Math.atan()

// Calculates angle of a right-angle triangle in radians
function calcAngle(opposite, adjacent) {
  return Math.atan(opposite / adjacent);
}
console.log(calcAngle(8, 10));
// Expected output: 0.6747409422235527
console.log(calcAngle(5, 3));
// Expected output: 1.0303768265243125

Math.atan(x)

Math.atan(-Infinity); // -1.5707963267948966 (-π/2)
Math.atan(-0); // -0
Math.atan(0); // 0
Math.atan(1); // 0.7853981633974483  (π/4)
Math.atan(Infinity); // 1.5707963267948966  (π/2)
// The angle that the line (0,0) -- (x,y) forms with the x-axis in a Cartesian coordinate system
const theta = (x, y) => Math.atan(y / x);