Скачать или смотреть Understanding Why n % 2 === 1 Fails for Negative Odd Numbers in JavaScript

Learn to filter negative odd numbers from an array in JavaScript, and understand the difference between `n % 2 === 1` and `n % 2 !== 0`.
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Filtering Negative Odd Numbers in JavaScript

When working with arrays in JavaScript, you might often need to filter specific values based on certain criteria. This guide addresses a common question among beginners: Why does n % 2 === 1 not work for filtering negative odd numbers, while n % 2 !== 0 does? Let’s break down this problem and find out how to filter negative odd numbers from an array effectively.

The Problem

You want to create a function that returns only the odd negative numbers from a given array. For instance, given the array const arr = [4, -7, -6], your goal is to extract numbers that are both negative and odd.

Initially, you might try using this code:

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

However, the result of this operation might not be what you expect—in this case, you would get an empty array [], even though -7 is indeed an odd negative number.

Understanding the Modulo Operator

To grasp why n % 2 === 1 fails, we need to understand the modulo operator % in JavaScript. This operator divides a number by a specified divisor—in our case, 2—and returns the remainder. Let's see how it works with both negative and positive numbers.

Examples of Modulo Operation

For a negative odd number:

Calculation:

-5 % 2:

When you divide -5 by 2, you get -2.5, which translates to -2 with a remainder of -1. Thus, -5 % 2 equals -1.

For a positive odd number:

Calculation:

5 % 2:

When you divide 5 by 2, you get 2.5, which translates to 2 with a remainder of 1. Hence, 5 % 2 equals 1.

Key Takeaway

From the evaluation above, we can conclude:

Odd positive numbers yield a remainder of 1 when divided by 2.

Odd negative numbers yield a remainder of -1 when divided by 2.

This means that using n % 2 === 1 will not correctly capture negative odd numbers, because the expression will not return true for negative odd values.

The Solution

To accurately filter negative odd numbers, you should use n % 2 !== 0. This expression correctly checks both odd positive and odd negative numbers, as it captures any number that has a remainder of either 1 or -1.

Correct Code Implementation

Here's how to modify your filtering function:

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

The Output

With this adjustment, the output will correctly reflect the negative odd numbers within your array, yielding [-7] as anticipated.

Conclusion

When working with number filtering in JavaScript, always consider how different operations affect your desired outcomes. In this case, understanding the modulo operation clarified why n % 2 !== 0 is the correct approach for identifying negative odd numbers. Happy coding!