Understanding the Use of Underscore (`_`) in JavaScript

When coding in JavaScript, you may come across the underscore character (_) used as a variable name, particularly in function parameters. While it may seem unusual at first glance, this practice is common among developers for various reasons. In this blog post, we'll explore what the underscore represents, why it's used, and how it appears in real-world examples, like the coalesceES6 function.

What Does the Underscore (_) Mean?

In JavaScript, the underscore (_) is often used as a placeholder for a variable, especially when the variable’s identity is not important to the logic of the code. This convention helps indicate that the variable is temporary and serves a specific purpose, usually for iteration or as a callback parameter.

Example: The coalesceES6 Function

To illustrate the use of the underscore, let’s look at a simple function called coalesceES6. This function takes multiple arguments and returns the first one that is neither null nor undefined.

Here’s how the function looks:

const coalesceES6 = (...args) => 
  args.find(_ => ![null, undefined].includes(_));

Breaking It Down:

1. Arrow Function:

   • This function is defined using the arrow function syntax. The (...args) allows it to accept any number of arguments, which are stored in the args array.

2. Finding Non-Nullish Values:

   • The core of the function uses args.find(). This method goes through each element in the args array to find the first one that meets a certain condition.
   • The condition checks if the current argument (represented by _) is not in the array [null, undefined].

3. Using _:

   • The underscore (_) here represents each individual argument as the function iterates over args.
   • It serves as a temporary placeholder, indicating that this variable is only relevant within the context of the find method.

Why Use the Underscore?

1. Indicates a Temporary Variable:

   • When using _, developers signal that the variable is not important outside of its immediate use. It helps other programmers understand that this variable will not be referenced later in the code.

2. Conciseness:

   • Using a single character like _ can make the code cleaner and shorter, especially in functional programming contexts where functions are often written inline.

3. Familiarity in the Community:

   • Many JavaScript libraries and frameworks use _ as a standard convention. This familiarity makes it easier for developers to read and understand the code.

Comparison with More Descriptive Names

While using _ is common, it's not the only option. Developers can also choose more descriptive variable names to enhance readability:

const coalesceDescriptive = (...args) => 
  args.find(arg => ![null, undefined].includes(arg));

In this version, arg is used instead of _. While this improves clarity, the function’s logic remains the same. The choice between using _ or a descriptive name often comes down to personal or team preference.

Conclusion

The underscore (_) is a simple yet powerful convention in JavaScript. It serves as a placeholder variable, making code cleaner and signaling to others that the variable's identity is not crucial to the overall logic. In functions like coalesceES6, using _ allows developers to focus on the functionality rather than the specifics of variable naming.

Next time you see the underscore in JavaScript, you'll know that it's not just a random choice, but a thoughtful decision that contributes to clear and concise coding practices.