line math

function lineEquation(ax, ay, bx, by) {
  const dx = bx - ax
  const dy = by - ay
  const k = dx === 0 ? 0 : dy / dx
  const n = -ax * k + ay
  return n === 0
    ? `y = ${k}x`
    : `y = ${k}x ${n < 0 ? '-' : '+'} ${Math.abs(n)}`
}

function lineIntersection(L1, L2) {
  const den = (L1[0].x - L1[1].x) * (L2[0].y - L2[1].y) - (L1[0].y - L1[1].y) * (L2[0].x - L2[1].x)
  if (den === 0) return 'No intersection'
  const X = ((L1[0].x * L1[1].y - L1[0].y * L1[1].x) * (L2[0].x - L2[1].x) - (L1[0].x - L1[1].x) * (L2[0].x * L2[1].y - L2[0].y * L2[1].x)) / den
  const Y = ((L1[0].x * L1[1].y - L1[0].y * L1[1].x) * (L2[0].y - L2[1].y) - (L1[0].y - L1[1].y) * (L2[0].x * L2[1].y - L2[0].y * L2[1].x)) / den
  return `x: ${X}, y: ${Y}`
}