---
title: math.asin
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/functions/math-trig/math-asin
---

```
FLOAT math.asin(FLOAT x)
```

**Available in:** all subroutines

Computes the principal value of the arc sine of the argument `x`.

## Parameters

`x` - Floating point value. The value of `x` should be in the range -1 to 1
inclusive.

## Return Value

Upon successful completion, this function returns the arc sine of `x`, in the
range -`math.PI_2` to
`math.PI_2` radians inclusive.

If `x` is `math.NAN`, a NaN will be returned.

If `x` is ±0, `x` will be returned.

If `x` is `math.POS_INFINITY` or
`math.NEG_INFINITY`, a domain error
occurs and a NaN will be returned.

If `x` is _subnormal_, a range error occurs and `x` will be returned.

For finite values of x not in the range -1 to 1 inclusive, a domain error
occurs and a NaN will be returned.

## Errors

- If the `x` argument is finite and is not in the range -1 to 1 inclusive, or is
  `math.POS_INFINITY` or
  `math.NEG_INFINITY`, then
  `fastly.error` will be set to `EDOM`.
- If the `x` argument is _subnormal_, then
  `fastly.error` will be set to `ERANGE`.

## Example

```vcl
declare local var.fo FLOAT;

set var.fo = math.asin(1.0);
```