request.header()

request.header(name[, value])

If value is specified, sets the request header with the specified name to the specified value and returns this request instance. If value is null, removes the request header with the specified name instead. If value is not specified, returns the current value of the request header with the specified name. Header names are case-insensitive.

Request headers can only be modified before the request is sent. Therefore, you cannot pass a callback to the request constructor if you wish to specify a header; use request.get or similar instead. For example:

d3.request(url)
    .header("Accept-Language", "en-US")
    .header("X-Requested-With", "XMLHttpRequest")
    .get(callback);

Note: this library does not set the X-Requested-With header to XMLHttpRequest by default. Some servers require this header to mitigate unwanted requests, but the presence of the header triggers CORS preflight checks; if necessary, set this header before sending the request.