Javascript array join does not work correctly

Question

I have the following array and I want to join it into a number

    const arr = [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,3]
    const digits = arr.join("") //6145390195186705543
    const digitsToNumber = +arr.join("") //6145390195186705000
    console.log(digits);
    console.log(digitsToNumber);

You can see that the join function works. However, when I try to convert it into a number, it shows a weird value. Do you guys know why it happened that way?

*when I try to convert it into a number, it shows a weird value* what weird value? You title and the actual question also appear to have very little in common — Liam, Jul 07 '21 at 11:59
I assume you mean `arr.join`, but check out `Number.MAX_SAFE_INTEGER`. — Dave Newton, Jul 07 '21 at 12:00
It's weird, that the number to convert is exactly the same as in the question 0stone0 has linked ..? — Teemu, Jul 07 '21 at 12:11
@Teemu https://forum.freecodecamp.org/t/javascript-coding-exercise/214499 via leetcode. — Dave Newton, Jul 07 '21 at 12:14

score 1 · Answer 1 · answered Jul 07 '21 at 12:14

As stated in the comments, the value is too large for JavaScript and is truncated.

We can use BigInt to prevent this. Use with caution!

const arr = [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,3]

const digits = arr.join(''); 
const digitsToNumber = +arr.join(""); 
const bigDigitsToNumber = BigInt(arr.join(''));


console.log(digits);                          // 6145390195186705543
console.log(digitsToNumber);                  // 6145390195186705000
console.log(bigDigitsToNumber.toString());    // 6145390195186705543

ruppy · Answer 2 · 2021-07-07T12:10:38.823

0

To convert your concatenated string into a number you could use parseInt("123") method.

const number= parseInt(digitsToNumber)

However because of your number is too big or could be bigger, Javascript can not handle that numbers which contains more than 16 digit. If you also have a problem like that you can use some external big number libraries like BigNumber.Js.

Edit: According to Teemu's comment, you could also use link native big integer handling library.

edited Jul 07 '21 at 12:10

answered Jul 07 '21 at 12:04

ruppy

45
7

The `+` is not a typo, it is one way to convert a string into a number. It's not the number of digits but the value of those digits. – Dave Newton Jul 07 '21 at 12:05
Oh sorry, i edited it – ruppy Jul 07 '21 at 12:06
The numbers still differ – Charlie Araya Jul 07 '21 at 12:11

Tushar Shahi · Accepted Answer · 2021-07-07T12:18:46.823

0

They will log different results because you are exceeding Number.MAX_SAFE_INTEGER - the highest value JS can safely compare and represent numbers.

One method to check if you are ever exceeding the limit (besides remembering the value) is Number.isSafeInteger()

Number.isSafeInteger(digitsToNumber); //false

From the docs: For larger integers, consider using the BigInt type.

edited Jul 07 '21 at 12:18

answered Jul 07 '21 at 12:09

Tushar Shahi

10,769
1
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Javascript array join does not work correctly

3 Answers3