files/en-us/web/javascript/reference/global_objects/bigint/asintn/index.md

metadata

title: BigInt.asIntN()
short-title: asIntN()
slug: Web/JavaScript/Reference/Global_Objects/BigInt/asIntN
page-type: javascript-static-method
browser-compat: javascript.builtins.BigInt.asIntN
sidebar: jsref

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.

{{InteractiveExample("JavaScript Demo: BigInt.asIntN()")}}

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

• {{jsxref("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

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

Unlike similar language APIs such as {{jsxref("Number.prototype.toExponential()")}}, asIntN is a static property of {{jsxref("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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{jsxref("BigInt")}}
• {{jsxref("BigInt.asUintN()")}}