libs/lib_futch.js

module.exports = function futch(url, opts = {}, onProgress) {
    return fetch(url, opts)
        .then(response => {
            if (!response.ok) {
                throw Error(response.status + ' ' + response.statusText)
            }
            if (!response.body) {
                throw Error('ReadableStream not yet supported in this browser.')
            }
            const contentLength = response.headers.get('content-length');
            if (!contentLength) {
                throw Error('Content-Length response header unavailable');
            }
            const total = parseInt(contentLength, 10);
            let loaded = 0;
            onProgress({
                loaded, total
            })
            return new Response(
                new ReadableStream({
                    start(controller) {
                        const reader = response.body.getReader();
                        read();

                        function read() {
                            reader.read().then(({
                                done, value
                            }) => {
                                if (done) {
                                    controller.close();
                                    return;
                                }
                                loaded += value.byteLength;
                                onProgress({
                                    loaded, total
                                })
                                controller.enqueue(value);
                                read();
                            }).catch(error => {
                                controller.error(error)
                            })
                        }
                    }
                })
            );
        })
}