How do you write a JSON file in Node.js without risking a half-written file?

If you overwrite a file directly and the power goes off halfway, you are left with a broken file, like a result sheet that was erased and only half rewritten. The safer habit is to prepare the new copy on a separate sheet, and only when it is complete, swap it in. Swapping is a single step, so nobody ever sees a half-finished version.

const fsp = require("fs/promises");
const path = require("path");

async function saveJson(file, data) {
  const tmp = path.join(
    path.dirname(file),
    `.${path.basename(file)}.${process.pid}.tmp`
  );
  await fsp.writeFile(tmp, JSON.stringify(data, null, 2), "utf8");
  await fsp.rename(tmp, file);   // atomic swap on the same filesystem
}

saveJson("./scores.json", { rohit: 183, virat: 152 });

Points to remember:

• The temp file must be on the same filesystem as the target, otherwise rename may copy instead of swap.
• Two processes writing at the same time still need locking or a database.
• For anything beyond a small config or a prototype, a real database is the better store.

• Writing directly with writeFile while other code reads the same file.
• Putting the temp file in /tmp, which may be on a different disk.