AWS signed url if the object exists using promises

Question

I have a code snippet for generating a signed url. The below return statement always returns empty url. Rest of the data is correctly resolved. When I debug I see that the return callback gets executed first then the resolve part of the function validSignedURL gets called.

awsHelper
        .s3vldSignedURL(s3Link)
        .then(function(signedURL) {
            data[1].url = signedURL;
            return callback(null, successResponse.getResponse(context, 'OK', data));
        });

The s3vldSignedURL maps to the function below. Here s3.headobject is promise based, used to check if a file exists in s3. I want this function to be generic, so that I can use it to generate a signed url, for any s3object.

function validSignedURL(bucket, path) {
console.log("Generating Presigned Link ... ");
const s3 = new aws.S3();

let params = {
    Bucket: bucket,
    Key: path
};

let checkObj = s3.getObject(params);
return new Promise(function(resolve, reject){
    s3.headObject(params).promise()
        .then(function (data) {
            console.log('s3 File exists' + data);
            resolve(getSignedURL(bucket, path));
        }).catch(function (err) {
        console.log('Generating Presigned Link ... Failed' + err);
        resolve('');
    });
});
}

The below function getSignedURL always returns a signed url irrespective of the object exists or not.

function getSignedURL(bucket, path) {
    console.log("Generating Presigned Link ... ");
    const s3 = new aws.S3();

    let params = {
        Bucket: bucket,
        Key: path
    };

return s3.getSignedUrl('getObject', params);
}

Also, how can I convert the function call s3.headObject(params) to a synchronous call which returns true or false?

Avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it)! — Bergi, Aug 25 '17 at 00:44
Also see the [difference between `.then(…, …)` and `.then(…).catch(…)`](https://stackoverflow.com/q/24662289/1048572), which doesn't however explain why you are seeing `resolve` after the `callback` either (unless that's not your actual code). — Bergi, Aug 25 '17 at 00:51
"*How can I convert the function call `s3.headObject(params)` to a synchronous call*" - you cannot. Don't try. — Bergi, Aug 25 '17 at 00:51
Another question is, do you really need to know if the object doesn't exist? Creating a pre-signed URL against a non-existent object **is a valid action** because the pre-signed URL will be valid if the object exists in the future, and will return a `404` error if the pre-signed URL is used used until then (or if the object is never created). — Michael - sqlbot, Aug 25 '17 at 02:33
`validSignedURL` always resolves, sure it resolves `''` on error, but you don't seem to check the resolved value in your code — Jaromanda X, Aug 25 '17 at 04:07
@JaromandaX Yes, I just assign it to `data[1].url` in the first code block. — user 923227, Aug 25 '17 at 18:11
@Bergi Hi, with the promise constructor anti pattern are you asking me to try `return s3.headObject(params).promise()` in place of `return new Promise(function(resolve, reject){` with the call as - `data[1].url = awsHelper. validSignedURL('invndata', s3Link + mapFileName);` this too behaves in the same nature. The return takes place before. — user 923227, Aug 25 '17 at 18:58
@SumitKumarGhosh Yes, that's what you should replace. No, this still returns a promise from `validSignedURL` (you can't prevent that), but then its written properly. — Bergi, Aug 25 '17 at 22:20

user 923227 · Accepted Answer · 2017-10-20T17:35:46.680

This is the complete function definition and calling for AWS signed url if the object exists using promises. If you are using AWS Lambda you need to add s3 permissions.

If there is no need to validate if the url exists then getSignedURL is the function to be used
If there is a need to check if the object exists then use validSignedURL it will return an empty url if not found.

Here is how I implemented it:

function getSignedURL(bucket, path) {
    console.log("Generating Presigned Link ... ");

    let params = {
        Bucket: bucket,
        Key: path
    };

    //expires in default 15 mins.
    return s3.getSignedUrl('getObject', params);
}

function validSignedURL(bucket, path) {
    console.log("Generating Presigned Link ... ");
    let params = {
        Bucket: bucket,
        Key: path
    };

    return new Promise(function(resolve, reject){
        s3.headObject(params).promise()
            .then(function (data) {
                console.log('s3 File exists' + data);
                resolve(getSignedURL(bucket, path));
            }).catch(function (err) {
            console.log('Generating Presigned Link ... Failed' + err);
            resolve('');
        });
    });
}

Calling the function and resolving the promise. I have some other fields populated before in data which is returned along with the signed url.

    validSignedURL(bucketName, s3Link + fileName).then(function(signedURL) {
        data.url = signedURL;
        return callback(null, successResponse.getResponse(context, 'OK', data));
        });

AWS signed url if the object exists using promises

1 Answers1

Linked