How to return error from Flask so JavaScript catches it

Question

I have an application running a Flask backend with a React frontend and I'm trying to do some error handling for the front end and display the error message to the user. Here is a simplified version of my files:

Flask

@app.route('/api/download', methods=['POST'])
def download():
    data = request.json
    #Get data from external API via a post request here
    try:
        req = urllib.request.Request(url, data, headers)
        with urllib.request.urlopen(req) as f:
            res = f.read()
        print("Success")

        data = json.loads(res.decode())
        df = pd.json_normalize(data['observationList'])
        #Do some pandas magic here
        retData = df.to_json(orient="records")
        return retData
    except Exception as e:
        pprint(e)
    
    return jsonify(message='password_update_error'),500

JavaScript

fetch("/api/download", {
            method:"POST",
            cache: "no-cache",
            headers:{
                'Content-Type': 'application/json'
            },
            body: JSON.stringify({
                values: this.state.values
              })
        }).then((response) => response.json())
        .then(data => { 
          this.setState({ sdata: data }, () => {
            // click the CSVLink component to trigger the CSV download
            setTimeout(() => {
              this.csvLink.current.link.click();
           });
          })
        }).catch(err => {
          errorBox.innerHTML = "<span style='color: red;'>"+ 
                        "Could not get data.</span>"
        })

This currently gives me two errors:

TypeError: Data should be a "String", "Array of arrays" OR "Array of objects" 
TypeError: Cannot read property 'link' of null

I understand what the problem is but I don't know how to return errors so javascript catches it and then displays it in the box like I want it to. Any ideas? Thank you!

Answer 1

At first glance, why don't you just return the error code? It should solve the errors you're getting.

@app.route('/api/download', methods=['POST'])
def download():
    data = request.json
    #Get data from external API via a post request here
    try:
        req = urllib.request.Request(url, data, headers)
        with urllib.request.urlopen(req) as f:
            res = f.read()
        print("Success")

        data = json.loads(res.decode())
        df = pd.json_normalize(data['observationList'])
        #Do some pandas magic here
        retData = df.to_json(orient="records")
        return retData
    except Exception as e:
        pprint(e)
    
    return 500