node js:add mysql query result to json in forloop

Question

I am trying to add query result from in for loop to JSON

function (req,res){
var result  = [{id:1
},{id:2},{id:3}];

for(var i=0;i<result.length;i++){
                        //query run
                       collection.getImages(result[i].id,function (status,error,image) {
                          //add query result to json
                          result[i]['images']=image;

                       });
                   }

 res.json(result);

}

But the final result doesn't contains the newly added key value(ie images) it because collection.getImages() is asynchronous so how can i
solve this?

Manuel Zametter · Accepted Answer · 2017-05-09 09:00:11Z

1

You could use Promises to handle your asynchronous calls. Then you can use Promise.all() to await all actions before sending your result back to the client.

var result = [
    {id: 1}, {id: 2}, {id: 3}
];

var promises = [];
for (var i = 0; i < result.length; i++) {
    //query run
    promises.push(new Promise(function (resolve, reject) {
        collection.getImages(result[i].id, function (status, error, image) {
            if (error) {
                reject(error);
            } else {
                resolve(image);
            }
        });
    }));
}

Promise.all(promises).then(function (images) {
    for (var i = 0; i < images.length; i++) {
        result[i]['image'] = images[i];
    }
    res.json(result)
}).catch(function (error) {
    // handle error
});

edited May 9, 2017 at 9:00

answered May 9, 2017 at 8:28

Manuel Zametter

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1 Comment

Jabaa Over a year ago

I got this error TypeError: Cannot set property 'images' of undefined

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