Cómo usar el relleno JSON (y otras opciones) para omitir la Política del mismo origen

En este artículo, veremos qué es JSONP, sus inconvenientes y algunas alternativas a JSONP.

Es posible que se haya encontrado con situaciones en las que realiza una llamada a la API de un origen a otro. Por ejemplo, tenemos una página servida desde localhost: 3000 que llama a una API desde localhost: 8000.

Nota : Nos referiremos a localhost: 3000 como nuestro servidor cliente. Nos referiremos a localhost: 8000 como nuestro servidor API.

Pero vemos este error intimidante.

Esta es la Política del mismo origen que nos protege. Esta política restringe cómo los recursos de un origen interactúan con los recursos de otro origen. Es un mecanismo de seguridad crítico en el navegador. Pero hay casos en los que queremos realizar solicitudes de origen cruzado a recursos confiables.

JSONP (JSON con relleno) proporciona una solución alternativa para este problema de política del mismo origen. Veamos cómo surgió JSONP.

Buceo técnico

Podemos ejecutar código JavaScript dentro de nuestro archivo HTML con pt> tags.

We can move our JavaScript code into a separate JavaScript file and reference it with our script tag. Our webpage now makes an external network call for the JavaScript file. But functionally, everything works the same.

The Javascript file doesn’t have to have a js extension. The browser will interpret content as JavaScript if the Response’s Content-Type is JavaScript. (text/javascript, application/javascript).

Most servers allow you to set the content type. In Express, you would do:

Your pt> tag can reference a URL that doesn’t h ave a js extension.

Script tags are not limited by the Same-Origin Policy. There are other tags, such as mg> ; and <video> tags, that are not limited by the Same-Origin Policy. So our JavaScript can live on a different origin.

The code inside the JavaScript file has access to everything that is in scope. You can use functions defined earlier in your HTML file.

You can pass arguments as you would for a normal function call.

In the above example, we passed a hard-coded string. But we could also pass in data coming from a database. Our API server can construct the JavaScript file with this dynamic information.

This is what JSONP is. Instead of using fetch or XMLHttpRequest to make an API call to retrieve data, we used a pt> tag. Because we u sed a <script> tag, we were able to bypass the Same-Origin Policy.

As I mentioned above, JSONP means JSON with Padding. What does the padding mean? Normal API responses return JSON. In JSONP responses, we return the JSON response surrounded (or padded) with a JavaScript function.

Most servers allow you to specify the name of your padding function.

The server takes your padding function name as a query. It invokes your padding function with the JSON data as an argument.

You are not limited to passing function names as your callback. You can pass inline JavaScript in your query.

I have not thought of a reason to do this.

Alternatives to using JSONP

There is no official spec for JSONP. I think of JSONP as more of a hack.

pt> tags can only make GET requests. So JSONP can only make GET requests.

Cross-Origin Resource Sharing has an official specification, and is the preferred way of getting around the Same-Origin Policy.

You can enable Cross-Origin Resource Sharing by adding a header to our Response.

This means all origins can use this resource without fear of the Same-Origin Policy.

Sometimes, you don’t have control over the server-code though. You would not be able to include the Access-Control-Allow-Origin header. An alternate solution is to make your own proxy server make the cross-origin request for you. The Same-Origin policy only applies to the browser. Servers are free to make cross-origin requests.

Questions? Comments? Please leave a message below.


  • Same Origin Policy
  • Github Repository with JSONP and CORS examples
  • Detailed explanation of JSONP