Understanding Why Your Array is Getting Bigger After a Loop in JavaScript

Discover the common pitfalls of looping through arrays in JavaScript and how to prevent your output array from unnecessarily expanding. Learn with a practical example from Google Sheets scripting.
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Understanding Why Your Array is Getting Bigger After a Loop in JavaScript

If you’ve ever delved into JavaScript, especially in the context of Google Apps Script for applications like Google Sheets, you may have encountered situations where your arrays get unexpectedly large after a loop. This can lead to errors and confusion, particularly when you’re trying to manipulate cell backgrounds to highlight duplicates.

The Problem

Imagine you have two arrays:

Values: Contains data from Google Sheets.

nowaArr: A smaller array containing specific values you want to check against the larger array.

You intended to loop through these arrays to check for duplicates and mark them. However, after your loop runs, you find that the output array, named calkiemNowa, is significantly larger than expected—growing from 200 elements to over 120,000. This exponential growth is causing errors due to too many values being generated.

Example Code

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

The Solution

The crux of your issue lies in how you are structuring your loops. Specifically, when you check for duplicates, you are adding a value to calkiemNowa for each comparison of nowaArr elements against the values instead of just once per values element.

Here’s How to Fix It

Change the Structure of Loops:
Use a continue statement to skip the inner loop once you find a match.

Push Values After the Inner Loop:
Instead of pushing an entry for every comparison, add your result after the inner comparison is complete.

Here’s the revised code:

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

Key Changes Explained

Using continue loop1:
This statement tells the function to exit the inner loop and go back to the start of the outer loop as soon as a match is found. This prevents calkiemNowa from being populated with multiple entries of 'null' for each comparison that fails.

Reducing Push Calls:
Instead of calling push for every comparison, you only push 'null' once after all comparisons for a single value are completed, which keeps your array size down.

Conclusion

Understanding the control flow of your loops is crucial in programming. By modifying your loops and carefully managing how results are stored, you can avoid creating unnecessarily large arrays and keep your script efficient. This is a common scenario when dealing with data comparison in JavaScript, especially in a spreadsheet context, so keeping these principles in mind can save you from many coding headaches in the future.

Now you can effectively check and highlight duplicates in your Google Sheets without running into size issues! Happy coding!