python - Identify if Flask request is from JavaScript or not

Question

I want to create a Flask error handler that returns a JSON response if the request was from JavaScript but returns a redirect otherwise. I tried using request.is_xhr, but it is false even for JavaScript requests. How can I check if the request is from JavaScript?

@app.errorhandler(Exception)
def unhandled_exception(error):
    if request.is_xhr:
        return flask.jsonify(error='yes')

    return redirect(url_for('error'))

Answer 1

There is no standard or reliable way to detect if a request comes from a particular source, such as JavaScript.

is_xhr was only true when a certain header was set by some JavaScript libraries, such as jQuery. The header is not sent by most JavaScript. is_xhr has been deprecated for that reason.

You can check the Accept header to see if the client is asking for application/json, but that too is unreliable.

if request.is_xhr or request.accept_mimetypes.accept_json:
    return jsonify(...)

return redirect(...)