let vehicle_row = i >> 1; // floored div by 2 (vehicles are 2 rows tall)
		b.innerText = out += '\n';
		for(let j = 100; j >= 0; j--) {

			// procedural vehicle position calculation.
			// to be honest I can't figure out how the longer vehicles come to be - very clever
			let is_empty_space = (
				(j + t * Math.sin(vehicle_row)/9 * level>>3) * vehicle_row * level
			)**4 % 97 < 89;

			if ( Y > 49 ) { // top row, time to level up
				Y=1;
				level++;
			} else { // do collision detection
				if (i-Y | j-X-50) { // if the player is not at this coordinate (X coordinate centered on column 50)
					out += is_empty_space ? '.' : '█'; // render road/vehicles
				} else {
					out += is_empty_space ? '♥' : Y=1; // render the player if no collision, or reset Y pos
				}
			}
		}
	}
}, 9); // run every 9ms