How do I convert base64 string image to blob in React Native?

I have a function in my project to convert image to a blob. Here it is. 1st function is to handle the camera. 2nd fuction is to create a blob and a image name.

addPicture = () => {
        ImagePicker.showImagePicker({ title: "Pick an Image", maxWidth: 800, maxHeight: 600 }, res => {
            if (res.didCancel) {
                console.log("User cancelled!");
            } else if (res.error) {
                console.log("Error", res.error);
            } else {
                this.updateProfilePicture(res.uri)
            }
        });
    }

This addPicture is used to launch the image picker. In above function, res means the output, that comes from showImagePicker. I had to pass the uri prop of the result(res.uri) to below function, in order to create the blob file

In below function, I wanted to name the image with userId. You can use anything you like.

updateProfilePicture = async (uri) => {
        var that = this;
        var userId = this.state.user.uid
        var re = /(?:\.([^.]+))?$/;
        var ext = re.exec(uri)[1];

        this.setState({
            currentFileType: ext
        });

        const blob = await new Promise((resolve, reject) => {
            const xhr = new XMLHttpRequest();
            xhr.onload = function () {
                resolve(xhr.response);
            };
            xhr.onerror = function (e) {
                console.log(e);
                reject(new TypeError('Network request failed'));
            };
            xhr.responseType = 'blob';
            xhr.open('GET', uri, true);
            xhr.send(null);
        });

        var filePath = userId + '.' + that.state.currentFileType;

    }

There are some other codes in above function, which are using to uplad the image to firebase storage. I did not include those codes.