BigInt.asUintN()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2020.

The BigInt.asUintN() static method truncates a BigInt value to the given number of least significant bits and returns that value as an unsigned integer.

Try it

Syntax

js
BigInt.asUintN(bits, bigint)

Parameters

bits

The amount of bits available for the returned BigInt. Should be an integer between 0 and 253 - 1, inclusive.

bigint

The BigInt value to truncate to fit into the supplied bits.

Return value

The value of bigint modulo 2^bits, as an unsigned integer.

Exceptions

RangeError

Thrown if bits is negative or greater than 253 - 1.

Description

The BigInt.asUintN method truncates a BigInt value to the given number of bits, and interprets the result as an unsigned integer. Unsigned integers have no sign bits and are always non-negative. For example, for BigInt.asUintN(4, 25n), the value 25n is truncated to 9n:

25n = 00011001 (base 2)
         ^==== Use only the four remaining bits
===>      1001 (base 2) = 9n

Note: BigInt values are always encoded as two's complement in binary.

Unlike similar language APIs such as Number.prototype.toExponential(), asUintN is a static property of BigInt, so you always use it as BigInt.asUintN(), rather than as a method of a BigInt value. Exposing asUintN() as a "standard library function" allows interop with asm.js.

Examples

Staying in 64-bit ranges

The BigInt.asUintN() method can be useful to stay in the range of 64-bit arithmetic.

js
const max = 2n ** 64n - 1n;

BigInt.asUintN(64, max); // 18446744073709551615n

BigInt.asUintN(64, max + 1n); // 0n
// zero because of overflow: the lowest 64 bits are all zeros

Specifications

Specification
ECMAScript Language Specification
# sec-bigint.asuintn

Browser compatibility

BCD tables only load in the browser

See also