What are the advantages of using JSONP and callbacks over specifying a custom variable name for the caller?

Although I grasp the concept of jsonp, I am quite intrigued as to why it is not common practice to create an API like the one described below:

What are the reasons (be it security concerns, ease of use, etc.) for avoiding this approach?

The response would be JavaScript containing:

the_var = {"my": "json", "content": 1};

On the client side, the code would appear as follows:

<script>
var the_var;
</script>
<script src="http://www.something.com/json?varname=the_var"></script>
// Now, the_var holds the requested JSON data

While this approach seems logical and works across domains, there must be specific reasons why standard JSONP practices differ. What could those reasons be?

Answer №1

Utilizing a "callback" function serves the purpose of receiving a callback request. The script loads asynchronously, and once it is ready, your function is automatically triggered - almost like the JSONP script activates a listener.

While using a variable approach can also be effective, especially for synchronously loading scripts, asynchronous loading requires polling for variable values or utilizing DOM events to execute scripts. The former method is not favored, and the latter can be complex due to inconsistent browser APIs.

If you decide to proceed with this method, conventional JSONP APIs can still be integrated by including ?callback=the_var%3D in the URL parameter to align with your "variable style."

Answer №2

In the typical scenario for JSONP, the script element is not hardcoded into the page. Instead, it is dynamically generated when needed.

By utilizing a function, you can execute a task with the data once the script has loaded.

Alternatively, you could insert the script element and continuously check the global namespace to determine if the variable has been populated yet or not.

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