Understanding Array.Prototype.every() Behavior: The Impact of Curly Braces in Arrow Functions

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The Problem

You may find yourself in a situation where your array-check logic is failing due to how your arrow function is structured. Let’s explore the following code example:

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

In this example:

isPresentInBothArrays1 returns false.

isPresentInBothArrays2 returns true.

This leads to a crucial question: Why does isPresentInBothArrays1 return false with curly braces, while isPresentInBothArrays2 returns true without them?

Understanding Arrow Functions

To explain this behavior, we first need to understand how arrow functions operate in JavaScript, particularly with respect to their return values.

Arrow Functions with Curly Braces

When you define an arrow function with curly braces, like so:

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

This syntax is equivalent to using a regular function declaration without an explicit return statement.

Since there’s no return statement in the code block, the function will return undefined by default.

Arrow Functions without Curly Braces

On the other hand, when you omit the curly braces and use a single expression:

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

This acts as a shorthand and automatically returns the result of that expression.

Analyzing the Example

In the provided example, the first case:

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

undefined is treated as false in the context of the every() method, resulting in an overall output of false.

Conversely, in the second case:

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

Thus, it accurately checks for the presence of each number in both arrays and returns true when all conditions are met.

Conclusion

By being mindful of how you structure your arrow functions, you can avoid unintended outcomes and ensure your JavaScript code behaves as expected.

Feel free to reach out if you have any further questions, or share your experiences with strange JavaScript behaviors! Happy coding!