BigInt.asIntN()

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

Try it

const I64_CEIL = 2n ** 63n;

console.log(BigInt.asIntN(64, I64_CEIL - 1n));
// 9223372036854775807n (2n ** 64n - 1n, the maximum non-wrapping value)
console.log(BigInt.asIntN(64, I64_CEIL));
// -9223372036854775808n (wraps to min value)
console.log(BigInt.asIntN(64, I64_CEIL + 1n));
// -9223372036854775807n (min value + 1n)
console.log(BigInt.asIntN(64, I64_CEIL * 2n));
// 0n (wrapped around to zero)
console.log(BigInt.asIntN(64, -I64_CEIL * -42n));
// 0n (also wraps on negative multiples)

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.

js
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