CORS错误随机发生

Question

所以我正在使用AJAX发送数据来表达，问题是即使每次都发送完全相同的数据，我有两种不同的结果：

1. 我收到了CORS错误
2. 流程没有错误，但服务器说数据未定义

为什么有时会出现CORS错误？ 为什么（当CORS没有失败时）服务器是否说数据未定义？

问题＃2的代码：

客户方：

document.getElementById('depositButton').onclick = function() {
    var steamid = getSteamID();
    if(steamid == "") {
        alert("Something went horribly wrong, please inform us about this issue by DM'ing us at https://twitter.com/csgobeararms");
    }

    console.log(JSON.stringify({arr:items}));
    $.ajax({
        type: 'POST',
        url: 'http://localhost:1337/deposit?steamid=' + steamid,
        data: {arr:items},
        success: function(data) {
            console.log("Tradeoffer has been sent");
        },
        error: function(data) {
            alert("Failed to call bot, please inform us about this issue by DM'ing us at https://twitter.com/csgobeararms (Additional info in console)");
            console.log("Failed to call bot, please inform us about this issue by DM'ing us at https://twitter.com/csgobeararms");
            console.log("This is what we know: ");
            console.log(data);
        }
    });
}

function getSteamID() {
    var cookie = getCookie("steamid");
    if(cookie != "") {
        return cookie;
    } else {
        return "";
    }
}

function getCookie(cookie) {
    var name = cookie + "=";
    var decoded = decodeURIComponent(document.cookie);
    var ca = decoded.split(';');
    for(var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while(c.charAt(0) == ' ') {
            c = c.substring(1);
        }

        if(c.indexOf(name) == 0) {
            return c.substring(name.length, c.length);
        }
    }
    return "";
}

服务器端：

var bodyParser = require('body-parser');
var express = require('express');
var app = express();
app.use(bodyParser.json());

app.use(function(err, req, res, next) {
    console.error(err.stack);
    console.log("ezpzlmnsqz")
    res.status(500).send("Something went horribly wrong!");
});

app.post('/deposit', function(req, res) {
    console.log('Deposit request recieved, info:');
    console.log('STEAM ID: ' + req.query.steamid);
    console.log('ITEMS: ' + req.params.arr);
    res.status(200).send('Success');
});

app.listen(1337);