@catamphetamine/id3js/lib/image/getBlobDataUrl.js

A modern ID3 parser written completely in JavaScript, making use of typed arrays and the HTML5 File API

/**
 * @param {Blob} blob Blob to read
 * @param {string} [options.contentType] Content Type
 * @return {Promise<string>}
 */
export default function getBlobDataUrl(blob, { contentType } = {}) {
    return new Promise((resolve, reject) => {
        const fileReader = new FileReader();
        fileReader.addEventListener('error', reject);
        fileReader.addEventListener('abort', reject);
        fileReader.addEventListener('load', () => {
            // Works around TypeScript error: "Object is possibly 'null'".
            // if (event.target === null) {
            //   return reject(new Error('No data'));
            // }
            // Works around TypeScript error: "Property 'result' does not exist
            // on type 'EventTarget'".
            // https://stackoverflow.com/questions/35789498/new-typescript-1-8-4-build-error-build-property-result-does-not-exist-on-t.
            // let dataUrl = event.target.result;
            if (fileReader.result === null) {
                // Shouldn't be possible.
                return reject(new Error('No data'));
            }
            let dataUrl = fileReader.result;
            if (dataUrl !== null) {
                if (contentType) {
                    dataUrl = dataUrl.replace('application/octet-stream', contentType);
                }
            }
            resolve(dataUrl);
        });
        fileReader.readAsDataURL(blob);
    });
}