How to convert BigInt to Number in JavaScript?

Question

I found myself in the situation where I wanted to convert a BigInt value to a Number value. Knowing that my value is a safe integer, how can I convert it?

Answer 1

Turns out it's as easy as passing it to the Number constructor:

const myBigInt = BigInt(10);  // `10n` also works
const myNumber = Number(myBigInt);

Of course, you should bear in mind that your BigInt value must be within [Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER] for the conversion to work properly, as stated in the question.

Answer 2

You can use parseInt or Number

const large =  BigInt(309);
const b = parseInt(large);
console.log(b);
const n = Number(large);
console.log(n);

Answer 3

Avoiding Caveats

I know the scope of the original question is knowing you already have a "safe integer", but I wanted to give scope to others that land here how they should try to handle it.

Number.isSafeInteger

One would think you could utilize Number.isSafeInteger, but something trivial like the following snippet will show that utilizing the function on a BigInt will always be false even if the value is within the minimum and maximum bounds of a Number.

console.log(Number.isSafeInteger(BigInt(1)))
// Returns false

An Implementation using Results

This utilizes the TS-Results library to return a success or error response, rather than doing something like throwing.

If you do not want to use the Results library, you could instead return the value and throw otherwise, or handle it however you see appropriate.

import { Err, Ok, Result } from 'ts-results'

export function bigIntToNumber(value: bigint): Result<number, Error> {
  if (validInterger(value)) {
    return Ok(Number(value))
  }

  return Err(new Error(`Value: "${value}" is out of bounds to be a Number`))
}

function validInteger(value: bigint): boolean {
  return value <= Number.MAX_SAFE_INTEGER && value >= Number.MIN_SAFE_INTEGER
}

An Aside on Data Types and Data Ranges

Something interesting to note is that a JS number go up to 9,007,199,254,740,991 (the Number.MAX_SAFE_INTEGER) while your classic 4-byte integer would only be 2,147,483,647, thus a JS Number can hold more, but not as much as an 8-byte integer which goes up to 9,223,372,036,854,775,807.

As noted in comments there can be numbers past Number.MAX_SAFE_INTEGER up to Number.MAX_VALUE that are still technically valid JS Numbers. You can read more here:

• Whats the difference between JS Number.MAX_SAFE_INTEGER and MAX_VALUE?

Answer 4

Edit: see the discussion in the comments below as to why this answer is not correct. I am leaving the answer up regardless for disambiguation.

You should use either of the static methods:

BigInt.asIntN() - Clamps a BigInt value to a signed integer value, and returns that value. BigInt.asUintN() - Clamps a BigInt value to an unsigned integer value, and returns that value.

as documented here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt#static_methods