Does JavaScript Software Embrace Classes? (SANER 2015)

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.

Transcript

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/ 2
3. 1995: Small Scripts 3
4. 2015: Complex Systems 4
5. Criticisms https://www.destroyallsoftware.com/talks/wat 5
6. Motivation How JavaScript software is organized? 6
7. Motivation How JavaScript software is organized? More specifically, Do JS developers use class-like structures? 7
8. JavaScript • Imperative • Objects maps strings to values – var person = { firstName: "Marco", lastName:"Valente" } • Functions are first-class objects – var fact = function factorial (n) { .... } 8
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 10
11. Prototype-based Inheritance 11 • 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 12
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() 13
14. JSClassFinder http://aserg.labsoft.dcc.ufmg.br/jsclasses/ 14
15. Evaluation
16. Research Questions 1. Do developers use classes? 2. Do developers use inheritance? 3. What is the size of the classes? 16
17. Dataset • 50 GitHub systems: –at least 1,000 starts –at least 150 commits –that are no forks of other projects 17
18. Dataset • Data cleaning: – compacted files (min.js.*) – third-party libraries 18
19. Dataset • Largest system (ace): 194 KLOC • Smallest system (masonry): 0.2 KLOC • Average size: 14 KLOC • Median is 2.5 KLOC 19
20. RQ #1: Do developers use classes? • Class Usage Ratio (CUR): 20
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% 21
22. Do large systems have more classes? 22 No correlation KLOC vs CUR
23. RQ #2: Do developers use inheritance? • Only 9 systems with subclasses (but very few) 23
24. RQ #3: What is the size of JS classes? 24 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 25
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 26
27. Implications
28. ECMAScript 6 • Syntactical support to classes • Conversion tool: “old JavaScript class styles” to ES 6 28
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) 29
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