x

A number representing a cosine, where x is between -1 and 1.

The arccosine (angle in radians) of the given number if it's between -1 and 1; otherwise, NaN.

The Math.acos() method returns a numeric value between 0 and π radians for x between -1 and 1. If the value of x is outside this range, it returns NaN.

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

Math.acos(-2);  // NaN
Math.acos(-1);  // 3.141592653589793
Math.acos(0);   // 1.5707963267948966
Math.acos(0.5); // 1.0471975511965979
Math.acos(1);   // 0
Math.acos(2);   // NaN

For values less than -1 or greater than 1, Math.acos() returns NaN.

• Math.asin()
• Math.atan()
• Math.atan2()
• Math.cos()
• Math.sin()
• Math.tan()