Why does the Number() method increment the value of a number when the length of the number is greater than 15?

Question

Whenever I run this, the number returned gets incremented, can anyone explain this to me?

let array = [9, 2, 2, 3, 3, 7, 2, 0, 3, 6, 8, 5, 4, 7, 7, 1]
return Number(array.join(''))

Outputs:

9223372036854772

Answer 1

The number is larger than Number.MAX_SAFE_INTEGER (2⁵³-1). You may want to use BigInt instead.

Example:

let array = [9, 2, 2, 3, 3, 7, 2, 0, 3, 6, 8, 5, 4, 7, 7, 1]
let num = BigInt(array.join(''));
console.log(num.toString());
console.log("Doubled:", (num * 2n).toString());
console.log("Squared:", (num ** 2n).toString());

You can use Number.isSafeInteger to check if a value is precise.

let array = [9, 2, 2, 3, 3, 7, 2, 0, 3, 6, 8, 5, 4, 7, 7, 1]
let num = Number(array.join(''));
console.log("Safe?", Number.isSafeInteger(num));

Answer 2

The result of array.join('') is "9223372036854772". That is much larger than Number.MAX_SAFE_INTEGER. Number constructor can't precisely hold numbers greater than Number.MAX_SAFE_INTEGER, and therefore you get this error. You might want to use something like a BigInt for handling such large numbers.