Скачать или смотреть Solving the ReferenceError: firstName is not defined in JavaScript

Fixing the common JavaScript error "ReferenceError: firstName is not defined" when checking an array of objects for properties and values. Learn how to correctly access object properties!
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This video is based on the question https://stackoverflow.com/q/63460051/ asked by the user 'Codestudio' ( https://stackoverflow.com/u/4206932/ ) and on the answer https://stackoverflow.com/a/63460081/ provided by the user 'hedy' ( https://stackoverflow.com/u/12041035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the ReferenceError: firstName is not defined in JavaScript

When working with JavaScript, encountering errors is part of the journey, especially for beginners. One of the common issues you might face is the ReferenceError: firstName is not defined. This error usually pops up when you try to access object properties incorrectly. In this guide, we will explore a specific example causing this error and how to solve it effectively.

The Problem

Imagine you are developing your own contact list application using an array of objects. Each object contains details about a contact, including their firstName, lastName, and number. Your goal is to create a function that can look up a contact's information based on the name and a property you're interested in. If the property isn't valid or the name isn't in the contact list, you want to return an appropriate message.

Here’s the given array of contact objects:

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

The Faulty Code

Here is the original function that leads to the error:

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

In this code, the line if (contacts[i][firstName] == name) is responsible for the ReferenceError because firstName should be quoted as a string.

The Solution

To fix the error, we can change our object property access method. Let's break down the solution step by step:

1. Access Object Properties Correctly

Instead of trying to use contacts[i][firstName], you should use contacts[i].firstName. The dot notation is a straightforward way to access an object's properties.

2. Revised Function Implementation

Here’s the corrected version of your lookUpProfile function:

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

3. Adding Property Check

In the revised function, there’s an additional check for whether the property exists. If contacts[i][prop] is defined, it will return the corresponding value; otherwise, it returns "No such property". This way, you handle both potential errors by returning clear messages for the user.

Conclusion

By ensuring that you're correctly accessing object properties with the appropriate syntax, you can avoid common JavaScript errors like the ReferenceError: firstName is not defined. Always pay attention to how you reference object properties, whether by dot notation or bracket notation with string keys. With these tips, you're on your way to writing cleaner, error-free code in JavaScript!