Guard Clauses

Here’s a very simple idea that can make your code a lot easier to read. First, take in the following function:

void describeEating(Item item) {
  if (!item.isFood) {
    print("Wait, that's not even food!");
  } else {
    if (item.isDelicious) {
      print("Hmm, that was good.");
    } else {
      print("Not bad. Nutritious.");
    }
  }
}

(Snippets on this blog are generally in Dart, but I take care that they’re simple enough that you’ll understand them even if you’ve never seen a single line of Dart code. Dart being a boring programming language helps.)

The code is not bad. There is no bug. But note how the main part of the function, the part that deals with food, is already indented, and follows an else keyword.

• The indentation is not a big deal now, but if the function grows in size, and if there are several if-statements like the one above, you can imagine it can become somewhat unwieldy.
• The fact that the “happy path” of the function (the case in which the item is edible) follows an else statement requires the reader to find the if statement all the way above, and then mentally flip the Boolean logic. Once again, you can imagine it only gets worse as the function grows in complexity.

11 . 1 Solution

In situations like this, try using a guard clause. Observe:

void describeEating(Item item) {
  if (!item.isFood) {
    print("Wait, that's not even food!");
    return;
  }

  if (item.isDelicious) {
    print("Hmm, that was good.");
  } else {
    print("Not bad. Nutritious.");
  }
}

The if-statement at the top is our guard clause. It checks for a condition that would make all the logic below it meaningless. If it’s not food we’re eating, then we obviously don’t want to spend a lot of time in a describeEating function. The guard clause just does what’s necessary (in our case, it prints a disgusted message), and bails out from the function with a return.

Because it bails out, there is no need for an else statement.

11 . 2 Benefits

• Guard clauses get rid of some of the indentation in your code, and some of the if-else spaghetti code.
• They make it clear to the reader what the invalid values and edge cases are.
• They can get rid of some very, very long if-statements (think if at the top of a function, and then twenty lines of code inside that conditional block).
• By having guard clauses at the beginning of the function, you avoid having return statements and throw statements in the middle of functions (those are really hard to reason about, and are often missed completely by readers, causing bugs down the line).
• As hinted above, you can also use guard clauses to throw instead of returning normally. That way, you take care of the invalid states at the top of your function.

11 . 3 The name

I see guard clauses as brave knights who defend the rest of the function from bad or irrelevant visitors. They stand at the gates (the top of the function) as guards. So, the name fits well.

11 . 4 Try it now

I encourage you to try using a guard clause right now, if you still have 5 minutes to spare. Research suggests that doing (instead of just reading about) something makes learning a lot more effective. If you don’t have an idea on how to apply your new knowledge, and if you don’t mind working in Dart, I made a small example for you to refactor using guard clauses.