Firefox Addon: XMLHttpRequest minor bug reading JSON

Question

I have noticed that XMLHttpRequest in my Firefox addon logs an error processing xyz.json if content is an array. The error does not halt the execution (hence a minor bug)

Example of xyz.json :

[
  "string1",
  "string2",
  "string3"
]

Although above is a valid JSON, XMLHttpRequest logs a syntax error at [

It is possible to avoid the error logging by setting responseType to 'text' .

Is this limited to Firefox Addon or Firefox or JavaScript in general?

Answer 1

Firefox will consult the response MIME type to see if the response should be parsed as XML. Reading some file: , chrome: , resource: , etc. that does not provide a response MIME type will make XHR default to XML (backwards-compatibility; after all the XML is right in the name), and attempt to parse it to provide .responseXML . This will of course fail if the file does not contain XML, and hence the warning.

Use .overrideMimeType() to provide an explicit proper MIME type, or just text/plain if in doubt, to stop XHR from attempting to parse data as XML, and the message will go away.