Javascript fetch handle both json and blob

Question

Using javascript fetch and invoking a rest service that returns a blob if successful, otherwise returns an error message as json. How would this be handled in the fetch? The actual service is a asp.net web api implementation that returns a FileStreamResult (or FileContentResult) when successful, otherwise returns an error code with json containing the error message. Below is an example of what I'm trying to do:

fetch('flowers.jpg').then(function(response) {
  if(response.ok) {
    return response.blob();
  } else {
    return response.json();
}

}).then(function(myBlob) {  // here I would also like to function(jsonError)
  var objectURL = URL.createObjectURL(myBlob); 
  myImage.src = objectURL; 
}).catch(function(error) {
  console.log('There has been a problem with your fetch operation: ', error.message);
});

Answer 1

Since you want to go down two fairly different paths, this is one of the relatively-rare situations where you probably want to nest handlers:

fetch('flowers.jpg').then(function(response) {
    if (response.ok) {
        return response.blob().then(function(myBlob) {
            var objectURL = URL.createObjectURL(myBlob);
            myImage.src = objectURL;
        });
    } else {
        return response.json().then(function(jsonError) {
            // ...
        });
    }
}).catch(function(error) {
    console.log('There has been a problem with your fetch operation: ', error.message);
});

Answer 2

You can use nested then block to pass literally whatever you want to your next then handler. Sample:

fetch('flowers.jpg').then(function (response) {
    if (response.ok) {
        return response.blob()
            .then(function (myBlob) {
                return {
                    blob: myBlob
                };
            });
    } else {
        return response.json()
            .then(function (myJson) {
                return {
                    json: myJson
                };
            });
    }

}).then(function (myData) { 
    if(myData.blob){
        // Handle blob case
    }else{
        // Handle JSON case
    }
}).catch(function (error) {
    console.log('There has been a problem with your fetch operation: ', error.message);
});

Perhaps a more semantically accurate way of handling this is rejecting when there is an error.

fetch('flowers.jpg').then(function (response) {
    if (response.ok) {
        return response.blob();
    } else {
        return response.json()
            .then(function (myJson) {
                return Promise.reject(myJson);
            });
    }

}).then(function (myData) {
    // Handle blob case
}).catch(function (error) {
    //Handle JSON case
});