BigInt.asIntN()

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

Syntax

BigInt.asIntN(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 a signed integer.

Exceptions

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

Description

The BigInt.asIntN method truncates a BigInt value to the given number of bits, and interprets the result as a signed integer. For example, for BigInt.asIntN(3, 25n), the value 25n is truncated to 1n:

25n = 00011001 (base 2)
          ^=== Use only the three remaining bits
===>       001 (base 2) = 1n

If the leading bit of the remaining number is 1, the result is negative. For example, BigInt.asIntN(4, 25n) yields -7n, because 1001 is the encoding of -7 under two's complement:

25n = 00011001 (base 2)
         ^==== Use only the four remaining bits
===>      1001 (base 2) = -7n

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

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

Examples

Staying in 64-bit ranges

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

const max = 2n ** (64n - 1n) - 1n;

BigInt.asIntN(64, max); // 9223372036854775807n

BigInt.asIntN(64, max + 1n); // -9223372036854775808n
// negative because the 64th bit of 2^63 is 1