How can you keep the leading zeros in a new Number() in JavaScript

Question

guys.

I'm having some issues with this function

const incrementString = str => {
  if (!str.match(/[\d+]$/)){
    return str += 1
  } else{
    return str.replace(/[\d+]$/, ch => new Number(ch) + 1)
  }
}

What I'm trying to do with that function is + 1 the number at the end of the string, and if the string doesn't has one, I'll add a 1 at the end.

string      expected
"foobar000" "foobar001"
"foo"       "foo1"
"foobar025" "foobar026"

I don't know if it's possible to do it with replace and regex, I have in mind a solution with loops, .length, split, etc..., but I want to do it with regex, if it's possible.

Problem: How can I take the number at the end of the string, with the leading zeros, and sum them a 1? this are some examples of the bad behavior of my function

Expected: 'foobar011', instead got: 'foobar11'
Test Passed: Value == 'foo1'
Expected: 'foobar002', instead got: 'foobar2'
Test Passed: Value == 'foobar100'
Test Passed: Value == 'foobar100'
Test Passed: Value == '1'

Thanks and happy holydays

Hint: Numbers don't have leading zeros. *Strings* of numbers formatted as such might. — user2864740, Dec 28 '18 at 18:19
String.prototype.padStart could be used with the original number's string length — Taplar, Dec 28 '18 at 18:20
See also: https://stackoverflow.com/search?q=%5Bjavascript%5D+number+leading+zero — user2864740, Dec 28 '18 at 18:21
wait. how come i get the expected value ? using Firefox browser and WebStorm — Tang Yun, Dec 28 '18 at 18:25

score 7 · Answer 1 · answered Dec 28 '18 at 18:27

7

You could store the length of the numerical string and apply after incrementing the wanted leading zeroes.

function increment(s) {
    var [left, right = '0'] = s.split(/(\d*$)/),
        length = right.length;

    return left + (+right + 1).toString().padStart(length, '0');
}

console.log(['foobar000', 'foo', 'foobar025'].map(increment));

answered Dec 28 '18 at 18:27

Nina Scholz

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Is line 2 is creating to `var`s and assigning the value `s.split(/(\d*$)/)` to left and `'0'` to right? How is called this syntax? I've never seen it before. – Mattia Dec 28 '18 at 18:42
2

it's a [destructuring assignment](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment). – Nina Scholz Dec 28 '18 at 18:45

score 1 · Accepted Answer · answered Dec 28 '18 at 19:43

I used all your hints and answers to check different options to solve my problem.

Finally, I solved with the "String of numbers" and the padStart hints.

const incrementString = str => {
  if (str.match(/[0-9+]+/g)) {
    let numbers = str.replace(/[a-z+]+/g, "")
    return str.replace(numbers, (Number(numbers) + 1 + '').padStart(numbers.length, 0))
  } else {
    return str + 1
  }
}

I hope this helps others as it helped to me.

Thanks and happy holydays

How can you keep the leading zeros in a new Number() in JavaScript

2 Answers2