Unraveling the Mysteries of JSONP: A Technique of the Past
The Origins of JSONP
JSONP, or JSON with Padding, has long been a misunderstood concept in web development. Its confusing name and questionable background have led to widespread confusion among developers. But before CORS (Cross-Origin Resource Sharing) became the standard, JSONP was the only way to retrieve JSON data from a server with a different origin.
The Same-Origin Policy: A Barrier to Data Retrieval
When a request is sent to a server with a different origin, the browser throws an error due to the same-origin policy. This policy restricts reading responses from websites with different origins, but allows sending requests. A website’s origin consists of three parts: the URI scheme, hostname, and port. For instance, http://logrocket.com and https://logrocket.com have different origins due to the URI scheme difference.
How JSONP Works
To bypass the same-origin policy, JSONP uses the <script> element’s ability to load and execute JavaScript from a foreign origin. By providing the API’s endpoint URL to the <script>‘s src attribute, the <script> fetches the response and executes it inside the browser context. However, the <script> element automatically parses and executes the returned code, which can lead to errors if the response is not a valid JavaScript.
The Solution: Wrapping JSON in a Function
To overcome this limitation, the server returns a JavaScript code with a function wrapped around the JSON object. The function name is provided by the client in the query parameter called callback. Once the response is parsed and executed, the function is called with the JSON object as an argument.
Implementation and Limitations
While JSONP can be implemented using a simple function, it has several limitations. For instance, it only supports GET requests and raises security concerns since the server can respond with arbitrary JavaScript code that has access to the window context, including localStorage and cookies.
A Relic of the Past: Why JSONP Should Be Avoided
With the advent of CORS, JSONP has become a relic of the past. Its limitations and security concerns make it an unreliable choice for cross-origin requests. Instead, developers should rely on CORS to provide safe and secure cross-origin requests.
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