What are the key points to understand about Error-First Callback Pattern (Node.js)?

First parameter is always an error object (or null if no error). Second parameter contains the successful result. Standard pattern in Node.js core modules (fs, http, etc.). Always check if error exists before using result. Promises and async/await have largely replaced this pattern

What are common mistakes with Error-First Callback Pattern (Node.js)?

Not checking the error before using the result. Passing non-Error objects as the first parameter. Forgetting to return after handling error. Checking result before error

What are the interview tips for Error-First Callback Pattern (Node.js)?

Know this is the Node.js standard pattern. Always check error first, return early on error. Understand why this pattern exists (consistent error handling). Know that Promises have largely replaced this in modern code

Error-First Callback Pattern (Node.js)

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The error-first callback pattern is a convention in Node.js where callbacks always receive an error object as the first argument. If an error occurred, the first argument contains the error; otherwise, it is null. The second argument contains the successful result. This pattern became the standard in Node.js core APIs.

Interactive Visualization

Path Taken

Error Path

(not taken)

Success Path

(not taken)

Callback Chain

readFile callbackpending

CodeCalling

1const fs = require('fs');
2 
3fs.readFile('data.txt', 'utf8', (err, data) => {
4  if (err) {
5    console.log("Error:", err.message);
6    return;
7  }
8  console.log("Data:", data);
9});

Output

We call fs.readFile with a callback that follows the (err, data) signature.

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Key Insight: The error-first pattern: always check err FIRST. If null/undefined, proceed with data.

Key Points

First parameter is always an error object (or null if no error)
Second parameter contains the successful result
Standard pattern in Node.js core modules (fs, http, etc.)
Always check if error exists before using result
Promises and async/await have largely replaced this pattern

Code Examples

Basic Error-First Pattern

// Node.js fs module example
const fs = require('fs');

fs.readFile('file.txt', 'utf8', function(error, data) {
  // First parameter: error (null if success)
  // Second parameter: result
  
  if (error) {
    console.error('Error reading file:', error.message);
    return;
  }
  
  console.log('File contents:', data);
});

The callback receives (error, result). Always check error first!

Writing Your Own Error-First Function

function divide(a, b, callback) {
  if (b === 0) {
    // Pass error as first argument
    callback(new Error('Cannot divide by zero'), null);
    return;
  }
  
  // Pass null as error, result as second argument
  callback(null, a / b);
}

// Usage
divide(10, 2, function(error, result) {
  if (error) {
    console.error(error.message);
    return;
  }
  console.log('Result:', result); // 5
});

divide(10, 0, function(error, result) {
  if (error) {
    console.error(error.message); // "Cannot divide by zero"
  }
});

When implementing, pass Error object first on failure, null first on success

Common Mistake: Ignoring Error

// ❌ WRONG: Not checking error first
fs.readFile('file.txt', 'utf8', function(err, data) {
  console.log(data); // Could be undefined if error!
});

// ✅ CORRECT: Always check error first
fs.readFile('file.txt', 'utf8', function(err, data) {
  if (err) {
    console.error('Failed:', err);
    return;
  }
  console.log(data); // Safe to use
});

Always check if error exists before accessing the result

Common Mistakes

Not checking the error before using the result
Passing non-Error objects as the first parameter
Forgetting to return after handling error
Checking result before error

Interview Tips

Know this is the Node.js standard pattern
Always check error first, return early on error
Understand why this pattern exists (consistent error handling)
Know that Promises have largely replaced this in modern code