Facebook Oauth CORS error - go

Question

I am using oauth2 to handle a user login via facebook. The error occurs when I call the authorization server in the golang api.
Here is the network error.
Fetch API cannot load https://www.facebook.com/dialog/oauth?client_id=1543358959292867&redirect_u…=email+public_profile&state=mUi4IpdY8yF5TNVVptMNNSn8IbVSZxJXTSEFM8Zg8LM%3D. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
network info (chrome)
Request URL:https://www.facebook.com/dialog/oauth?client_id=1543358959292867&redirect_uri=http%3A%2F%2Flocalhost%3A7001%2FFBLogin%2FCallback&response_type=code&scope=email+public_profile&state=lkmenB4FjNOShUQzL0Gpymi1xsvauaL7TawmUjCyvI4%3D
Request Method:GET
Status Code:302
Remote Address:[2a03:2880:f003:c1f:face:b00c:0:25de]:443
request headers
:authority:www.facebook.com
:method:GET
:path:/dialog/oauth?client_id=1543358959292867&redirect_uri=http%3A%2F%2Flocalhost%3A7001%2FFBLogin%2FCallback&response_type=code&scope=email+public_profile&state=KPbwnGWQoI7PHvwhJ_JvZ7RowPthjqpaDTgKVI5NHrM%3D
:scheme:https
accept:*/*
accept-encoding:gzip, deflate, sdch
accept-language:en-US,en;q=0.8
origin:null
referer:http://localhost:3000/
user-agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.86 Safari/537.36
Javascript function that calls server api.
let loginUrl = "http://localhost:7001/FBLogin" //server url
let config = {
method: 'GET',
mode: 'cors',
}
fetch(loginUrl, config).then(response => {
...
}
Golang function that calls oauth authorization server (facebook).
func FBLogin(w http.ResponseWriter, r *http.Request) {
state, err := hashutil.GenerateRandomState()
logutil.Fatal(err)
url := config["fb"].AuthCodeURL(state)
logutil.OauthSessionLogger(state)
http.Redirect(w, r, url, 302)
}
I set the proper headers on the server side following this guide. Setting HTTP headers in Golang
I can't seem to find where the error comes from. (facebook API developer console, the callback headers in the server code, or the javascript initiating the call? I spent a lot of time changing the server headers but the error still persists as a CORS issue.
The origin in my request header is null, is that something to consider?
edit added no-cors mode result.
FBLogin 302 text/html SessionActions.js?524b:35 736 B 5 ms
oauth?client_id=154335&redirect_uri=http%3A%2F%2Flocalhost%3A7001%2FFBLogin%2FCallback&D
302 text/html
http://localhost:7001/FBLogin 611 B 57 ms
login.php?skip_api_login=1&api_key=154335 200
All three calls were successful, but was never redirected to the oauth page even when it said so. The response type was 'opaque' with a status 0.