How to replace the first occurrence of a character with a string in JavaScript?

Question

I have a string which contains characters that should be replaced once (at first appearance).

These characters are:

• L => will be replaced with the lecture name
• N => will be replaced with a name
• D => will be replaced with a date

Example input:

L_N_L_D

Desired result (note that only the first L is replaced):

Math_Ex01_L_2018-10-05

My current code (simplified for the example):

let res = file_string.replace(/L|N|D/, x => {
  switch (x) {
    case 'L': return lecture;
    case 'N': return name;
    case 'D': return date;
    default: return x;
  }
});

What I get is this:

Math_L_N_D

If I change the regex to /L|N|D/g the second L will also be replaced which is not what I want.

How could this be implemented?

Answer 1

Answering your general question "replace only the first occurrence of a character", you could do it like this:

 var lecture = "Math"; var name = "Ex01"; var date = "2018-10-05"; var found = {}; var file_string = "L_N_L_D"; var filename_result = file_string.split("").map(function (character) { if (!found[character]) { found[character] = true; switch (character) { case "L": return lecture; case "N": return name; case "D": return date; } } return character; }).join(""); console.log(filename_result); 

You should probably explain where that odd format of the file_string comes from. Are there other similar use cases? Or is this just about this specific example? Understanding the original requirements would help a lot.

If the file_string is supposed to be a configurable format string, then it should be improved. A sequence which serves as a placeholder to be replaced should (or must ) be different from a literal, eg:

var filename_format = "{L}_{N}_L_{D}";

Answer 2

Put your replacements in a map, once a replacement is made, set map[x] to x :

 let lecture = "Math"; let name = "Ex01"; let date = "2018-10-05"; let repl = { 'L': lecture, 'N': name, 'D': date }; let file_string = "L_N_L_D" let result = file_string.replace(/[LND]/g, x => { let r = repl[x]; repl[x] = x; return r; }); console.log(result) 

Apart from solving the problem at hand, this also greatly simplifies your replacement function (think adding new placeholders, for example).

That being said, a real solution to your problem would be to follow the @marsze's advice and use unambiguous placeholders, like {...} , in which case the whole enterprise becomes simply

 repl = {...as before...}
 result = subject.replace(/{(.+?)}/g, (_, x) => repl[x])