How can I convert a Blob to a base64 string in JavaScript for uploading to a server?

<!-- BEGIN COPY / PASTE -->
        function blobToBase64(blob) {
            return new Promise((resolve, reject) => {
                const reader = new FileReader();
                reader.onloadend = () => resolve(reader.result.split(",")[1]);
                reader.onerror = reject;
                reader.readAsDataURL(blob);
            });
        }

        // Example usage:
        const blob = new Blob(["Hello, world!"], { type: "text/plain" });
        blobToBase64(blob).then(base64String => {
            console.log(base64String); // Outputs the base64 string
        });
        <!-- END COPY / PASTE -->