The benchmark on MeasureThat.net compares the performance of two ways to define a class in JavaScript: class declaration and class expression. Both of these syntax forms allow developers to create classes in JavaScript, but there are differences in how they are used and potentially their performance.

Test Cases Overview

1. Class Declaration:

   • Code: class a { static b = 'c'; d = 'e' }
   • Test Name: "class declaration"

   This is a traditional way to declare a class in JavaScript. It introduces a class a with a static property b and an instance property d. Class declarations are hoisted, meaning they can be used before they are defined in the code.

2. Class Expression:

   • Code: let a = class { static b = 'c'; d = 'e' }
   • Test Name: "class expression"

   Here, the class is defined as an expression. It's assigned to a variable a. Like classic declarations, it also has a static property b and an instance property d. However, class expressions are not hoisted, which means they can only be used after their definition in the code.

Performance Results

• The benchmark results show that the class expression achieved approximately 242,669.28 executions per second, while the class declaration achieved 230,969.44 executions per second. A clear conclusion drawn from this would be that, at least in this test run on Chrome 132, class expressions performed better than class declarations in this specific context.

Pros and Cons

Class Declaration:

• Pros:

  • Hoisting allows you to reference the class before its definition.
  • Easier to read and use for large class definitions or when organizing class hierarchies.

• Cons:

  • Potentially less flexible in terms of variable scope due to hoisting.
  • Similar performance characteristics but could be slower under certain conditions.

Class Expression:

• Pros:

  • More flexible as it can be assigned to variables, passed around like functions or other objects.
  • Can be utilized in contexts where classes need to be defined conditionally or within a function scope.

• Cons:

  • Not hoisted, which means it can't be used until its declaration is executed in the code.
  • Might be less familiar to those coming from classical object-oriented programming languages.

Other Considerations

• Usage Context: The choice between these two often comes down to the specific requirement of the code being written. If the class is to be used conditionally, a class expression might be necessary. Conversely, in a straightforward case where a class can be fully defined and used, a class declaration may suffice.

• Browser and Environment Differences: The results can vary based on the JavaScript engine (V8 in Chrome, SpiderMonkey in Firefox, etc.) and the optimization techniques they apply. What might perform well in one environment might not in another.

Alternatives

In addition to class declarations and expressions, alternative design patterns include:

• Function Constructors: Before classes were introduced in ES6, JavaScript relied heavily on function-based constructors to create object instances.
• Object Factory Functions: Instead of using classes, you might use factory functions that return objects. They offer flexibility in instantiation but don't provide classical inheritance.
• ES5 Prototypal Inheritance: Using prototype manipulation for inheritance, which allows you to extend objects without using the formal class syntax.

Understanding these distinctions between class declaration vs. class expression is critical for efficiently writing JavaScript applications, impacting both code organization and performance.