Magic Number

In programming, a magic number refers to a hardcoded value that appears directly in the code, without any explanation of its meaning. These values are often seen as a bad practice because they reduce code readability and maintainability. Instead of using magic numbers, it’s better to define constants or enums to give meaning to the values.

Example

Problem

function calculateCircleArea(radius: number): number {
  return 3.14159 * radius * radius;  // Magic number: 3.14159
}

function calculateDiscount(price: number): number {
  if (price > 1000) {
    return price * 0.2;  // Magic number: 0.2
  }
  return 0;
}

In this case:

• The value 3.14159 is a magic number for π (Pi), which can be hard to understand without context.
• The value 0.2 represents a 20% discount, which is not immediately clear from just looking at the code.

Solution

Instead of using magic numbers, you can define constants to improve readability and maintainability:

const PI = 3.14159;
const DISCOUNT_THRESHOLD = 1000;
const DISCOUNT_RATE = 0.2;

function calculateCircleArea(radius: number): number {
  return PI * radius * radius;  // No magic number, use the constant PI
}

function calculateDiscount(price: number): number {
  if (price > DISCOUNT_THRESHOLD) {
    return price * DISCOUNT_RATE;  // No magic number, use constants
  }
  return 0;
}

• Readability: It's immediately clear what the constant represents (e.g., PI is the mathematical constant for π, DISCOUNT_RATE is the discount percentage).
• Maintainability: If the value of PI or DISCOUNT_RATE needs to change, you only need to update it in one place rather than hunting through the code to find all instances of the magic number.
• Avoids Errors: It's easier to avoid mistakes by referring to constants rather than reusing magic numbers in multiple places.