Node.js: readline inside replace() - node.js

Question

I have a string with the alphabet and certain letters are in parenthesis. It means these letters should be uppercase. But the user has to be asked first if he wants it.
The problem is, I have to call readline inside a loop and this doesn't work.
const readline = require('readline');
const text = 'a(b)cdefg(h)ijklmnopqrst(u)vwxyz';
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
text.replace(/\((\w)\)/g, function (match, p1) {
let answer;
rl.question(`Do you want to change ${p1}? [yes/no] `, input => {
answer = input;
});
return answer === "yes" ? p1.toUpperCase() : p1;
});
console.log("Result:", text);
It just prints out Do you want to change b? [yes/no] Result: a(b)cdefg(h)ijklmnopqrst(u)vwxyz.
How can this be solved?

Answer 1

As Nick has explained in the comments the main issue of your code it that it does not handle the async readline operation correctly - here's how I would do this:
use a promisifed version of readline to prevent nested callbacks
use a while loop to step through all regex matches and await the user input in order to convert the chars to upper case or not
Something like (still needs error handling, but should give you an idea):
const readline = require('readline').createInterface({
input: process.stdin,
output: process.stdout
});
function question(query) {
return new Promise(resolve => {
readline.question(query, resolve);
})
}
const specialCharsRegex = /\((\w)\)/g;
const text = 'a(b)cdefg(h)ijklmnopqrst(u)vwxyz';
(async () => {
let result = text;
let currMatch;
while ((currMatch = specialCharsRegex.exec(text)) !== null) {
const convertInput = await question(`Do you want to change ${currMatch[1]}? [yes/no] `);
result = result.replace(currMatch[0], v => {
return convertInput === "yes" ? currMatch[1].toUpperCase() : currMatch[1];
});
}
console.log("Result: ", result);
readline.close();
})();

Answer 2

The main issue with your code is that you're trying to do something asynchronous in a callback that is expected to be synchronous - that is, you're trying to read user input in the .replace() callback, but reading user input doesn't happen immediately, so the callback to .question() occurs sometime after your question has been asked and your replace callback has terminated/finished. Making rl.question() synchronous would fix this (Node.js synchronous prompt) and would allow you to prompt against the exact words you're replacing.
If you want to keep things asynchronous, you could first match the letters you want to replace using .matchAll(), that way you'll have the text/groups, and the indexes of the characters you want to change. Then, you can use a loop to iterate the matches, and prompt the user whether they want to change the group. For each group, you can update the text accordingly by updating the text at the specified index. You could use replace, but if your text has duplicates eg: "a(b)c(b)e", and you said "no" for the first (b) but yes for the second (b), then you'll either need to replace both occurrences, or the first match. By using the index, you can specify exactly which group to change:
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin, //or fileStream
output: process.stdout
});
const question = q => new Promise(res => rl.question(q, res));
let text = 'a(b)cdefg(h)ijklmnopqrst(u)vwxyz';
(async () => {
const matches = text.matchAll(/\((\w)\)/g);
for(const {0: match, 1: p1, index} of matches) {
const ans = await question(`Do you want to change ${p1}? [yes/no] `);
if(ans === "yes")
text = text.slice(0, index) + match.toUpperCase() + text.slice(index + match.length);
}
console.log(text);
})();