How to convert BigInt to Number in JavaScript?

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.

You can use parseInt or Number

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

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