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TypeError: Cannot read properties of undefined (reading 'customer')

import React, { useState, useEffect } from 'react';
import axios from 'axios';

{const CustomerList = () => {
  const [information, setInformation] = useState([]);
  const [isLoading, setIsLoading] = useState(true);

  useEffect(() => {
    if (isLoading) {
      axios.get('').then((response) => {
        setInformation((prevInfo) => {
          return [...prevInfo, response.data];
        });
      });
    }
    return () => {
      setIsLoading(false);
    };
  }, [information, isLoading]);

  return (
    <>
      <h2>Name: </h2>
      {information.map((item, index) => (
        <div>
          <div>{item[index].customer.firstName}</div>
          <div>{item[index].customer.lastName}</div>
        </div>
      ))}
    </>
  );
};

export default CustomerList;

I am fetching data from an api for work using axios and useeffect
I map through the response array of data and if no info is in the arraythe app breaks
How can I map through only the specific amount of items in an array?
For example if I have 3 items in my array of data, it will show them on the browser down from the return statement BUT... when it maps all 3 it will continue to try again and break hence 'customer is undefined' because there is no more customers 5.I have tried to do {item.customer.firstName and get an error of undefined first name}

Any way to get around this? Thanks!

enter image description here

about 4 years ago · Juan Pablo Isaza

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Answer 1 · 2022-07-12T07:52:44.621Z

That's not how map works. With these arguments to the callback:

information.map((item, index) => ...

index is the current index when iterating over the information array, and item is the element at that index. It's not another copy of the entire array. So instead of this:

item[index].customer.firstName

You would just do this:

item.customer.firstName

As an aside, when iterating over an array to output a list in React you would want to add a key to the top-level element. You can use index for this purpose if you like:

{information.map((item, index) => (
    <div key={index}>
      ...
    </div>
))}

Or if item has a unique identifier then you can use that instead and remove index entirely.

Answer 2 · 2022-07-12T07:52:44.630Z

item[index] is not correct, since item refers to the element of the array at that index anyway.

Correct Way:

const data = [1,2,3,4,5,6,7,8,9,10]

const mappedData = data.map((d, index) => {
    console.log("index is", index)
    console.log("d is", d)
    console.log("-----------------------------")
    return d
})

What you are trying to do:

const data = [1,2,3,4,5,6,7,8,9,10]

const mappedData = data.map((d, index) => {
  console.log("index is", index)
  console.log("d[index] is", d[index]) //undefined since d is not an array 
  console.log("------------------------------")
  return d
})

Answer 3 · 2022-07-12T07:52:44.637Z

When you map() an array of items, you are handling each item separately, therefore you don't need the index, you could use the index as key, read more about lists and keys here.

{information.map((item, i) => (
  <div key={i}>
    <div>{item.customer.firstName}</div>
    <div>{item.customer.lastName}</div>
  </div>
))}

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TypeError: Cannot read properties of undefined (reading 'customer')

3 Respuestas

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Correct Way:

What you are trying to do:

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Andres GPT