Even though there is data stored in the array, the React Native array.length appears to be returning a value

I am struggling with what appears to be a simple issue, and it's frustrating that I've had to seek help for this.

The problem lies in iterating through an array messages: Message[] = [...]. No matter what method of iteration I try, it doesn't seem to work as expected.

Here is the snippet of code causing me trouble:

const getMessages = (messages: Message[]) => {
  const cards = []
  // eslint-disable-next-line no-console
  console.log("Test 1: ", messages);
  // eslint-disable-next-line no-console
  console.log("Test 2: ", messages.length);
  // eslint-disable-next-line no-console
  console.log("Test 3: ", messages[0]);
  let i = 0;
  for (const msg of messages) {

    // eslint-disable-next-line no-console
    console.log("ADD TO LIST");
    cards.push(
      <MessageSummary
        key={i}
        council={msg.council}
        latestMessage={msg.content}
        date={msg.date"}
        hasBeenRead={false}
      />
    );
    i += 1;
  };
  if (cards.length > 0) {
    return messages;
  }
  return (
    <Text style={styles.noApplicationsMessage}>
      You don&apos;t have any messages yet
    </Text>
  );
};

The terminal output reveals that although logging messages gives correct data, attempting to check its length or access elements poses an issue.

EDIT

Below is where I fetch the messages from the database:

const processNewMessages = useCallback((newMessages: Message[]) => {
    setMessages(newMessages);
  }, []);

  useFocusEffect(
    useCallback(() => {
      if (userExternalId < 0) {
        return;
      }
      setLoading(true);
      getMessages(userExternalId)
        .then((messagesQuery) => processNewMessages(messagesQuery))
        .catch(() => {
          setError("Messages could not be fetched. Please try again later.");
          setShowBanner(true);
        })
        .finally(() => {
          setLoading(false);
        });
    }, [processNewMessages, userExternalId])
  );

Your assistance in resolving this matter would be greatly appreciated. Thank you.

Answer №1

Array is always passed by reference. When you log all three items,

  1. messages was initially an empty array // passed by reference
  2. therefore messages.length was 0 // passed by value
  3. therefore messages[0] was undefined. // passed by value

However, since console.log(messages) logs by reference, it later updates to the correct data, but the other two logs do not update.

It appears that the messages array being passed is asynchronously retrieved, consider using async await and only execute the function once the messages data has been received.

Similar questions

If you have not found the answer to your question or you are interested in this topic, then look at other similar questions below or use the search

Observing changes in a parent component variable with Angular

One feature of my form is that it consists of a parent component and a child component. To disable a button within the form, I utilize a function called isDatasetFilesValid() which checks a specific variable (datasetList[i].fileValid). This variable is mo ...

How do I insert items into an ES6 Map using a specific object key/value type in Typescript?

I'm looking to utilize Maps instead of object maps to define keys and values. However, it appears that Typescript doesn't fully support index types for ES6 Maps. Are there any alternatives or workarounds available? Furthermore, I want to enforce ...

Enhance your Angular application with lazy loading and nested children components using named outlets

Let me start by explaining that the example provided below is a simplified version of my routes that are not functioning properly. I am working on an angular project, specifically a nativescript angular project, and I suspect the issue lies within Angular ...

Are there any Java alternatives comparable to PHP arrays with non-numeric keys?

How can we convert this PHP array to Java? HashMap<String, String> user = new HashMap<>(); user.put("name", "John"); user.put("email", "<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="92f8fdfafcd2fff3fbfebcf1fdff" ...

Issue with NgFor nested component not refreshing after @Input modification

When populating a component called ContactUpdatableItem within a NgFor, the code looks like this: <section class="plContactCreation-listItem" *ngFor="let contact of contacts$ | async; index as idx" > <contact-updatable-item [c ...

What Causes the "Do Not Push Route with Duplicated Key" Error in React Native with Redux?

I have successfully integrated Redux into my React Native project, specifically for navigation purposes. Below is the code snippet from my navigation reducer file (navReducer.js): import { PUSH_ROUTE, POP_ROUTE } from '../Constants/ActionTypes' ...

The grid items fail to refresh even after I have made changes to the useState in my React Native application

I am currently working on a react native project that includes a grid component. My goal is to update an array in the state called selectedHomeTypes whenever a user clicks on an item within the grid. Initially, the array is set to contain only one element: ...

Overtaking a property in a node_module interface: a guide

I am facing a situation where I need to modify an existing interface property in my custom type declaration file, rather than creating a new one. Despite trying various approaches, such as attempting to omit the property, I have not been successful in ach ...

Tips for creating a test to choose a movie from the MuiAutocomplete-root interface

I am currently utilizing Material UI with React using Typescript and I am looking to create a test for the autocomplete functionality using Cypress. Here is the approach I have taken: Identifying the Autocomplete component and opening it, Choosing an opti ...

Exploring the power of Vue CLI service in conjunction with TypeScript

I've recently set up a Vue project using the Vue CLI, but now I am looking to incorporate TypeScript into it. While exploring options, I came across this helpful guide. However, it suggests adding a Webpack configuration and replacing vue-cli-service ...

select elements based on specific keys in the array

I have been working on a function that filters my array by keys: private function filterMyArray( ) { function evaluateKey( $value ) { return $value['type'] == 'video'; } return array_filter( $array, ...

`Understanding the outcome type by assessing the (potential) attributes with TypeScript`

Here is a detailed example of a problem with some code: type Mapper<T, R> = (data: T) => R; interface Config<T, R> { readonly mapper?: Mapper<T, R>; } function transform<T, R>(config: Config<T, R>, data: T) { return c ...

I am continuously receiving the message "Alert: It is important for every child in a list to possess a distinct "key" prop." while working with the <option> list

Having trouble generating unique keys for my elements. I've tried different values and even Math.random() but still can't seem to get it right. I know the key should also be added to the outer element, but in this case, I'm not sure which on ...

Tips for managing events in a custom form control that does not involve the use of inputText

I am currently working on developing a custom form control that functions as a list picker within a screen, without using a modal or native iOS picker. This custom control does not involve any TextInput or Picker control components. How can I manage the st ...

Tips for encapsulating a promise while maintaining the original return type

In my code, there is a function that utilizes an axios instance and is of type async function register(data: RegisterData): Promise<AxiosResponse<UserResponse, any>> export const register = (data: RegisterData) => api.post<UserResponse> ...

How to add a service to a static function in Angular

After incorporating a logger service into my project, I have encountered an issue with using it in NGXS static selectors. The selectors in NGXS are static methods, which prevent me from accessing the logger service injected via Angular DI. Are there any e ...

Iterate through every multi-dimensional array and output a single array

This is a multidimensional array: Array ( [CalculateOverheadDoorSpringsResult] => Array ( [SpringForce] => Array ( [InputData] => Array ( ...

A common error known as a segmentation fault occurs when attempting to access an array within a Fortran

My fortran program is experiencing issues with a subroutine causing a segmentation fault. The fault occurs after executing line 1434 and printing the message: i: 115 256 2 Segmentation fault (core dumped) The program has pa ...

Importing Byte[] into Database via API

I've been exploring different ways to achieve this task using LINQ following this resource. However, I'm facing challenges in extracting data from the API. Whenever I try to output resource.Data.Body, it displays as Byte[]. Similarly, when I at ...

Strategies for effectively choosing this specific entity from the repository

Is it possible to choose the right entity when crafting a repository method using typeorm? I'm facing an issue where I need to select the password property specifically from the Admin entity, however, the "this" keyword selects the Repository instead ...