Javascript promise 链 - ES6

Question

我正在尝试链接一些 promise ，但由于理解这些概念而遇到麻烦。

我试图从数据库返回一些数据，只有当行数小于选定的数量时，否则它将返回错误。

到目前为止，我有一种获取数据并返回 promise 的方法，如下所示:

fetchDataPoints(dataset,fields,startTimestamp,endTimestamp){

        let self = this;
        let queryBody =  " FROM [bi].[" + dataset + "] " +
            "WHERE [timestamp]  >= '" + startTimestamp.format("YYYY-MM-DD HH:mm:ss") + "' "  +
            "AND   [timestamp]  <  '" + endTimestamp.format("YYYY-MM-DD HH:mm:ss") + "' " ;


        let rowCountCheck = new Promise(function(resolve,reject) {
            let request = new self.sql.Request();

            let rowCheckQueryString = "SELECT COUNT(*) AS row_count " + queryBody;
            request.query(rowCheckQueryString).then(function(recordSet){
                if (recordSet[0].row_count > self._rowLimit) {
                    reject("Too many rows");
                } else {
                    resolve();
                }
            });
        });

        let request = new self.sql.Request();

        let fieldsString = "[timestamp],[etl_timestamp]";
        for(let i = 0; i < fields.length; i++){
            fieldsString += ",[" + fields[i] + "]";
        }
        let  resultQueryString = "SELECT " + fieldsString + queryBody + " ORDER BY [timestamp] DESC";
        let resultQuery = request.query(resultQueryString);

        return rowCountCheck.then(resultQuery);

    }

但是这并没有按预期方式工作。更熟悉 Promise 的人能否澄清它们如何用于此类事情？

Answer 1

您可以返回request.query，而不是将其用作回调函数:

return rowCountCheck.then(function(response){
        let request = new self.sql.Request();
        let fieldsString = "[timestamp],[etl_timestamp]";

        for(let i = 0;i < fields.length; i++){
            fieldsString += ",[" + fields[i] + "]";
        }

        let  resultQueryString = "SELECT " + fieldsString + queryBody + " ORDER BY [timestamp] DESC";

        return request.query(resultQueryString);
});
`