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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/atan below:

Math.atan() - JavaScript | MDN

Math.atan()

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The Math.atan() static method returns the inverse tangent (in radians) of a number, that is

𝙼𝚊𝚝𝚑.𝚊𝚝𝚊𝚗 ( 𝚡 ) = arctan ( x ) = the unique  y ∊ [ − π 2 , π 2 ]  such that  tan ( y ) = x \mathtt{\operatorname{Math.atan}(x)} = \arctan(x) = \text{the unique } y \in \left[-\frac{\pi}{2}, \frac{\pi}{2}\right] \text{ such that } \tan(y) = x Try it 
// 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
Syntax Parameters Return value

The inverse tangent (angle in radians between - π 2 -\frac{\pi}{2} and π 2 \frac{\pi}{2} , inclusive) of x. If x is Infinity, it returns π 2 \frac{\pi}{2} . If x is -Infinity, it returns - π 2 -\frac{\pi}{2} .

Description

Because atan() is a static method of Math, you always use it as Math.atan(), rather than as a method of a Math object you created (Math is not a constructor).

Examples Using Math.atan() 
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);

Note that you may want to avoid the theta function and use Math.atan2() instead, which has a wider range (between -π and π) and avoids outputting NaN for cases such as when x is 0.

Specifications Browser compatibility See also

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