当PHP响应头包含“application / json”时，JSON.parse在jQuery中失败

Question

I'm getting a bit of a headache trying to figure this one out. To request some json-data from a PHP-script via Ajax, I'm using the jQuery function:

$.ajax({
    type: 'GET',
    cache: 'false',
    url: ajaxUrl,
    data: dataString,
    success: updatePage
});

If I don't set content-type in the PHP header to:

header('Content-type: application/json');

Then my response from the server looks like this:

{"content":"new content"}

And the content-type is automatically set to text/html. When dataType in the jQuery ajax options is unset, it uses a default of 'intelligent guessing'. I'm strongly assuming that jQuery recognizes the response-data as json because updatePage is parsed an object. updatePage uses the JSON js library(json2.js), and does this:

function updatePage(data) {
  $dataObj = JSON.parse(data); 
}

When the function is called upon ajax succes, everything works fine. No errors. Here's the strange thing, if I set my header to application/json as above, JSON.parse suddenly reports an error. The exact same error happens if i set my dataType to 'json' in the jQuery ajax request. The response I get from the PHP script when changing these things looks exactly like the one above. The error looks like this in Firebug:

JSON.parse
  $dataObj = JSON.parse(data); 

Kind of a long one, sorry, but If anyone knows what is wrong their help is greatly appreciated. Thanks for your time.

Answer 1

It's because you end up trying to double-parse the return value.

Both the explicit json data type and usage of the application/json MIME type cause jQuery to parse the returned string into a JavaScript object for you.

So, your usage of JSON.parse(), in these cases, is superfluous.