javascript static method
BigInt.asUintN()
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.
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
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
bitsis negative or greater than 253 - 1.
- : Thrown if
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]
BigIntvalues are always encoded as two’s complement in binary.
Unlike similar language APIs such as 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.
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