How to Fix bcrypt Compare Error in Node.js

In this tutorial, you'll learn about How to Fix bcrypt Compare Error in Node.js. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Your Node.js application logs:

Error: data and hash arguments required

Or:

Error: Invalid salt. Salt must be in the form of: $2b$10$...

Or bcrypt.compare() always returns false even with the correct password.

Quick Fix

1. Check the bcrypt package

There are two popular bcrypt packages for Node.js:

// bcrypt — native C++ bindings (faster)
const bcrypt = require('bcrypt')

// bcryptjs — pure JavaScript (no native deps)
const bcryptjs = require('bcryptjs')

If you install bcrypt but it fails to compile native bindings, switch to bcryptjs:

npm uninstall bcrypt
npm install bcryptjs

// Both have the same API
const bcrypt = require('bcryptjs')
const hash = await bcrypt.hash('password', 10)
const match = await bcrypt.compare('password', hash)

2. Validate inputs to compare()

// Wrong — undefined or null values
const result = await bcrypt.compare(password, storedHash)
// Error: data and hash arguments required

// Right — validate inputs
async function verifyPassword(plainPassword, hashedPassword) {
  if (!plainPassword || !hashedPassword) {
    throw new Error('Password and hash are required')
  }
  if (typeof plainPassword !== 'string') {
    throw new Error('Password must be a string')
  }
  return bcrypt.compare(plainPassword, hashedPassword)
}

3. Check the hash format

Valid bcrypt hashes start with $2b$, $2a$, or $2y$:

function isValidBcryptHash(hash) {
  return /^\$2[aby]\$\d{2}\$[./A-Za-z0-9]{53}$/.test(hash)
}

console.log(isValidBcryptHash('$2b$10$...'))  // true
console.log(isValidBcryptHash('5e884898da28047151d0e56f8dc62927'))  // false (SHA-256)

4. Fix mix of synchronous and async calls

// Wrong — mixing sync and async
const bcrypt = require('bcrypt')

// Async hash
const hash = await bcrypt.hash('password', 10)

// Sync compare (works but blocks the event loop)
const match = bcrypt.compareSync('password', hash)

// Right — use async consistently
const match = await bcrypt.compare('password', hash)

5. Rebuild native bindings

If bcrypt (native) fails to load, rebuild it:

# Rebuild bcrypt from source
npm rebuild bcrypt

# Or install build tools first
sudo apt install build-essential python3
npm rebuild bcrypt

6. Fix Buffer vs string comparison

// Wrong — comparing a string hash with a Buffer
const hashBuffer = Buffer.from(storedHash, 'utf-8')
const result = await bcrypt.compare('password', hashBuffer)
// bcrypt.compare expects both arguments to be strings

// Right — ensure both are strings
const result = await bcrypt.compare('password', String(storedHash))

Prevention

• Validate both arguments to bcrypt.compare() before calling.
• Use bcryptjs if native compilation is problematic.
• Store hashes as strings in the database, not buffers.
• Test password verification in your test suite with known test vectors.
• Log error details (but not the password) when verification fails.

Common Mistakes with error

1. Using head and tail instead of pattern matching, causing runtime errors on empty lists
2. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
3. Using return to exit a function early instead of wrapping a pure value in the monad

These mistakes appear frequently in real-world BCRYPT code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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