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

const U64_CEIL = 2n ** 64n; console.log(BigInt.asUintN(64, U64_CEIL - 1n)); // 18446744073709551615n (2n ** 64n - 1n, the maximum non-wrapping value) console.log(BigInt.asUintN(64, U64_CEIL)); // 0n (wraps to zero) console.log(BigInt.asUintN(64, U64_CEIL + 1n)); // 1n console.log(BigInt.asUintN(64, U64_CEIL * 2n)); // 0n (wraps on multiples) console.log(BigInt.asUintN(64, U64_CEIL * -42n)); // 0n (also wraps on negative multiples)

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® 2026 Language Specification
# sec-bigint.asuintn

Browser compatibility

See also