The XMLHttpRequest method getResponseHeader() returns the string containing the text of a particular header's value.
If there are multiple response headers with the same name, then their values are returned as a single concatenated string, where each value is separated from the previous one by a pair of comma and space. The getResponseHeader() method returns the value as a UTF byte sequence.
Note: The search for the header name is case-insensitive.
If you need to get the raw string of all of the headers, use the getAllResponseHeaders() method, which returns the entire raw header string.
getResponseHeader(headerName)
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 !== my_expected_type) {
client.abort();
}
}
};