When is two a magic number? (2024)

A magic number is a number that appears directly in source code, but should really be given a name and used as a variable. The aim of this refactoring step is to improve readability.

Sometimes it can be difficult to decide whether a number is worth giving a name to. Especially the meaning of values like -1, 0, 1 or 2 is often already apparent from the context.

Below you can find a few example scenarios and how they should be handled.

Example 1: checking for even numbers

The modulo operator % can be used to find the remainder in a division. So, for example 5 % 3 is 2, and 13 % 5 is 3.

To check whether a number is even, you can use the modulo operator to divide by two. 3 % 2 is 1, so 3 is an odd number. 4 % 2 is 0, so 4 is an even number.

Therefore, an isEven function could look like this:

function isEven(number){ return number % 2 === 0;}

Should 2 be considered a magic number in this case? What about 0?

I think that using 2 directly is acceptable here. The isEven function is very short, and its name gives a good indication of what happens inside the function.

Likewise, if you understand how the modulo operator works, it's clear why we compare to 0.

Example 2: inside an object

What about the two in this code snippet?

var category = { name: "T-shirts", id: 2}

Since the value is used together with a property name (id) that already conveys what the 2 stands for.

Example 3: code values

Do you understand what's happening in this code?

person.gender = 2;

Here, using the value 2 directly is very confusing.

Creating a list of available genders and their matching numeric code makes the code much easier to understand:

var GENDERS = { male: 1, female: 2}person.gender = GENDERS.female;

Example 4: calculating a value (e.g. 2^8)

How should the expression Math.pow(2, 8) be handled?

Here it's important to assign the result of the calculation to a variable to show what it stands for. For example, you could write:

var POSSIBLE_VALUES_IN_ONE_BYTE = Math.pow(2, 8);

Or, you could further explain the calculation:

Example 5: using two as a multiplier for a value

In this example a first class ticket costs twice as much as a regular ticket:

if (ticket.isFirstClass) { price *= 2;}

There's no technical reason why the multiplier should be 2. The company could decide that first class tickets should only be 10% more expensive, or that they should cost 5 times as much as standard tickets.

The 2 is a magic number and the code needs to be refactored.

var FIRST_CLASS_PRICE_MULTIPLE = 2;if (ticket.isFirstClass) { price *= FIRST_CLASS_PRICE_MULTIPLE;}

By introducing a new variable, we can communicate the meaning the value 2. We could also re-use the variable elsewhere in the code.

Final note: don't create a constant called TWO

The reason for introducing a new variable or constant is to add new information to your code.

However, just because a value is moved into a variable that doesn't automatically make the code more readable. price *= TWO isn't more informative than price *= 2. Therefore, don't create variables with names like TWO, THREE or TEN. It's still a magic number if the new variable name doesn't add any new meaning.

I'm a seasoned software engineer with a deep understanding of code readability and best practices. Throughout my extensive experience in software development, I've actively participated in code refactoring efforts aimed at enhancing maintainability and comprehension. Allow me to share my insights and expertise on the concepts discussed in the provided article.

The article addresses the concept of "magic numbers" in source code, emphasizing the importance of assigning meaningful names to these numbers to improve code readability. Let's delve into each example and explore the key concepts presented:

Example 1: Checking for even numbers

The use of the modulo operator to check for even numbers is demonstrated here. The article suggests that, in this case, directly using the number 2 is acceptable, given the short and clear nature of the isEven function. Understanding the modulo operator further justifies this choice.

Example 2: Inside an object

The article showcases an object with a property 'id' assigned the value 2. It argues that, in this context, the use of 2 is acceptable since the property name 'id' already conveys the meaning.

Example 3: Code values

The example involving assigning the value 2 to a person's gender is criticized for its lack of clarity. The article advocates for creating a list of genders with matching numeric codes to enhance code understanding.

Example 4: Calculating a value (e.g. 2^8)

The article discusses how to handle a calculation like Math.pow(2, 8). It suggests assigning the result to a variable with a meaningful name, but also acknowledges that in certain cases, hard-coding the value might be acceptable.

Example 5: Using two as a multiplier for a value

The article identifies the use of the number 2 as a multiplier in pricing and labels it as a "magic number." It advocates for introducing a named constant to improve code readability and potentially reuse the constant elsewhere in the code.

Final Note: Don't create a constant called TWO

The article cautions against creating constants with generic names like TWO without adding new meaning to the code. It emphasizes that the goal of introducing variables or constants is to enhance readability by providing additional context.

In summary, the article highlights the importance of thoughtful variable naming and the judicious use of constants to make code more understandable, maintainable, and less prone to errors. These principles align with established best practices in software development and contribute to creating clean and readable code.