1 JavaScript 2.0: Evolving a Language for Evolving Systems Waldemar Horwat Netscape

2 Outline Background Motivation Construct the JavaScript 2.0 language Process Conclusion

3 JavaScript History Web page scripting language invented in 1996 by Brendan Eich at Netscape Used in over 25% of web pages More than an order of magnitude more widely used than all other web client languages combined

4 Terminology LiveScript JavaScript Jscript ECMAScript All the same language No relation to Java

5 JavaScript Evolution JavaScript 1.0 in Navigator 2.0 JavaScript 1.5 is most recent JavaScript 2.0 in 2003

6 JavaScript and ECMAScript JavaScript standardized by the ECMA TC39TG1 committee –ECMA 262 –ISO JavaScript 1.5 = ECMAScript Edition 3 JavaScript 2.0 = ECMAScript Edition 4 TC39TG1 leading the design of Edition 4

7 Outline Background Motivation Construct the JavaScript 2.0 language Process Conclusion

8 JavaScript 2.0 Motivation Evolving programs –Compatibility –Modularity –Safety –Interoperability with other languages Simplify common patterns –Class declarations –Type declarations –Attributes and conditional compilation Compatible with JavaScript 1.5

9 Programming in the Large Programs written by more than one person Programs assembled from components (packages) Programs in heterogeneous environments Programs that use evolving interfaces Long-lived, evolving programs

10 Non-Goals High-performance applications General language Language for writing huge programs Stripped-down version of existing language

11 Outline Background Motivation Construct the JavaScript 2.0 language Process Conclusion

12 JavaScript 2.0 JavaScript Packages

13 Scenario 1 Library package Searcher { public function find(x) { while (x >= 0) if (a[x]) return a[x]; else x -= 2; return null; }

14 Scenario 1 LibraryWeb page package Searcher { public function find(x) { while (x >= 0) if (a[x]) return a[x]; else x -= 2; return null; } import Searcher; var c = find(34); var d = find("42");

15 Scenario 1 LibraryWeb page package Searcher { public function find(x) { x += 2; while ((x -= 2) >= 0) if (a[x]) return a[x]; return null; } import Searcher; var c = find(34); var d = find("42");

16 Scenario 1 LibraryWeb page package Searcher { public function find(x) { x += 2; while ((x -= 2) >= 0) if (a[x]) return a[x]; return null; } import Searcher; var c = find(34); var d = find("42"); // same as find(420)

17 Solution: Type Annotations LibraryWeb page package Searcher { public function find(x: Integer) { x += 2; while ((x -= 2) >= 0) if (a[x]) return a[x]; return null; } import Searcher; var c = find(34); var d = find("42"); // now same as find(42)

18 Type Annotations Optional Strongly typed — type annotations are enforced Not statically typed — only dynamic type of an expression matters class A { function f() } class B extends A { function g() } var a:A = new B; a.f(); // OK a.g(); // OK var x = new A; // OK var b:B = x; // Error

19 JavaScript 2.0 JavaScript Packages Strong dynamic typing

20 Scenario 2 LibraryWeb page package ColorLib { var red = 1; var green = 2; var blue = 4; function set(color) … function drawPoint(x,y) … } import ColorLib; set(red); drawPoint(3,8);

21 Scenario 2 LibraryWeb page package ColorLib { var red = 1; var green = 2; var blue = 4; function set(color) … function drawPoint(x,y) … } import ColorLib; set(red); red = blue; drawPoint(3,8);

22 Solution: const LibraryWeb page package ColorLib { const red = 1; const green = 2; const blue = 4; function set(color) … function drawPoint(x,y) … } import ColorLib; set(red); red = blue; drawPoint(3,8);

23 JavaScript 2.0 JavaScript Packages Strong dynamic typing const

24 JavaScript 1.5 Hoists Declarations Scenario 3Equivalent Code function f(x) { if (g(x)) { var a = x+3; b = 18; } else { var b = x; var a = "Hello"; } return a+b; } function f(x) { var a; var b; if (g(x)) { a = x+3; b = 18; } else { b = x; a = "Hello"; } return a+b; }

25 Hoisting Problems Scenario 3Equivalent Code function f(x) { if (…) { const a = …; var b:Integer = …; } else { var b:String = …; const a = …; } return a+b; } function f(x) { const a = ?; var b:?; if (…) { a = …; b = …; } else { b = …; a = …; } return a+b; }

26 Solution: Local Scoping const declarations are locally scoped Typed declarations are locally scoped Class member declarations are locally scoped Declarations with attributes are locally scoped That leaves plain var declarations –Locally scoping would be an incompatible change –Not locally scoped except in strict mode

27 Solution: Local Scoping Scenario 3 function f(x) { if (…) { const a = …; var b:Integer = …; } else { var b:String = …; const a = …; } return a+b; }

