React + Fetch+ Json. TypeError: Cannot read property of undefined

Question

When I used SuperAgent I didn't have this problem, but I decided to use Window.fetch polifyl and I met this problem. I see all data was loaded, but it still shows error. Could your help me identify this error please:

In render() I genereate a list of components based on an obtained list:

    render() {

    if (this.state.data.list) {
        console.log("render: " + this.state.data.list);
        var counter = 0;
        const list = this.state.data.list.map((item) => {
....

I imagine it like that: after obtaining response, sent this response to a next function and the result of that function I send to a next function, but I didn't return any result. It means I don't transfer any data without return value from functions in the chain of functions.... — Alex Pilugin
– Alex Pilugin, Commented Feb 6, 2017 at 0:29

Josh Kelley · Accepted Answer · 2017-02-03 21:17:56Z

6

The promise handlers in your screenshot won't work:

.then((json) => console.log('parsed json: ', json))
.then((json) => { this.setState({ data: json }); })

"Take the value from resolving this promise and pass it to console.log. Then, take console.log's return value (which is undefined) and pass it to this.setState."

answered Feb 3, 2017 at 21:17

Josh Kelley

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1 Comment

Alex Pilugin Over a year ago

Thank you for hint!

fetch(url, {             headers: {                 'Accept': 'application/json',             },         }).then((response) => response.json()             .catch(err => {                 console.err(

'${err}' happened!

);                 return {};             }))             .then((json) => {                 console.log('parsed json: ', json);                 this.setState({ data: json })             })             .catch((err) => { console.log('fetch request failed: ', err) }             )

Alex Pilugin · Accepted Answer · 2017-02-03 21:58:54Z

3

    fetch(url, {
        headers: {
            'Accept': 'application/json',
        },
    }).then((response) => response.json()
        .catch(err => {
            console.err(`'${err}' happened!`);
            return {};
        }))
        .then((json) => {
            console.log('parsed json: ', json);
            this.setState({ data: json })
        })
        .catch((err) => { console.log('fetch request failed: ', err) }
        )

answered Feb 3, 2017 at 21:58

Alex Pilugin

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