JSFiddle - React, Tailwind, and code Playground

JavaScript

/*

If dataType: "json" is commented out, the $.ajax sends the correct string. If not, the value of message will be something like:

test jQuery151020082441558501163_1300269082410

*/

$.ajax({
    url: "/echo/json/",
    contentType: "application/json; charset=utf-8",
    //dataType: "json",
    data: "{\"message\":\"test ??\"}",
    processData: false,
    type: "POST",
    success: function(data) {
        console.log(data);
    }
});