28 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode

29 Scenario 4 LibraryWeb page package ColorLib { function drawPoint(x,y) … } import ColorLib; drawPoint(3,8);

30 Scenario 4 LibraryWeb page package ColorLib { function drawPoint(x,y) … } import ColorLib; drawPoint(3,8,true);

31 Scenario 4 Library 1.1Web page package ColorLib { function drawPoint( x, y, color = black) … } import ColorLib; drawPoint(3,8,true);

32 Solution: Argument Checking Calls to functions with typed arguments are checked Calls to functions with typed result are checked Arguments may be optional Use... for variable argument lists That leaves plain, untyped function declarations –Argument checking would be an incompatible change –Not checked unless defined in strict mode

33 Scenario 4 LibraryWeb page package ColorLib { function drawPoint( x:Integer, y:Integer) … } import ColorLib; drawPoint(3,8,true);

34 Named Function Parameters function f(a: Integer, b = 5, named c = 17, named d = 3) f(4) f(4, d:6) Why is the named attribute needed?

35 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode Named and optional arguments Argument checking

36 Why Classes? Ubiquitous idiom, even in prototype-based languages Few users get it right in JS 1.5 with prototypes Difficult to evolve a program that uses prototypes –Little knowledge of invariants Classes model cross-language interaction Basis for access control Potential efficiency gains One of most-requested features

37 Class Definition Syntax class Dance { const kind; var name: String; static var count; constructor function start(partner, length: Integer) {…} }; class Swing extends Dance { var speed: Integer; function acrobatics(): Boolean {…} };

38 Classes and Prototypes Classes and prototypes will co-exist Language avoids early-binding the decision of whether to use classes or prototypes –Access syntax identical: a.b, a["b"] –The dynamic keyword on a class enables its instances to dynamically acquire new properties Classes can’t inherit from prototypes or vice versa

39 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode Named and optional arguments Argument checking Classes

40 Scenario 5 Name conflicts

Library package BitTracker { public class Data { public var author; public var contents; function save() {…} }; function store(d) {... storeOnFastDisk(d); } 41

Library Web page package BitTracker { public class Data { public var author; public var contents; function save() {…} }; function store(d) {... storeOnFastDisk(d); } import BitTracker; class Picture extends Data { function size() {…} var palette; }; function orientation(d) { if (d.size().h >= d.size().v) return "Landscape"; else return "Portrait"; } 42

Library rev2 Web page package BitTracker { public class Data { public var author; public var contents; public function size() {…} function save() {…} }; function store(d) {... if (d.size() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker; class Picture extends Data { function size() {…} var palette; }; function orientation(d) { if (d.size().h >= d.size().v) return "Landscape"; else return "Portrait"; } 43

Library rev2 Web page package BitTracker { public class Data { public var author; public var contents; public function size() {…} function save() {…} }; function store(d) {... if (d.size() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker; class Picture extends Data { function size() {…} var palette; }; function orientation(d) { if (d.size().h >= d.size().v) return "Landscape"; else return "Portrait"; } 44

Library rev2 Web page package BitTracker { public class Data { public var author; public var contents; public function ?() {…} function save() {…} }; function store(d) {... if (d.?() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker; class … extends Data { ? }; 45

46 Non-Solutions Assume it won’t happen –It does, especially in DOM Have programmer detect/fix conflicts (C++) –Old web pages linked with new libraries Have compiler bind identifier to definition (Java) –Programs distributed in source form Explicit overrides + static type system (C#) –Doesn’t work in dynamic languages –Burden on library user instead of library developer

47 Solution: Namespaces Each name is actually an ordered pair namespace :: identifier A use namespace( n ) statement allows unqualified access to n’s identifiers within a scope Default namespace is public Namespaces are values

Library Web page package BitTracker { public class Data { public var author; public var contents; function save() {…} }; function store(d) {... storeOnFastDisk(d); } import BitTracker; class Picture extends Data { function size() {…} var palette; }; function orientation(d) { if (d.size().h >= d.size().v) return "Landscape"; else return "Portrait"; } 48

Library rev2 Web page package BitTracker { namespace v2; use namespace(v2); public class Data { public var author; public var contents; v2 function size() {…} function save() {…} }; function store(d) {... if (d.size() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker; class Picture extends Data { function size() {…} var palette; }; function orientation(d) { if (d.size().h >= d.size().v) return "Landscape"; else return "Portrait"; } 49

Library rev2 Web page rev2 package BitTracker { namespace v2; use namespace(v2); public class Data { public var author; public var contents; v2 function size() {…} function save() {…} }; function store(d) {... if (d.size() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker; use namespace(v2); class Picture extends Data { function dims() {…} var palette; }; function orientation(d) { if (d.dims().h >= d.dims().v) return "Landscape"; else return "Portrait"; } … d.size() … 50

