Sådan bruges JSON-polstring (og andre muligheder) til at omgå samme oprindelsespolitik

I denne artikel vil vi se på, hvad JSONP er, dens ulemper og nogle alternativer til JSONP.

Du er muligvis kommet i situationer, hvor du foretager et API-opkald fra en oprindelse til en anden. For eksempel har vi en side, der serveres fra localhost: 3000, der kalder en API fra localhost: 8000.

Bemærk : Vi vil henvise til localhost: 3000 som vores klientserver. Vi vil henvise til localhost: 8000 som vores API-server.

Men vi ser denne truende fejl.

Dette er den samme oprindelsespolitik, der beskytter os. Denne politik begrænser, hvordan ressourcer fra en oprindelse interagerer med ressourcer fra en anden oprindelse. Det er en kritisk sikkerhedsmekanisme i browseren. Men der er tilfælde, hvor vi ønsker at stille krydsoprindelsesanmodninger til pålidelige ressourcer.

JSONP (JSON med polstring) giver en løsning på dette problem med samme oprindelsespolitik. Lad os se på, hvordan JSONP blev til.

Teknisk dykning

Vi kan køre JavaScript-kode inde i vores HTML-fil med 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