/JavaScript

# Math.fround

The `Math.fround()` function returns the nearest 32-bit single precision float representation of a `Number`.

## Syntax

`var singleFloat = Math.fround(doubleFloat);`

### Parameters

`doubleFloat`
A `Number`. If the parameter is of a different type, it will get converted to a number or to `NaN` if it cannot be converted.

### Return value

The nearest 32-bit single precision float representation of the given number.

## Description

JavaScript uses 64-bit double floating-point numbers internally, which offer a very high precision. However, sometimes you may be working with 32-bit floating-point numbers, for example if you are reading values from a `Float32Array`. This can create confusion: Checking a 64-bit float and a 32-bit float for equality may fail even though the numbers are seemingly identical.

To solve this, `Math.fround()` can be used to cast the 64-bit float to a 32-bit float. Internally, JavaScript continues to treat the number as a 64-bit float, it just performs a "round to even" on the 23rd bit of the mantissa, and sets all following mantissa bits to `0`. If the number is outside the range of a 32-bit float, `Infinity` or `-Infinity` is returned.

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

## Examples

### Using Math.fround()

The number 1.5 can be precisely represented in the binary numeral system, and is identical in 32-bit and 64-bit:

```Math.fround(1.5); // 1.5
Math.fround(1.5) === 1.5; // true
```

However, the number 1.337 cannot be precisely represented in the binary numeral system, so it differs in 32-bit and 64-bit:

```Math.fround(1.337); // 1.3370000123977661
Math.fround(1.337) === 1.337; // false
```

$2^150$ is too big for a 32-bit float, so `Infinity` is returned:

```2 ** 150; // 1.42724769270596e+45
Math.fround(2 ** 150); // Infinity
```

If the parameter cannot be converted to a number, or it is not-a-number (`NaN`), `Math.fround()` will return `NaN`:

```Math.fround('abc'); // NaN
Math.fround(NaN); // NaN
```

## Polyfill

This can be emulated with the following function, if `Float32Array` are supported:

```Math.fround = Math.fround || (function (array) {
return function(x) {
return array[0] = x, array[0];
};
})(new Float32Array(1));
```

Supporting older browsers is slower, but also possible:

```if (!Math.fround) Math.fround = function(arg) {
arg = Number(arg);
// Return early for ±0 and NaN.
if (!arg) return arg;
var sign = arg < 0 ? -1 : 1;
if (sign < 0) arg = -arg;
// Compute the exponent (8 bits, signed).
var exp = Math.floor(Math.log(arg) / Math.LN2);
var powexp = Math.pow(2, Math.max(-126, Math.min(exp, 127)));
// Handle subnormals: leading digit is zero if exponent bits are all zero.
var leading = exp < -127 ? 0 : 1;
// Compute 23 bits of mantissa, inverted to round toward zero.
var mantissa = Math.round((leading - arg / powexp) * 0x800000);
if (mantissa <= -0x800000) return sign * Infinity;
return sign * powexp * (leading - mantissa / 0x800000);
};```

## Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
`fround` 38 12 26 No 25 8
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
`fround` 38 38 26 25 8 3.0
Server
Node.js
`fround` 0.12