In this talk we report on a large and in-depth study to understand how class emulation is employed in JavaScript applications. We propose a strategy to statically detect class-based abstractions in the source code of JavaScript systems. We used this strategy in a dataset of 50 popular JavaScript applications available from GitHub. We found four types of JavaScript software: class-free (systems that do not make any usage of classes), class-aware (systems that use classes, but marginally), class-friendly (systems that make a relevant usage of classes), and class-oriented (systems that have most of their data structures implemented as classes). The systems in these categories represent, respectively, 26%, 36%, 30%, and 8% of the systems we studied

1. Does JavaScript Software
Embrace Classes?
L. Silva, M. Ramos,
M. T. Valente
UFMG, Brazil
A. Bergel
Univ. Chile
N. Anquetil
INRIA, France
SANER, Montreal, March 2015

2. JavaScript is gaining popularity
http://redmonk.com/dberkholz/2014/05/02/github-
language-trends-and-the-fragmenting-landscape/
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3. 1995: Small Scripts
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4. 2015: Complex Systems
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5. Criticisms
https://www.destroyallsoftware.com/talks/wat 5

6. Motivation
How JavaScript software is organized?
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7. Motivation
How JavaScript software is organized?
More specifically,
Do JS developers use class-like structures?
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8. JavaScript
• Imperative
• Objects maps strings to values
– var person = { firstName: "Marco", lastName:"Valente" }
• Functions are first-class objects
– var fact = function factorial (n) { .... }
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9. Emulating Classes
• new:
–creates a new object
–calls the constructor function
–with this bound to the new object 9

10. Emulating Inheritance
• Each object has an implicit prototype property
• To eval obj.p, the runtime searches for p in:
–The current object obj
–The prototype of obj
–The prototype of the prototype of obj
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11. Prototype-based Inheritance
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• Prototype chaining emulates inheritance

12. Heuristic to Detect Classes
• A class is a tuple (C,A,M)
–C is the class name
–A= {a1, a2, ..., ap} are attributes
–M= {m1, m2, ..., mq} are methods
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13. Heuristic to Detect Classes
• A program P has a class (C,A,M) if:
–P has a function called C
• For each a in A,
– C includes this.a = Exp;
• For each m in M
– C includes this.m = function {...}
–P has a new C()
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14. JSClassFinder
http://aserg.labsoft.dcc.ufmg.br/jsclasses/
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15. Evaluation

16. Research Questions
1. Do developers use classes?
2. Do developers use inheritance?
3. What is the size of the classes?
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17. Dataset
• 50 GitHub systems:
–at least 1,000 starts
–at least 150 commits
–that are no forks of other projects
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18. Dataset
• Data cleaning:
– compacted files (min.js.*)
– third-party libraries
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19. Dataset
• Largest system (ace): 194 KLOC
• Smallest system (masonry): 0.2 KLOC
• Average size: 14 KLOC
• Median is 2.5 KLOC
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20. RQ #1: Do developers use classes?
• Class Usage Ratio (CUR):
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21. Key Finding: Four Types of Systems
• Class-free systems (CUR = 0)
– 13 systems, 26%
• Class-aware systems (0 < CUR ≤ 0.25)
– 18 systems, 36%
• Class-friendly systems (0.25 < CUR ≤ 0.75)
– 15 systems, 30%
• Class-oriented systems (CUR > 0.75)
– 4 systems, 8%
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22. Do large systems have more classes?
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No correlation KLOC vs CUR

23. RQ #2: Do developers use inheritance?
• Only 9 systems with subclasses (but very few)
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24. RQ #3: What is the size of JS classes?
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Heavy-tailed distribution,
with NOA, NOM < 10 for most classes

25. Few methods
• In Java:
– Classes have more methods than attributes
• In JavaScript:
– Classes with more attributes are common
– Possible reason: lack of getters and setters
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26. In Summary
• For many JavaScript developers:
• Objects are not enough... they need classes
• We found classes in 74% of the systems
• But there is controversy :
• “This is the first time a research paper has ever
called me classless, and boy am I proud.”
• (prototype-based) inheritance is rarely used
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27. Implications

28. ECMAScript 6
• Syntactical support to classes
• Conversion tool: “old JavaScript class styles” to ES 6
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29. Class-Oriented Tools and Techniques
1. Reverse engineering (class diagrams)
2. Bad smells (large class, feature envy etc)
3. IDEs with class browsers (package explorer)
4. Metrics (coupling, cohesion etc)
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30. Thanks!

31. Does JavaScript Software
Embrace Classes?
L. Silva, M. Ramos,
M. T. Valente
UFMG, Brazil
A. Bergel
Univ. Chile
N. Anquetil
INRIA, France
SANER, Montreal, March 2015