CORS 策略已阻止访问从源“http://localhost:3000”获取

Question

Adding to the database shows the error. what should I do?

Access to fetch at 'http:xxx' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

my function:

addItem = (e) => {
    e.preventDefault();
    const ob = {
        X: 53.0331258,
        Y: 18.7155611,
    }
    fetch("http://xxx", {
            method: "post",
            headers: {
                "Content-Type": "application/json"
            },
            body: JSON.stringify(ob)
        })
        .then(res => res.json())
        .then(res => {
            console.log(res);
        })
}

Answer 1

try using ''no-cors' mode.

fetch('http://localhost:8080/example', {
            mode: 'no-cors',
            method: "post",
            headers: {
                 "Content-Type": "application/json"
            },
            body: JSON.stringify(ob)
 })

Answer 2

Your server should respond with a response that looks like below

Access-Control-Allow-Origin: https://localhost:3000

Untill you are able to configure that as a workaround you can install the below chrome extension to resume your testing

https://chrome.google.com/webstore/detail/who-cors/hnlimanpjeeomjnpdglldcnpkppmndbp?hl=en-GB

But the above is only a workaround to continue development

I would suggest you read this article for understanding CORS https://javascript.info/fetch-crossorigin

Answer 3

I resolved the issue by installing the cors node module and adding this on the requested server

const cors = require("cors");
app.use(cors());

Answer 4

For browser CORS is enabled by default and you need to tell the Browser it's ok for send a request to server that not served your client-side app ( static files ).

if you use RestFul API with node and express add this middleware to your file

app.use((req, res, next) => {
  res.header("Access-Control-Allow-Origin", "*")
}) 

Answer 5

You must install cors.

npm install cors

inside the main scripts index.js or app.js

const cors = require("cors");
app.use(cors());

Answer 6

The middleware should be like this.

app.use((req, res, next) => {
  res.header({"Access-Control-Allow-Origin": "*"});
  next();
}) 

Answer 7

const cors = require("cors"); app.use(cors());

i use this but can't solve this problem.

app.use((req, res, next) => { res.header({"Access-Control-Allow-Origin": "*"}); next(); }) when use this code than solve this problem

Answer 8

make a new variable called header:

const header = new Headers({ "Access-Control-Allow-Origin": "*" });

pass this variable as an object in fetch:

fetch(url, { header: header })
      .then((res) => res.json())
      .then((data) => console.log(data));

This will resolve the issue

Answer 9

Try using the following code:

var corsOptions = {
  origin: "http://localhost:3000"
};

app.use(cors(corsOptions));

You should copy in server.js(or app.js)

(I've experienced the exact same problem as this one. I was able to solve the problem by adding the following code to server.js.)