How to read a JSON file in Node.js

Application settings are often loaded before a Node.js process can start its real work. A JSON reader needs to turn the file into a JavaScript value while keeping a missing path distinguishable from invalid JSON.

The promise-based readFile() function from node:fs/promises reads the file without blocking the event loop. Passing utf8 returns a string for JSON.parse() instead of a Buffer.

Runtime file paths suit an explicit read-and-parse function because the application controls when the bytes are read and how failures are reported. Static JSON module imports require with { type: “json” } and cache the imported value, so they serve a different use case.

1. Create the application settings in app-config.json.

   app-config.json

   {
     "service": "orders-api",
     "port": 8080,
     "features": {
       "payments": true,
       "exports": false
     }
   }

   JSON copied from payloads or changed by hand may contain syntax mistakes.
   Tool: JSON Validator

2. Create read-json.mjs with the runtime path and asynchronous JSON loader.

   read-json.mjs

   import { readFile } from "node:fs/promises";
    
   const filePath = process.argv[2] ?? "app-config.json";
    
   async function readJson(path) {
     const text = await readFile(path, "utf8");
     return JSON.parse(text);
   }

3. Append the selected configuration output below readJson().

   read-json.mjs

   function printConfig(config, path) {
     console.log(`file=${path}`);
     console.log(`service=${config.service}`);
     console.log(`port=${config.port}`);
     console.log(`payments=${config.features.payments}`);
   }

   Selected fields keep configuration output focused because complete settings may contain credentials or tokens.

4. Append the error classifier below printConfig().

   read-json.mjs

   function reportError(error, path) {
     if (error.code === "ENOENT") {
       console.error(`missing=${path}`);
     } else if (error instanceof SyntaxError) {
       console.error(`invalid-json=${path}`);
     } else {
       throw error;
     }
    
     process.exitCode = 1;
   }

   ENOENT identifies a path that readFile() could not find, while SyntaxError identifies text that JSON.parse() rejected.

5. Append the top-level read operation below reportError().

   read-json.mjs

   try {
     const config = await readJson(filePath);
     printConfig(config, filePath);
   } catch (error) {
     reportError(error, filePath);
   }

6. Compare the assembled read-json.mjs with the complete source.

   read-json.mjs

   import { readFile } from "node:fs/promises";
    
   const filePath = process.argv[2] ?? "app-config.json";
    
   async function readJson(path) {
     const text = await readFile(path, "utf8");
     return JSON.parse(text);
   }
    
   function printConfig(config, path) {
     console.log(`file=${path}`);
     console.log(`service=${config.service}`);
     console.log(`port=${config.port}`);
     console.log(`payments=${config.features.payments}`);
   }
    
   function reportError(error, path) {
     if (error.code === "ENOENT") {
       console.error(`missing=${path}`);
     } else if (error instanceof SyntaxError) {
       console.error(`invalid-json=${path}`);
     } else {
       throw error;
     }
    
     process.exitCode = 1;
   }
    
   try {
     const config = await readJson(filePath);
     printConfig(config, filePath);
   } catch (error) {
     reportError(error, filePath);
   }

7. Run the reader against a missing JSON path.

   $ node read-json.mjs missing.json
   missing=missing.json

8. Create malformed input in broken-config.json.

   broken-config.json

   {
     "service": "orders-api",
     "port": 8080,
   }

   Strict JSON rejects the trailing comma after 8080.

9. Run the reader against broken-config.json.

   $ node read-json.mjs broken-config.json
   invalid-json=broken-config.json

10. Remove the malformed test file.

    $ rm broken-config.json

11. Run the reader against app-config.json to confirm the parsed settings.

    $ node read-json.mjs app-config.json
    file=app-config.json
    service=orders-api
    port=8080
    payments=true