How to Fix Cors in Koa.js

In this tutorial, you'll learn about How to Fix Cors in Koa.js. We cover key concepts, practical examples, and best practices.

Koa CORS with @koa/cors enables cross-origin requests. Missing origin whitelist opens the API to any domain. DodaTech configures strict CORS in production Koa apps.

The Problem

Developers working with cors in Koa.js often encounter runtime errors, unexpected behavior, and production failures. These issues commonly stem from incorrect API usage, missing configuration, wrong middleware ordering, or misunderstanding the framework's design patterns.

Error: Cors failed
    at Object.<anonymous> (/app/src/routes.js:15:3)

Quick Fix

1. Apply the correct pattern

// Wrong — incorrect cors usage in Koa
app.use(ctx => {
  // Missing await or wrong ctx access
})

// Right — correct cors pattern with Koa
app.use(async (ctx, next) => {
  try {
    const result = await processRequest(ctx.request)
    ctx.body = { success: true, data: result }
    ctx.status = 200
  } catch (err) {
    ctx.status = err.status || 500
    ctx.body = { error: err.message }
  }
  await next()
})

// Example response
// {"success":true,"data":{"processed":true}}

2. Handle async errors properly

// Wrong — uncaught async rejection
async function handleRequest(data) {
  const result = await processData(data)
  return result
}
// If processData throws, the error is unhandled

// Right — wrap async operations in try-catch
async function handleRequestSafe(data) {
  try {
    if (!data) throw new Error('Input required')
    const result = await processData(data)
    if (!result) throw new Error('Processing returned empty')
    return { success: true, data: result }
  } catch (err) {
    console.error('Cors failed:', err.message)
    return { success: false, error: err.message }
  }
}
const response = await handleRequestSafe(input)
console.log('Cors status:', response.success)
// Output: Cors status: true

3. Validate inputs and configuration

// Wrong — assuming inputs are always valid
function processcors(input) {
  return input.value.toUpperCase()
}

// Right — validate before processing
function safecors(input) {
  if (!input || typeof input !== 'object') {
    return { error: 'Input must be an object' }
  }
  if (!input.value || typeof input.value !== 'string') {
    return { error: 'Input.value must be a string' }
  }
  return { result: input.value.toUpperCase(), processed: true }
}
const result = safecors({ value: 'hello' })
console.log('Cors:', result)
// Output: Cors: {result: "HELLO", processed: true}

Prevention

• Always read the Koa.js documentation for the correct cors API before writing code
• Use TypeScript for better type safety when working with Koa.js applications
• Wrap cors operations in try-catch blocks to handle runtime errors gracefully
• Write integration tests that cover request-response cycles for your API
• Follow DodaTech coding standards for consistent patterns across your codebase
• Monitor production with structured logging to catch cors issues early
• Use Koa.js's built-in error handling as a safety net for unexpected failures

Common Mistakes with cors

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 KOA 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.

FAQ