I am loading a txt file and obtaining the values from the 'source' tags and trying to replace the 'destination tags'. Example, the source lines should replace the destination lines but it's not setting the destination lines correctly. Source <AddtlRmtInf>Hello 1</AddtlRmtInf> <AddtlRmtInf>Hello 2</AddtlRmtInf> <AddtlRmtInf>Hello 3</AddtlRmtInf> Destination <AddtlNtryInf>Hello 2</AddtlNtryInf> <AddtlNtryInf>Hello 3</AddtlNtryInf> <AddtlNtryInf>Test 3</AddtlNtryInf>

const sourceTagStart = '<AddtlRmtInf>';
        const sourceTagEnd = '</AddtlRmtInf>';
        const destinationTagStart = '<AddtlNtryInf>';
        const destinationTagEnd = '</AddtlNtryInf>';

        // Extract values between source tags
        const tagValues = [];
        let startIndex = fileContent.indexOf(sourceTagStart);
        let endIndex;
        while (startIndex !== -1) {
            endIndex = fileContent.indexOf(sourceTagEnd, startIndex + sourceTagStart.length);
            if (endIndex === -1) {
                log.error({title: 'End tag not found', details: 'The end tag was not found in the file content.'});
                return;
            }
            tagValues.push(fileContent.substring(startIndex + sourceTagStart.length, endIndex));
            startIndex = fileContent.indexOf(sourceTagStart, endIndex + sourceTagEnd.length);
        }
        log.debug({title: 'Tag Values', details: tagValues}); //
[
   "Hello 1",
   "Hello 2",
   "Hello 3"
]

        // Replace destination tags with values
        startIndex = fileContent.indexOf(destinationTagStart);
        let tagIndex = 0;
        while (startIndex !== -1 && tagIndex < tagValues.length) {
            endIndex = fileContent.indexOf(destinationTagEnd, startIndex + destinationTagStart.length);
            if (endIndex === -1) {
                log.error({title: 'End tag not found', details: 'The end tag was not found in the file content.'});
                return;
            }
            fileContent = fileContent.substring(0, startIndex + destinationTagStart.length) +
                tagValues[tagIndex] +
                fileContent.substring(endIndex);
            tagIndex++;
            startIndex = fileContent.indexOf(destinationTagStart, endIndex + destinationTagEnd.length);
        }

        // Save the updated file
        const updatedFile = file.create({
            name: myFile.name,
            fileType: myFile.fileType,
            contents: fileContent,
            folder: myFile.folder
        });
        const updatedFileId = updatedFile.save();
        log.debug({title: 'Updated File ID', details: updatedFileId});

Seems like a good opportunity to leverage N/xml so you don't have to reinvent the wheel of parsing/generating xml and can instead focus on your business logic

we reach for JSONIX for this.