JavaScript » Math » acos

Syntax:
Math.acos(x)

This method returns the arccosine of a number as a numeric value between 0 and PI radians. Passing it a value for 'x' which is outsite the range -1 to 1 will cause the Netscape browser to return NaN, and the Internet Explorer browser to return an error message. Passing it -1 will return the value of PI.