gitignore-synthesizer/lib/mergeGitignore.js

Generate smart .gitignore files based on your project's actual contents

export function mergeGitignore(existing, incoming) {
  const existingLines = new Set(existing.split(/\r?\n/).map(line => line.trim()).filter(Boolean));
  const incomingLines = incoming.split(/\r?\n/).map(line => line.trim()).filter(Boolean);

  const mergedLines = [...existingLines];

  for (const line of incomingLines) {
    if (!existingLines.has(line)) {
      mergedLines.push(line);
    }
  }

  return mergedLines.join('\n') + '\n';
}