Node.js写入JSON文件报错：无法为不存在的对象创建新字段

Hey there, let's break down why you're hitting this error and how to fix it quickly.

Error: Can't make new fields in object if it doesn't exist

The Root Cause #

The core issue here is that fs.readFile is asynchronous — it doesn't pause your code while it reads the file from disk. By the time your code reaches these lines:

if (!xp[usr]) xp[usr] = {xpp: 0};
xp[usr].xpp++;

the xp variable is still undefined (since the file read hasn't finished yet). Trying to access properties on an undefined value throws that frustrating error.

Solution 1: Wrap Dependent Code in the Read Callback #

The simplest fix is to move all code that uses xp inside the fs.readFile callback. This guarantees xp is only accessed after the file has been successfully read and parsed:

const fs = require('fs'); // Don't forget to require fs if you haven't already

fs.readFile('./xp.json', 'utf8', (err, data ) => {
  if (err) {
    console.error(err);
    return; // Exit early if file read fails
  }
  
  let xp = JSON.parse(data);
  const usr = m['id'];
  
  // Clean up the log with template literals
  console.log(`${m['id']} ${m['channel_id']} ${m['username']}`);
  
  if (!xp[usr]) {
    xp[usr] = {xpp: 0};
  }
  xp[usr].xpp++;

  fs.writeFile("./xp.json", JSON.stringify(xp), (err) => {
    if (err) console.error(err);
  });
});

Solution 2: Use Async/Await for Cleaner Code (Node.js 10+) #

If you prefer more linear, readable code over nested callbacks, use Node.js's promise-based file system API with async/await:

const fs = require('fs').promises; // Use promise-powered fs methods

async function updateUserXP(m) {
  try {
    // Wait for the file to read before proceeding
    const data = await fs.readFile('./xp.json', 'utf8');
    let xp = JSON.parse(data);
    const usr = m['id'];
    
    console.log(`${m['id']} ${m['channel_id']} ${m['username']}`);
    
    if (!xp[usr]) {
      xp[usr] = {xpp: 0};
    }
    xp[usr].xpp++;
    
    // Wait for the file to write before finishing
    await fs.writeFile("./xp.json", JSON.stringify(xp));
  } catch (err) {
    console.error(err); // Catch any errors in one place
  }
}

// Call the function with your message object
updateUserXP(m);

Quick Extra Tip #

Make sure your xp.json file exists and starts with a valid empty object {} if there are no users yet — otherwise the initial JSON.parse will throw an error when the file is empty.

内容的提问来源于stack exchange，提问作者Bjelke