How To Check If A Number Is A BigInt In JavaScript?

Question

I want to check if a number is a BigInt in an acceptable size if statement

I know there is this try/catch solution:

function isBigInt(x) {
    try {
        return BigInt(x) === x;
    } catch(error) {
        return false;
    }
}

I want to implement the test directly in an if statement and not as a function.

Can someone help me with this?

Answer 1

You can use typeof. If the number is a BigInt, the typeof would be "bigint". Otherwise, it will be "number"

var numbers = [7, BigInt(7), "seven"];
numbers.forEach(num => {
  if (typeof num === "bigint") {
    console.log("It's a BigInt!");
  } else if (typeof num === "number") {
    console.log("It's a number!");
  } else {
    console.log("It's not a number or a BigInt!");
  }
});

Answer 2

An easy solution would simply be (assuming you only work with numbers):

Number.prototype.bigInt = false
BigInt.prototype.bigInt = true

And then you could simply call myNum.bigInt to check.

NOTE: 3.bigInt will not work because 3. is considered a number, so you could do:

(3).bigInt
 3 .bigInt

or use it on a variable

Answer 3

Quick suggestion:

if (BigInt(n) === n.valueOf()) {
  // is BigInt
} else {
  // not a BigInt
}

Handles both literal BigInt such as 123n, and an Object-wrapped one: Object(123n). Both forms are valid BigInts with different typeof results: "bigint" vs an "object".

Answer 4

If you're only interested to see if the value is a native BigInt, I think @Rojo's suggestion to use

typeof value === 'bigint'

is the best.

But if the BigInt is provided by a custom class (a 'polyfill') then the typeof expression will evaluate to 'object'.

But the following does work:

value.constructor === BigInt

(Note: value instanceof BigInt does not work for the native BigInt type)

Answer 5

A ternary operator can help you, if I understood your question well:

const isBigInt = BigInt(x) === x;

Your variable isBigInt will either be true or false depending on whether your number is a BigInt or not.

And a BigInt object is not strictly equal to Number, but can be in the sense of weak equality.

0n === 0
// ↪ false
    
0n == 0
// ↪ true

Visit : https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Global_Objects/BigInt for more about BigInt.