Скачать или смотреть Resolving TypeError: Cannot read property 'locationList' of undefined in Vue.js

Discover the solution to the common JavaScript error in Vue.js related to the context of `this`. Learn how to properly manage scopes in asynchronous functions.
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This video is based on the question https://stackoverflow.com/q/62290903/ asked by the user 'Eugene Chefranov' ( https://stackoverflow.com/u/11541482/ ) and on the answer https://stackoverflow.com/a/62290973/ provided by the user 'Jakub A Suplicki' ( https://stackoverflow.com/u/6170233/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the TypeError: Cannot read property 'locationList' of undefined in Vue.js

Introduction

When building applications with Vue.js, you might occasionally encounter errors that can be frustrating to diagnose, especially when dealing with asynchronous data fetching. One common issue is the TypeError: Cannot read property 'locationList' of undefined. This error typically occurs when you're trying to access a property of an object that has not been defined, particularly within the context of an asynchronous function. In this post, we'll explore the cause of this error and how to resolve it effectively.

The Problem: Understanding the Error

Let's break down the specific scenario that leads to this error. In the given code snippet, a Vue.js component fetches data from an external API and attempts to update its locationList property.

Here’s a simplified version of the relevant code:

[[See Video to Reveal this Text or Code Snippet]]

When the getLocations method is called, the context of this inside the .then() block does not refer to the Vue component instance anymore, leading to the error when trying to access this.locationList.

The Solution: Fixing Context Issues

To rectify this issue, we need to ensure that we correctly bind the context of this so that it points to the Vue component instance even within the asynchronous function. There are several ways to achieve this. One effective method is using a variable to store the context prior to entering the chain of asynchronous calls.

Step-by-Step Fix

Store the Context: Before calling the fetch function, store the current context of this in a variable. We will use let vm = this.

Use the Stored Context: Replace instances of this within the .then() block with the new variable (vm).

Here’s the corrected version of the getLocations method:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Understanding Context: In JavaScript, especially inside functions, the value of this can change based on how the function is called. For asynchronous callbacks like those used with Promises, it's crucial to maintain the intended scope.

Using Scope Variables: If you find yourself losing the context of this, storing it in a variable (like vm) is a simple and effective technique.

Conclusion

By following the steps highlighted in this post, you should now be able to resolve the TypeError: Cannot read property 'locationList' of undefined. It's an essential skill for any JavaScript developer, especially when working with frameworks like Vue.js that rely on reactive data binding. Happy coding!