what is ajax jquery used for code example

Example 1: jquery ajax

$.ajax({
        url: url,
        dataType: "json",
        type: "Post",
        async: true,
        data: { },
        success: function (data) {
           
        },
        error: function (xhr, exception) {
            var msg = "";
            if (xhr.status === 0) {
                msg = "Not connect.\n Verify Network." + xhr.responseText;
            } else if (xhr.status == 404) {
                msg = "Requested page not found. [404]" + xhr.responseText;
            } else if (xhr.status == 500) {
                msg = "Internal Server Error [500]." +  xhr.responseText;
            } else if (exception === "parsererror") {
                msg = "Requested JSON parse failed.";
            } else if (exception === "timeout") {
                msg = "Time out error." + xhr.responseText;
            } else if (exception === "abort") {
                msg = "Ajax request aborted.";
            } else {
                msg = "Error:" + xhr.status + " " + xhr.responseText;
            }
           
        }
    });

Example 2: jquery ajax

$.ajax({

   url     : "file.php",
   method  : "POST",

       data: { 
         //key                      :   value 
         action                   	:   action , 
         key_1   					:   value_key_1,
         key_2   					:   value_key_2
       }
   })

   .fail(function() { return false; })
	// Appel OK
   .done(function(data) {

   console.log(data);

 });