javascript - 发送错误后无法设置 header 。我没有调用 render()/json() 两次

Question

在向数据库输入条目后，我尝试向客户端返回响应。我编写了以下代码来实现所需的结果，但是我得到了

错误:发送后无法设置 header 。

我不会在任何地方为此响应调用两次 response.json() 或 response.render() 。那么为什么会出现这个错误呢？

router.post('/yolo', jsonParser, function(req, res, next) {

    response = res;
    shortUrl = shortenTheUrl(req.body.textBoxValue);
    var insertDataInTableQuery = "INSERT INTO `WinGoku`.`ShortenedUrlsTable` (`OriginalUrl`, `ShortUrl`) VALUES('"+req.body.textBoxValue+ "', '"+ shortUrl +"')";
    runSQLQuery(insertDataInTableQuery, sendResponseToClient);
});

function runSQLQuery(query, callbackFunction) {
    connection.query(query, function(error) {
        if(error) {
            console.log(query);
            console.log("QueryError: "+ error);
            if(callbackFunction != undefined)
                callbackFunction({type: "alert", errorMessage: error});
        }

        if(callbackFunction != undefined)
            callbackFunction({type: "alert", errorMessage: "successful"});
    });
}

function sendResponseToClient(queryExecutionResult) {
    console.log("1: "+ queryExecutionResult["type"] + " 2: "+ queryExecutionResult["errorMessage"]);
    response.json({shortenedUrl: shortUrl, type: queryExecutionResult["type"], message: queryExecutionResult["errorMessage"]});
            ^
            error here. Can't set headers after they are sent
}

Answer 1

您可能会调用response.json()两次，如果查询失败并且它不会从错误子句返回，而是也会执行查询成功子句。所以你必须在错误子句的末尾放置一个返回语句。这应该可以解决问题。

function runSQLQuery(query, callbackFunction) {
    connection.query(query, function(error) {
        if(error) {
            console.log(query);
            console.log("QueryError: "+ error);
            if(callbackFunction != undefined)
                callbackFunction({type: "alert", errorMessage: error});

            return; // this should fix the problem!
        }

        if(callbackFunction != undefined)
            callbackFunction({type: "alert", errorMessage: "successful"});
    });
}