Access Control Allow Origin header not present with fetch api call

问题

So I'm trying to use isomorphic-fetch https://www.npmjs.com/package/isomorphic-fetch

I have a server written in go that is giving back JSON data. This is how I'm making the call -

export function fetchDistricts(geoState) {

    return function (dispatch) {
        dispatch(requestDistricts(geoState));


        return fetch(`http://localhost:8100/districts/`)
            .then(response => {console.log(response);})
            .then(json => {
                console.log("json");
            });
}

I get this error in the chrome console

Fetch API cannot load http://localhost:8100/districts/. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:8200' is therefore not allowed access.

This is weird, because in my handler I am doing this

func getDistricts(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/jsonp;charset=UTF-8")
    w.WriteHeader(http.StatusOK)
    w.Header().Set("Access-Control-Allow-Origin", "*")
    rows, err := db.Query("SELECT * from districts")
    //other code here

Further, this was working

var activitiesDfD =  $.ajax({
    url: "http://localhost:8100/district/1/activities",
    type: "GET",
    dataType: "json"
});

$.when(activitiesDfD).then(function(data, textStatus, jqXhr) {

Why would this fail when using the fetch API and how do I get around this?

Edit-

I've now tried this

func getDistricts(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/jsonp;charset=UTF-8")
    w.Header().Set("Access-Control-Allow-Origin", r.Header.Get(`origin`))
    w.WriteHeader(http.StatusOK)

Incorporating the two suggestions below - but the error is the same.

回答1:

Almost all web browsers reject the origin "*". Therefore sending "*" as the Access-Control-Allow-Origin header results in a same-origin-policy violation.

Fortunately there is a work-around. If you look at the gin-cors code that handles this, what it does instead is to re-send the "origin" header sent by the browser. So to make * work, you'd have to do this:

w.Header().Set("Access-Control-Allow-Origin", r.Header.Get(`origin`))

回答2:

I ended up using this middleware https://github.com/rs/cors, and that got everything working correctly.

来源：https://stackoverflow.com/questions/32722409/access-control-allow-origin-header-not-present-with-fetch-api-call

标签

ajax

same-origin-policy