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How to Fix That Pesky CORS Error

This past week, I was working on building a new app. I had just finished setting up the React frontend and Rails API backend, but when I tried to make my first API request I came across an error I had never seen before:

I remembered learning about CORS the first time I built a Rails API. For anyone unfamiliar with it, here’s a quick breakdown:

CORS stands for “cross origin resource sharing.” It’s what allows web applications to make “cross-domain” requests under secure conditions. In other words, it gives scripts running on one browser permission to access resources from a different origin, hence the term “cross origin.”

What is an origin?

An origin refers to a URL’s:

· scheme or protocol (http://, https://)

· port (80, 81)

· host or domain name (

Normally, JavaScript can’t access resources from other origins due to the same-origin security policy, which allows one webpage to access data from another as long as they both have the same origin. This works as a security feature, which helps prevent malicious scripts on one web page from accessing sensitive data (ie., your bank account) on another.

CORS, on the contrary, provides developers with more flexibility (albeit, less secure) so they can allow cross-origin requests and data transfers under predetermined conditions. So, in my case, I can make requests from my React frontend to my Rails API backend.

So, what about that error?

For starters, “Access-Control-Allow-Origin” is an HTTP response header that the server sends in response to an HTTP request. Its main job is to tell the browser whether or not it’s allowed to access a resource based on the origin of the request. The error message I received stated that no header was present, so the browser blocked the resource.

To resolve that, I decided to take a look at my CORS settings on the backend. When I first created the Rails API, I uncommented the “rack-cors” gem from my Gemfile and bundle installed. I then uncommented everything out in my cors.rb file. Take a look below and see if you can figure out what else needed to be changed.

Rails.application.config.middleware.insert_before 0, Rack::Cors do allow do  origins ''  resource '*',   headers: :any,   methods: [:get, :post, :put, :patch, :delete, :options, :head] endend

As you can see above, my origins were set to allow “” (essentially, nothing). “Origins” refer to the website(s) making a request. So I changed “” to “*” because I wanted to allow any client to make a request, or all origins. And I didn’t need to specify the exact headers, since my middleware (rack-cors gem) took care of serving them for me. Then, I restarted my server, and voilà! All fixed.

origins '*'

Quick note: I could have strictly specified origins to only allow “localhost:3001,” which is the origin of my client-side app, but I chose all (“*”) because web fonts rely on CORS too. Since I was using a couple Google fonts, I wanted that flexibility. In the future when I decide to deploy my app, I’ll edit my cors.rb file to be a bit more specific (and secure) with whitelisted origins.




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