JavaScript substring not working as expected when adding another string

Question

Can someone help me check this code below and tell me what I'm doing wrong.

Expected result

'2022-04-02'

Output result

"02022-04-2"

code

const str = '2022-04-2';

console.log(str.substring(8));


console.log(str.replace(str.substring(8, 9), "0" + str.substring(8,9)));
// expected output: "2022-04-02"

Answer 1

replace with a string target will find and replace the first occurrence of the target. The first occurence of 2 in 2022-04-2 is here:

2022-04-2
^

and not here:

2022-04-2
        ^

To do what you want, work with indices, don't use replace:

const str = '2022-04-2';

console.log(str.substring(0, 8) + "0" + str.substring(8));

Or be more precise about your targetting, if you do use it:

const str = '2022-04-2';

console.log(str.replace(/\b(\d)\b/g, "0$1"))

Answer 2

It's because replace doesn't work as you think. In the code you wrote it searches for the first occurrence of the string "2" and it replaces it with "02". In your case for "2022-04-02" the first "2" is the very first character.

I suggest a different approach:

const str = "2022-04-2";
const [year, month, day] = str.split("-");
const formattedDate = `${year}-${month.padStart(2, "0")}-${day.padStart(2, "0")}`;
console.log(formattedDate);

Answer 3

You can simply achieve it by using string.split() method.

Demo :

const str = '2022-04-2';

const splittedStr = str.split('-');

splittedStr[2] = '0' + splittedStr[2]

console.log(splittedStr.join('-'))