How to write a file in Node.js

Programs that generate reports or export data need an explicit filesystem boundary between in-memory values and a durable artifact. Node.js provides promise-based filesystem methods that let a program wait for a completed write without using synchronous file calls.

The writeFile() method from node:fs/promises suits small payloads that are already held in memory. Resolving the destination from import.meta.url keeps the output beside the script, while flag: “wx” refuses an existing path instead of truncating its content.

Large or incremental payloads belong in a writable stream. For a small text file, reading the completed file back and comparing it with the prepared string exposes a partial or altered write before the program reports success.

1. Create write-report.mjs with the filesystem boundary for the generated report.

   write-report.mjs

   import { mkdir, readFile, writeFile } from "node:fs/promises";
    
   const outputDirectory = new URL("./output/", import.meta.url);
   const reportUrl = new URL("daily-report.txt", outputDirectory);

2. Append UTF-8 report content derived from an in-memory record array to write-report.mjs.

   write-report.mjs

   const records = [
     { id: 101, status: "ready" },
     { id: 102, status: "ready" },
     { id: 103, status: "queued" },
   ];
    
   const body = [
     "Daily job report",
     `Items written: ${records.length}`,
     `Ready items: ${records.filter((record) => record.status === "ready").length}`,
     "",
   ].join("\n");

3. Append the create-only filesystem writer to write-report.mjs.

   write-report.mjs

   async function writeReport() {
     await mkdir(outputDirectory, { recursive: true });
     await writeFile(reportUrl, body, { encoding: "utf8", flag: "wx" });
   }

   flag: “wx” rejects an existing destination with EEXIST. The default flag: “w” would replace its content.

4. Append the read-back verification function to write-report.mjs.

   write-report.mjs

   async function verifyReport() {
     const saved = await readFile(reportUrl, "utf8");
    
     if (saved !== body) {
       throw new Error("Saved report does not match generated text");
     }
    
     return saved;
   }

5. Append the main execution block with existing-file error handling to write-report.mjs.

   write-report.mjs

   try {
     await writeReport();
     const saved = await verifyReport();
    
     console.log("wrote=output/daily-report.txt");
     console.log(`bytes=${Buffer.byteLength(saved)}`);
     console.log(saved.trimEnd());
   } catch (error) {
     if (error.code === "EEXIST") {
       console.error("output/daily-report.txt already exists");
       process.exitCode = 1;
     } else {
       throw error;
     }
   }

6. Run write-report.mjs to create output/daily-report.txt with immediate read-back verification.

   $ node write-report.mjs
   wrote=output/daily-report.txt
   bytes=49
   Daily job report
   Items written: 3
   Ready items: 2

7. Run write-report.mjs again to confirm that create-only mode refuses the existing file.

   $ node write-report.mjs
   output/daily-report.txt already exists

   The second run exits with status 1 and leaves the first file unchanged.

8. Read output/daily-report.txt after the refused write to confirm that its original content remains.

   $ cat output/daily-report.txt
   Daily job report
   Items written: 3
   Ready items: 2