Скачать или смотреть How to Determine If an Argument is an Array in JavaScript

Discover how to check if the argument passed into a JavaScript function is an array, and learn about useful methods like `isArray()`.
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How to Determine If an Argument is an Array in JavaScript

In JavaScript, working with functions often involves verifying the types of arguments passed to them. One common challenge developers face is ensuring that an argument is an array. In this guide, we’ll dive deep into how to check if the argument passed into a function is indeed an array and provide practical methods to do this.

The Problem Statement

When developing functions in JavaScript, you might encounter a scenario where you need to manipulate or validate an array passed to your function. For instance, consider the following code snippet:

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Here, if the last item popped from arr (checkableArr) is not an array, you'll receive an error stating that .push() is not a function. The function fails because it assumes checkableArr should be an array.

The Solution

To address this issue effectively, you can leverage JavaScript's built-in methods that are designed for type-checking. The recommended way to check if an argument is an array is to use Array.isArray().

Basic Check with Array.isArray()

This method returns a boolean indicating whether the object passed to it is an array. Here's how you can use it in your function:

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In this example, if the last item is not an array, an informative error message is thrown. This helps you understand what went wrong without the function failing silently.

Using a Fallback Array

If you want your function to proceed without throwing an error when the last item is not an array, you can define a fallback. Here’s an improved version of the function:

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In this updated snippet, if lastItem isn't an array, checkableArr defaults to an empty array, allowing your function to continue executing without interruption.

Keep the Value if Last Item is an Array

If the last item is expected to sometimes contain a value (like a nested array), you should copy that into your new array:

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This change ensures that if the last item isn't an array, it will at least be an array containing its value.

Beyond Basic Validation

If the primary aim of your function is to reverse and flatten an array, you can implement this efficiently using modern JavaScript methods. Here are a two effective one-liners:

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Considering Older Environments

For environments that do not support flat(), you can utilize reduce and concat for compatibility:

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Conclusion

Checking if an argument is an array in JavaScript can be accomplished effectively with Array.isArray(), allowing developers to avoid type-related errors and ensure their functions operate correctly. With this knowledge, you can enhance your code's robustness while handling arrays in JavaScript.

Now you’re equipped with the tools and understanding needed to ensure the arguments of your functions are the right types, making your code cleaner and more maintainable!