Javascript Replace the first character

How do I replace the first character of a string with another character? I am casting the number as string.

Example: String 12341
New string: 92341

I tried this script but cant figure a way to only replace the first character>

var oldStr =12341;
var newStr = oldStr.replace(1, 9);

>Solution :

1. oldStr is not a string, it’s a number.

2. replace has to replace characters, not numbers.

3. So coerce the number to a string, and then do the replacement.

const oldStr = 12341;
const newStr = oldStr.toString().replace('1', '9');
console.log(newStr);