在node.js中使用ES6 Promise返回空响应

Question

how can I call a query using ES6 native promises in node.js. Below is the code.

let arr= [];
    conn.query('select * from table1', (err, b) => {
    for (let i = 0; i < b.length; i++) {
        console.log(b[i]["id"])
        let promiseGetData = new Promise((resolve, reject) => {
            conn.query('select * from table2 where id = ?', [b[i]["id"]], (err, ce) => {
                if (err) {
                    const response = {
                        statusCode: 500,
                        body: JSON.stringify({
                            message: err
                        }),
                    }
                    reject(response);
                } else {
                    if (ce.length != 0) {
                        resolve(ce);
                    }
                }
            });
        });

        promiseGetData .then((data) => {
            b[i]['data'] = data;
            arr.push(b[i])
        }).catch((err) => {
            console.log(err);
        });
    }
console.log(b)
    })

I see an empty response when i do console.log(b). I dont know if I have used a promise in correct way, I think for first query I should execute in the promise as well. Any help is very much appreciated

Answer 1

Wrapping an async callback-based function into a promise is called Promisifying .

You can of course use a library for that, but basically what it does is :

const queryAsPromise = function( ...args ) {
    return new Promise( function( resolve, reject ) {
       try { 
           conn.query(...args, function( error, result ) {
               if ( error ) { 
                   reject( error );
               } else {
                   resolve( result );
               }
           } ); 
       } catch( error ) {
           reject( error );
       }
    } )
} );

By doing this one time you will keep your code DRY and you can always use that promise for making queries now :

queryAsPromise('select * from table1')
     .then( result => {
          return Promise.all(
              result.map( b => {
                 return queryAsPromise('select * from table2 where id = ?', b["id"])
                             .then( data => b["data"] = data )
              } )
          )
     )
     .catch( err => res.send(500) )
     .then( console.log )

Answer 2

You got empty response from console.log(b) because the promise from querying database aren't finished. You have to wait all of them to finish before you can get the full result.

Sample:

let arr = [];
conn.query('select * from table1', (err, b) => {

  var promiseArr = [];

    for (let i = 0; i < b.length; i++) {
        let promiseGetData = new Promise((resolve, reject) => {
            conn.query('select * from table2 where id = ?', [b[i]["id"]], (err, ce) => {
                if (err) {
                    const response = {
                        statusCode: 500,
                        body: JSON.stringify({
                            message: err
                        }),
                    }
                    reject(response);
                } else {
                    if (ce.length != 0) {
                        resolve(ce);
                    }
                }
            });
        });

            promiseArr.push(promiseGetData);
    }

    Promise.all(promiseArr).then((resultArr) => {
        //resultArr is all the resolved value returned from the promise in promiseArr
        for (let i = 0; i < resultArr.length; i++) {
            b[i]['data'] = resultArr[i];
            arr.push(b[i]);
        }
    }).then(() => {
        console.log(arr);
    }).catch((err) => {
        //if any promise throw/reject with error, it will go here directly
            console.log(err);
    });
})

Edit: Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise