angle.js

/**
 * @param {Number} px - Player X
 * @param {Number} py - Player Y
 * @param {Number} pvx - Player Velocity-X
 * @param {Number} pvy - Player Velocity-Y
 * @param {Number} tx - Turret X
 * @param {Number} ty - Turret Y
 * @param {Number} ts - Turret Projectile Speed
**/
function getProjVel (px, py, pvx, pvy, tx, ty, ts) {
  const x = (px + pvx * (tx - px) - tx) / ((tx - px) * ts)
  const y = (py + pvy * (ty - py) - ty) / ((ty - py) * ts)
  return [x, y]
}

// Example use
const targetVelocity = getProjVel(0, 0, 0, 0, 5, 5, 1)
const targetAngle = Math.atan2(targetVelocity[1], targetVelocity[0]) * (180 / Math.PI)
console.log(targetVelocity, targetAngle)