A wrapper for XMLHttpRequest that imitates the fetch() API, but allows you to set `onprogress` to track the progress of a request!

README.md

Somewhat inspired by https://github.com/JakeChampion/fetch/blob/main/fetch.js

fetch.ts

type Fetch = typeof globalThis.fetch;
type FetchResponse = ReturnType<Fetch>;

type CustomOptions = {
    body?: XMLHttpRequestBodyInit | null;
    onprogress?: (event: ProgressEvent) => void;
};

const parseHeaders = (rawHeaders: string) => {
    const headers = new Headers();

    for (let line of rawHeaders.split("\n")) {
        line = line.trim();

        const colon = line.indexOf(":");
        if (colon === -1) continue;

        const key = line.slice(0, colon).trim();
        const value = line.slice(colon + 1).trim();

        headers.append(key, value);
    }

    return headers;
};

export const fetch = async (req: URL | string, init?: Omit<RequestInit, "body"> & CustomOptions): FetchResponse => {
    const { promise, resolve, reject } = Promise.withResolvers<Response>();

    const xhr = new XMLHttpRequest();
    xhr.addEventListener("load", () => {
        const headers = xhr.getAllResponseHeaders();
        const body = xhr.response as Blob;

        resolve(
            new Response(body, {
                status: xhr.status,
                statusText: xhr.statusText,
                headers: parseHeaders(headers),
            })
        );
    });

    xhr.addEventListener("error", reject);
    xhr.addEventListener("abort", reject);
    xhr.addEventListener("timeout", reject);

    if (init?.onprogress) {
        xhr.addEventListener("progress", init.onprogress);
    }

    let url: string;
    if (typeof req === "string") url = req;
    else if (req instanceof URL) url = req.toString();
    else throw new TypeError("Invalid URL");

    let method = "GET";
    if (init?.method) method = init.method;

    if (["GET", "HEAD"].includes(method) && init?.body) {
        throw new TypeError("Body not allowed for GET or HEAD requests");
    }

    xhr.open(method, url, true);
    xhr.responseType = "blob";

    let credentials: "omit" | "same-origin" | "include" = "same-origin";
    if (req instanceof Request) credentials = req.credentials;
    if (init?.credentials) credentials = init.credentials;

    switch (credentials) {
        case "include":
        case "same-origin": {
            xhr.withCredentials = true;
            break;
        }

        case "omit": {
            xhr.withCredentials = false;
            break;
        }
    }

    const reqHeaders = req instanceof Request ? req.headers : undefined;
    for (const [key, value] of Object.entries(init?.headers ?? reqHeaders ?? {})) {
        if (value instanceof Array) {
            for (const v of value) {
                xhr.setRequestHeader(key, v);
            }
        } else {
            xhr.setRequestHeader(key, value);
        }
    }

    xhr.send(init?.body);

    return await promise;
};

Note: This is not a 1:1 implementation of fetch, as in, it doesn't accept RequestInfo | URL, but just URL | string.