React Typeerror: Cannot Read Property '0' of Undefined
This is one of the more than common errors yous will encounter when starting out with React:
Cannot read property 'map' of undefined In this post, nosotros'll learn how to fix it.
Why It's Happening
The variable you are trying to map over is undefined. It volition probably eventually be an array, only due to the asynchronous nature of React, you are experiencing at least one return when the variable is undefined.
Allow's take this example lawmaking. In it, nosotros fetch some data from an API and fix state with that information.
function MyComponent ( ) { const [data, setData] = useState ( ) ; useEffect ( ( ) => { fetch ( '/api/data' ) . then ( ( res ) => res. json ( ) ) . then ( ( data ) => { setData (data) ; } ) . catch ( ( err ) => { panel. log (err) ; } ) ; } , [ ] ) ; render ( <ul > {data. map ( ( particular ) => ( <li key = {particular.id} > {item.proper noun} </li > ) ) } </ul > ) ; } This lawmaking might seem fine, but our data fetching takes some time and React does not wait for the information fetching (or any async action) to happen before information technology showtime renders your JSX. That means, while the information is being fetched, React is trying to run data.map(...).
Since we provided no initial value for information in our useState hook, data is undefined. As we know from the error message, it'south problematic to effort to call map on undefined!
Fix Choice 1: Default the Variable to an Empty Array
This offset quick fix might be enough for your use case: just default your stateful variable to an array while you lot're waiting for your information to fetch. For example:
function MyComponent ( ) { // Empty array in useState! const [data, setData] = useState ( [ ] ) ; useEffect ( ( ) => { fetch ( '/api/data' ) . and so ( ( res ) => res. json ( ) ) . so ( ( data ) => { setData (data) ; } ) . catch ( ( err ) => { console. log (err) ; } ) ; } , [ ] ) ; return ( <ul > {information. map ( ( item ) => ( <li key = {detail.id} > {item.name} </li > ) ) } </ul > ) ; } The reason this works is that, while your data fetching is happening, React will call the map method on an empty data array. This is fine—nothing volition be rendered and at that place volition be no errors. Once the data loads from the API call, our data state volition be set up and our list will correctly render.
Ready Option 2: Showing a Loading Indicator
While the previous fix is simple, it might not be the all-time user feel to brandish nothing until the data loads. Instead, nosotros might choose to brandish a loading indicator. There are a few ways we can do this, simply one of the simpler ways is to just add another stateful variable chosen loading.
function MyComponent ( ) { const [data, setData] = useState ( [ ] ) ; const [loading, setLoading] = useState ( false ) ; useEffect ( ( ) => { setLoading ( true ) ; fetch ( '/api/data' ) . then ( ( res ) => res. json ( ) ) . then ( ( data ) => { setData (data) ; } ) . take hold of ( ( err ) => { console. log (err) ; } ) . finally ( ( ) => { setLoading ( faux ) ; } ) ; } , [ ] ) ; if (loading) { render <p > Data is loading... </p > ; } return ( <ul > {data. map ( ( item ) => ( <li key = {detail.id} > {item.proper noun} </li > ) ) } </ul > ) ; } This is pretty simple and effective! When our information starts to fetch, we set loading to true. When it'southward washed fetching, nosotros set loading to false. Note that we use the finally method on our Promise since that will run regardless of whether the fetch succeeds or fails.
Speaking of Fetch Failures…
Nosotros should probably handle the state of affairs in which our fetch fails. Additionally, we can show the user an error message if our data variable is not an array. This latter point is important in making sure that nosotros never endeavour to access the map property on a non-array since information technology just won't work.
function MyComponent ( ) { const [data, setData] = useState ( [ ] ) ; const [loading, setLoading] = useState ( false ) ; const [mistake, setError] = useState ( ) ; useEffect ( ( ) => { setLoading ( true ) ; fetch ( '/api/information' ) . then ( ( res ) => res. json ( ) ) . then ( ( information ) => { setData (data) ; } ) . catch ( ( err ) => { setError (err) ; } ) . finally ( ( ) => { setLoading ( false ) ; } ) ; } , [ ] ) ; if (loading) { render <p > Data is loading... </p > ; } if (error || !Array. isArray (data) ) { return <p > There was an mistake loading your data! </p > ; } return ( <ul > {information. map ( ( item ) => ( <li key = {item.id} > {detail.name} </li > ) ) } </ul > ) ; } And now we have a pretty condom way of treatment our async performance without getting the dreaded "cannot read holding 'map' of undefined" error!
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Source: https://typeofnan.dev/fix-cannot-read-property-map-of-undefined-error-in-react/
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