How to/why use guard clauses in JavaScript

A guard clause is a code structuring programming technique where conditional statements are added to the beginning of a function to check for pre-conditions and handle exceptions. If the pre-conditions are passed then the rest of the code in the function will execute, otherwise the function will exit early. Guard clauses can help improve code readability, reduce nesting levels, and make it easier to reason about the behavior of a function.

Note: Guard clauses are a common practice in almost all programming languages, but their use is fundamentally optional.

How to use guard clauses in JavaScript

So if guard clauses are optional, why do we use them?

Many consider guard clauses a best practice for writing clean and maintainable code. They help to improve the clarity and readability of a function by giving error handling more visibility. They also improve performance by avoiding unnecessary processing when pre-conditions are not met.

Therefore, in most cases a guard clause will provide a developer with cleaner code. It will cut down on nested if statements and general spaghetti, making your work a lot more human-readable. This will present itself in less indented and flatter code.

Basic guard clause use

The basic premise is that if preconditions aren’t met then we will exit a function early. This will improve code safety, but technically it should also improve performance if used correctly.

function printMessage(message) {
    if (message === '') {
        console.log('Message is empty');
    } else {
        console.log(message);
    }
}

If you look at the above code, we have to use an else block in our if statement. This isn’t ideal, especially if we had to nest more if statements.

function printMessage(message) {
    if (!message) {
        console.log('Message is empty');
        return;
    }

    console.log(message);
}

Now we’ve rewritten the code to use a guard clause. In this case the guard clause simplifies the code by removing the else block. If the required condition of having a message isn’t met (remember that empty strings are falsy in JavaScript), then the message isn’t logged because we exit the function early.

Essentially, both pieces of code work the same so the guard clause isn’t really a necessity. It is a choice in style with an aim to make your code more readable.

Multiple exit points using guard clauses

Let’s look at an example with multiple exit points, though:

function addNumbers(x, y) {
    if (typeof x !== "number") return "Error: x is not a number";
    if (typeof y !== "number") return "Error: y is not a number";

    return x + y;
}

Now we have an example of a function with multiple exit points, one for each guard clause. We can improve the robustness of a function by using guard clauses to validate input parameters and handle errors in a concise and clear way, as demonstrated here.

Finally, by replacing the returns with a throw we start to get something a lot more useful:

function addNumbers(x, y) {
    if (typeof x !== "number") throw new Error("Error: x is not a number");
    if (typeof y !== "number") throw new Error("Error: y is not a number");

    return x + y;
}

try {
    addNumbers(5, "teddy"); // Throws an error on y
} catch (error) {
    console.log(error.message);
}

This shows how guard clauses can be used to handle errors in a more concise and clear way.

Things to consider when using a guard clause

Remember, guard clauses aren’t ‘necessary’, they are a suggestion. Keep the following points in mind so that when you use them they actually improve your code.

• Use guard clauses in functions that have pre-conditions or multiple exit points.
• Keep your guard clause simple and easy to read by using clear and concise expressions.
• Avoid using guard clauses for complex error handling or validation.
• Use descriptive error messages to make it clear why the guard clause failed.