http Post request with Typescript

Update 2020:

Note that as of now, the global fetch is available on all modern browsers and covers 95% of all web users. If you need support for IE, read the original answer.

MDN Doc | TypeScript Definition

Where the function is available in the window or global contexts, looks like:

    fetch(input: RequestInfo, init?: RequestInit): Promise<Response>;

so you can do:

const response = await fetch(myUrl, {
  method: 'POST',
  body: content,
  headers: {'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8'} });

if (!response.ok) { /* Handle */ }

// If you care about a response:
if (response.body !== null) {
  // body is ReadableStream<Uint8Array>
  // parse as needed, e.g. reading directly, or
  const asString = new TextDecoder("utf-8").decode(response.body);
  // and further:
  const asJSON = JSON.parse(asString);  // implicitly 'any', make sure to verify type on runtime.
}

Original Answer:

If you want to use native JavaScript functions in TypeScript for your HTTP POST request, take a look at the JSON and POST examples on YouMightNotNeedJQuery.com. Using that, you can implement your own:

// Using callbacks:
function request<Request, Response>(
        method: 'GET' | 'POST',
        url: string,
        content?: Request,
        callback?: (response: Response) => void,
        errorCallback?: (err: any) => void) {

    const request = new XMLHttpRequest();
    request.open(method, url, true);
    request.onload = function () {
        if (this.status >= 200 && this.status < 400) {
            // Success!
            const data = JSON.parse(this.response) as Response;
            callback && callback(data);
        } else {
            // We reached our target server, but it returned an error
        }
    };

    request.onerror = function (err) {
        // There was a connection error of some sort
        errorCallback && errorCallback(err);
    };
    if (method === 'POST') {
        request.setRequestHeader(
            'Content-Type',
            'application/x-www-form-urlencoded; charset=UTF-8');
    }
    request.send(content);
}

// Using promises:
function request2<Request, Response>(
    method: 'GET' | 'POST',
    url: string,
    content?: Request
): Promise<Response> {
    return new Promise<Response>((resolve, reject) => {
        request(method, url, content, resolve, reject);
    });
}

XMLHttpRequest is a built-in JavaScript class and included in the TypeScript typings.