XMLHttpRequest: getResponseHeader() method - Web APIs

XMLHttpRequest: getResponseHeader() method - Web APIs | MDN

Syntax

getResponseHeader(headerName)

Parameters

headerName: A string indicating the name of the header you want to return the text value of.

Return value

A string representing the header's text value, or null if either the response has not yet been received or the header doesn't exist in the response.

In this example, a request is created and sent, and a readystatechange handler is established to look for the readyState to indicate that the headers have been received; when that is the case, the value of the Content-Type header is fetched. If the Content-Type isn't the desired value, the XMLHttpRequest is canceled by calling abort().

const client = new XMLHttpRequest();
client.open("GET", "unicorns-are-awesome.txt", true);
client.send();

client.onreadystatechange = () => {
  if (client.readyState === client.HEADERS_RECEIVED) {
    const contentType = client.getResponseHeader("Content-Type");
    if (contentType !== myExpectedType) {
      client.abort();
    }
  }
};