15.8.2.4 Math.atan(x)

number Math.atan(x:number|mixed)

Return arc tangent of x.

Result expressed in radians! In range of -π/2 to +π/2.

Math.atan = function(x){
x = ToNumber(x);
if (isNaN(x)) return NaN;
if (x === +0) return +0;
if (x === -0) return -0;
if (x === +Infinity) return Math.PI/2;
if (x === -Infinity) return -Math.PI/2;
return Math.acos(x);
}