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.

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BigInt.asUintN(bits, bigint)



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


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

Return value

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



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


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.


Staying in 64-bit ranges

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

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


ECMAScript Language Specification
# sec-bigint.asuintn

Browser compatibility

BCD tables only load in the browser

See also