我如何收到JSON数组？

I am trying to receive an array from a PHP document in JavaScript using JSON. The PHP document sends the array (it is printed when I open the PHP file with my browser) but the success() or done() methods are not called. What am I doing wrong here?

Code:

JavaScript:

function updateHighscores() {

    //var functionName = "showData";
    $.ajax({        
        //type: "GET",
        dataType: "json",
        url: "gesallprov.php",
        //data: {functionName: functionName},
    }).done(function(data) {
        window.alert("GGGG");

        var table = "<table style='width:100%'>" +
        "<tr>" +
        "<th>No.</th>" +
        "<th>Name</th>" +
        "<th>Date & Time</th>"  +
        "<th>Score</th>" +
        "</tr>";

        $(data).each(function(index, value) {           
            var i = 1;

            table += "<tr>" +
            "<td>" + i + "</td>" +
            "<td>" + value.name + "</td>" + 
            "<td>" + value.when + "</td>" +
            "<td>" + value.score + "</td>" +
            "</tr>";

            i++;
            });
}

PHP:

function showData() {
    try {

        $dbh = new PDO(...);
        $st = $dbh->prepare("SELECT `name` , `when` , `score` FROM `snake` ORDER BY `score` DESC");
        $st->execute();

        $result = $st->fetchAll(PDO::FETCH_ASSOC);

        return $result;
        $st = null;
        $dbh = null;
    }
    catch (PDOException $ex) {
        echo 'Connection failed: ' . $ex->getMessage();
    }
}

echo json_encode(showData());

why don't you try the following code it might help you fine the problem

     jQuery.ajax({
            type: "POST",
            url: "gesallprov.php",///contain the url of ajax 
           // data:formData,
            dataType:"json",
            success : function (data)
                  {
          window.alert("GGGG");

          var table = "<table style='width:100%'>" +
          "<tr>" +
         "<th>No.</th>" +
         "<th>Name</th>" +
         "<th>Date & Time</th>"  +
        "<th>Score</th>" +
         "</tr>";

    $(data).each(function(index, value) {           
        var i = 1;

        table += "<tr>" +
        "<td>" + i + "</td>" +
        "<td>" + value.name + "</td>" + 
        "<td>" + value.when + "</td>" +
        "<td>" + value.score + "</td>" +
        "</tr>";

        i++;
        });


        },
      error :function(xhr, ajaxOptions, thrownError){
         console.log(xhr);
        console.log(ajaxOptions);
       // console.log(thrownError);
          //alert(xhr.status);
          //alert(ajaxOptions);
      }
    })

If I test your code, I see this message in the browser console:

SyntaxError: missing ) after argument list

Maybe it will help if you add the closing parentheses and curly brace in your updateHighscores function:

function updateHighscores() {

    //var functionName = "showData";
    $.ajax({
        //type: "GET",
        dataType: "json",
        url: "gesallprov.php",
        //data: {functionName: functionName},
    }).done(function (data) {
        window.alert("GGGG");

        var table = "<table style='width:100%'>" +
                "<tr>" +
                "<th>No.</th>" +
                "<th>Name</th>" +
                "<th>Date & Time</th>" +
                "<th>Score</th>" +
                "</tr>";

        $(data).each(function (index, value) {
            var i = 1;

            table += "<tr>" +
                    "<td>" + i + "</td>" +
                    "<td>" + value.name + "</td>" +
                    "<td>" + value.when + "</td>" +
                    "<td>" + value.score + "</td>" +
                    "</tr>";

            i++;
        });
    });
}