Simple fetch GET request in javascript to a flask server

by TheSmartMonkey   Last Updated September 11, 2019 17:26 PM

I'm trying to display some json data from a flask server to my html page but I have a TypeError: NetworkError when attempting to fetch resource. with a Promise { <state>: "rejected" }.

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/hello', methods=['GET'])
def hello():
    jsonResp = {'jack': 4098, 'sape': 4139}
    return jsonify(jsonResp)

if __name__ == '__main__':'localhost', port=8989)


function getHello() {
    const url = 'http://localhost:8989/hello'
    const response = fetch(url)
    document.getElementById("demo").innerHTML = response;


<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <button onclick="getHello()">click</button>
    <label id="demo"></label>

    <script src="script.js"></script>

I also have a [object Promise] in the label section when I click on the button.

I did the simplest code possible but it doesn't work.

Answers 2

This could be caused by Same-Origin Policy. Browser does not allow making calls to different origin unless server sets special HTTP header. If you are opening this html file from your browser, the origin of server which is localhost does not match with the origin in your browser which is probably a file path. You can make it work by adding Access-Control-Allow-Origin header to the response as follows:

from flask import after_this_request

@app.route('/hello', methods=['GET'])
def hello():
    def add_header(response):
        response.headers['Access-Control-Allow-Origin'] = '*'
        return response

    jsonResp = {'jack': 4098, 'sape': 4139}
    return jsonify(jsonResp)

Now there is no network error but your promise is pending so you need to add then for it.

Alternatively you can serve index.html file by your Flask server so that origins match.

You can read more about CORS and SOP here:

September 11, 2019 15:35 PM

fetch() returns a promise, if you want to use .json() it returns another promise. You cound use .then() to get the json response.

  .then(response => response.json())  
  .then(json => console.log(json))

But you have another error probably related to cors in your flask server, try this before the return:

response.headers.add('Access-Control-Allow-Origin', '*')
September 11, 2019 17:25 PM

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