How I Get return Variable if get function javascript?

Question

how i get return javascript blob data ? i have the code like this . i want upload file via blob to ajax callback

function getBase64(file) {
        var reader = new FileReader();
        reader.readAsDataURL(file);
        reader.onload = function () {
            //console.log(reader.result);
            var data = reader.result;
        };
        reader.onerror = function (error) {
            //console.log('Error: ', error);
        };

        return reader.result;
    }