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


var singleFloat = Math.fround(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.


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).


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

21502^150 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


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);


ECMAScript (ECMA-262)
The definition of 'Math.fround' in that specification.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
froundChrome Full support 38Edge Full support 12Firefox Full support 26IE No support NoOpera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Firefox Android Full support 26Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support 3.0nodejs Full support 0.12


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See also