Скачать или смотреть Debugging JavaScript: Resolving the Cannot read property -1 of Undefined Error

Learn how to fix the `Cannot read property -1 of Undefined` error in JavaScript, especially in the context of the Diagonal Difference challenge on Hackerrank. Explore step-by-step solutions and tips for effective debugging!
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This video is based on the question https://stackoverflow.com/q/63020922/ asked by the user 'JellySquare2' ( https://stackoverflow.com/u/13915356/ ) and on the answer https://stackoverflow.com/a/63021515/ provided by the user 'crosen9999' ( https://stackoverflow.com/u/13909402/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding and Fixing the Cannot read property -1 of Undefined Error

When diving into programming challenges, errors can sometimes throw us off our game. One common issue that developers, especially those tackling algorithms, encounter is the dreaded Cannot read property -1 of Undefined error. You might find yourself wondering: what is this error, and why is it causing problems in my code? Let's unpack a real-world scenario—specifically, the Diagonal Difference challenge on Hackerrank.

The Problem

In the challenge, a programmer implemented an algorithm to compute the difference between the sums of the diagonals in a square matrix. However, when running their code, they received this error message:

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

The developer was perplexed, especially since they couldn't track down the source of the -1 that was supposedly causing the issue.

Here's a simplified version of the code that leads to the error:

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

The Solution: Fixing the Error

The error message points to an issue in the second inner loop where val2 is summing values from the array. This happens because of two fundamental mistakes in the code which we will correct below.

1. Wrong Comparison Operator

In the line:

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

The problem stems from using => (which is not a valid operator in this context) instead of the correct >=. This mistake leads to the loop not functioning as intended, and effectively attempting to access an index that does not exist in the array.

Correct line:

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

2. Incrementing Index i Incorrectly

Another critical error arises from incrementing i within the inner loop. This action causes the outer loop to run only once because i is increased while still in its first iteration.

To keep the control flow intact, the i+ + statement should not be included inside the inner loops. The outer loop will handle i’s progression through the matrix.

Putting it All Together

Here’s the corrected version of the function:

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

Conclusion

By fixing the comparison operator and correctly handling the index increments, we resolved the error that caused confusion. These debugging steps not only help you understand the underlying issues in your code but also foster a greater understanding of JavaScript itself.

Remember, encountering errors is a natural part of programming. With the right approach, you can turn these obstacles into learning opportunities. Happy coding!