files/en-us/web/javascript/reference/global_objects/math/acos/index.md

metadata

title: Math.acos()
short-title: acos()
slug: Web/JavaScript/Reference/Global_Objects/Math/acos
page-type: javascript-static-method
browser-compat: javascript.builtins.Math.acos
sidebar: jsref

The Math.acos() static method returns the inverse cosine (in radians) of a number. That is,

$$\forall x∊[-1,1],𝙼𝚊𝚝𝚑.𝚊𝚌𝚘𝚜(𝚡)=arccos(x)=\text{the unique }y∊[0,\pi ]\text{ such that }cos(y)=x\backslash forall\; x\; \backslash in\; [\{-1\},\; 1],\backslash ;\backslash mathtt\{\backslash operatorname\{Math.acos\}(x)\}\; =\; \backslash arccos(x)\; =\; \backslash text\{the\; unique\; \}\; y\; \backslash in\; [0,\; \backslash pi]\; \backslash text\{\; such\; that\; \}\; \backslash cos(y)\; =\; x$$

{{InteractiveExample("JavaScript Demo: Math.acos()")}}

// Calculates angle of a right-angle triangle in radians
function calcAngle(adjacent, hypotenuse) {
  return Math.acos(adjacent / hypotenuse);
}

console.log(calcAngle(8, 10));
// Expected output: 0.6435011087932843

console.log(calcAngle(5, 3));
// Expected output: NaN

Syntax

Math.acos(x)

Parameters

• x
  • : A number between -1 and 1, inclusive, representing the angle's cosine value.

Return value

The inverse cosine (angle in radians between 0 and π, inclusive) of x. If x is less than -1 or greater than 1, returns {{jsxref("NaN")}}.

Description

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).

Examples

Using Math.acos()

Math.acos(-2); // NaN
Math.acos(-1); // 3.141592653589793 (π)
Math.acos(0); // 1.5707963267948966 (π/2)
Math.acos(0.5); // 1.0471975511965979 (π/3)
Math.acos(1); // 0
Math.acos(2); // NaN

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{jsxref("Math.asin()")}}
• {{jsxref("Math.atan()")}}
• {{jsxref("Math.atan2()")}}
• {{jsxref("Math.cos()")}}
• {{jsxref("Math.sin()")}}
• {{jsxref("Math.tan()")}}
• CSS {{cssxref("acos()")}} function