How to Prevent Object Values from Being Overwritten in JavaScript's forEach Loop

Learn how to effectively manage object properties in JavaScript and avoid issues with data being overwritten in loops.
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How to Prevent Object Values from Being Overwritten in JavaScript's forEach Loop

When working with arrays in JavaScript, especially when using the forEach method, developers often encounter problems related to object properties being unintentionally overwritten. A common scenario involves iterating through a list of structured data, and inadvertently resetting objects during each iteration. This guide will break down an issue where a JavaScript array loop overwrites values in an object and provide a concrete solution to it.

The Problem

Let's consider a simple example where we have an array of file names structured as follows:

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

As the developer loops through the planList and parses each file name, they want to store information about each week in a single object. However, when doing so, they noticed that the entries for week 1 were being overwritten by week 2. Here is a snippet of the relevant code causing the issue:

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

This line of code resets the entire weeks object on every iteration, leading to the final object containing only the last iteration's data.

Understanding the Cause

The core of the issue lies in initializing the weeks property for the particular plan title (_planTitle) during each loop iteration. Every time the loop runs, the code defines a new object for that title which means that prior information is lost.

What Happens in Your Code

On the first iteration (for week 1), it adds the week 1 data to _completePlan.

On the second iteration (for week 2), it overwrites the entire _completePlan[_planTitle] object, effectively discarding week 1 data.

As a result, at the end of the loop, the data reflects only the last week's information.

The Solution

To prevent the values from being overwritten, the key is to check if the weeks object already exists for that _planTitle before initializing it. If it exists, you simply update it. If it does not exist, you create a new object. Here’s how that looks in code:

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

Breakdown of Key Changes

Checking for Existing Properties:

Instead of resetting _completePlan[_planTitle], use || to assign it if it doesn’t already exist.

Maintaining Object State:

Similarly, do the same for both the info and weeks properties to avoid losing previous data.

Conclusion

By following this structured approach, you can effectively manage your objects and prevent them from losing data during iterations. This pattern of checking for existing properties is quite useful not just in this specific context, but can be applied broadly in JavaScript programming when dealing with complex data structures. Remember, with great power comes great responsibility; managing your data carefully is key to writing robust JavaScript applications.