Library rev2 Web page rev2 package BitTracker { explicit namespace v2; use namespace(v2); public class Data { public var author; public var contents; v2 function size() {…} function save() {…} }; function store(d) {... if (d.size() > limit) storeOnSlowDisk(d); else storeOnFastDisk(d); } import BitTracker, namespace(v2); class Picture extends Data { function dims() {…} var palette; function size() {…} }; function orientation(d) { if (d.dims().h >= d.dims().v) return "Landscape"; else return "Portrait"; } 51

52 Versioning Namespaces distinguish between accidental and intentional identifier matches Library publisher annotates new functionality with new namespaces Web page imports a specific namespace Versioning affects name visibility only — there is only one library implementation

53 Other Uses of Namespaces private, internal, etc. are syntactic sugars for anonymous namespaces Export private functionality to privileged clients Can use namespaces to add methods to an already existing class (future feature)

54 Property Lookup

Classes Lookup class A { function f() // fA } class B extends A { function f() // fB } class C extends B { function f() // fC } use namespace(public); c = new C; c.f(); // fC 55

Classes Lookup 1 class A { N function f() // fA } class B extends A { N function f() // fB } class C extends B { function f() // fC } use namespace(public); c = new C; c.f(); // fC use namespace(public); use namespace(N); c = new C; c.f(); // fB, not fC! 56 Lookup 2

57 Property Lookup c.f Find highest (least derived) class that defines a member f that’s visible in the currently used namespaces to pick a namespace n Find lowest (most derived) definition of c. n ::f –Gives object-oriented semantics

58 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode Named and optional arguments Argument checking Classes Namespaces, versioning, and protection

59 Attributes public var a; v2 v3 v4 var x; v2 v3 v4 function y(…) …; v2 v3 v4 function z(…) …; public const d; v2 v3 v4 const c;

60 Attributes public var a; v2 v3 v4 var x; v2 v3 v4 function y(…) …; v2 v3 v4 function z(…) …; public const d; v2 v3 v4 const c; public var a; v2 v3 v4 { var x; function y(…) …; function z(…) …; const c; } public const d; Distributed Attributes

61 Attributes public var a; v2 v3 v4 var x; v2 v3 v4 function y(…) …; v2 v3 v4 function z(…) …; public const d; v2 v3 v4 const c; const v2_4 = v2 v3 v4; public var a; v2_4 var x; v2_4 function y(…) …; v2_4 function z(…) …; public const d; v2_4 const c; User-Defined Attributes

62 Attributes static debug v3 final foo(3) bar var x:Integer = 5; Restricted expressions that evaluate to: Built-in attributes such as final and static Namespaces true or false (conditional compilation) User-defined attributes

63 Attributes const debug = true; static debug v3 final foo(3) bar var x:Integer = 5; const A = static debug v3; A bar var y:Integer; private { var z; function f(); }

64 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode Named and optional arguments Argument checking Classes Namespaces, versioning, and protection Flexible attributes

65 Additional Types sbyte, byte short, ushort int, uint Subsets of Number (IEEE double) All arithmetic done as though on Number : int(1e9) * int(1e9)  1e18

66 Additional Types long, ulong Not subsets of Number Values distinct from mathematically equal Number values, but coercable Exactly represent all integers between and 2 64 –1

67 Additional Types float Subset of Number

68 Additional Types Sparse arrays as in JavaScript 1.5 Dense arrays Typed arrays

69 JavaScript 2.0 JavaScript Packages Strong dynamic typing const Local scoping Strict mode Named and optional arguments Argument checking Classes Namespaces, versioning, and protection Flexible attributes Types for interacting with other languages

70 Omitted Features Overloading Interfaces Units Operator overriding Wraparound integer arithmetic Lots of built-in libraries

71 Strict Mode Revisited An implementation will run three kinds of programs: JavaScript 1.5 –Run unchanged Non-strict JavaScript 2.0 –Almost all JavaScript 1.5 programs run unchanged Strict JavaScript 2.0 –Turns off troublesome quirks (scope hoisting, newline dependencies, etc.)

72 Strict Mode Variables must be declared Function argument checking Function declarations are immutable Semicolon insertion changes

73 Outline Background Motivation Construct the JavaScript 2.0 language Process Conclusion

74 Process JavaScript 1.0 developed at Netscape –Later adopted by Microsoft –Later standardized by ECMA JavaScript 1.5 developed and standardized concurrently JavaScript 2.0 design discussions occur mainly within ECMA

75 Tools ECMAScript standards use semantic descriptions of the language Use extended typed lambda calculus with Algol- like syntax for ECMAScript Edition 4 Wrote Common Lisp engine to: –Run and test grammar and semantics directly –Auto-generate web page descriptions –Auto-generate chapters of draft ECMAScript standard Freely available

76 ListExpression   AssignmentExpression  | ListExpression , AssignmentExpression  proc Validate [ ListExpression  ] (cxt: Context, env: Environment) [ ListExpression   AssignmentExpression  ] do Validate [ AssignmentExpression  ](cxt, env); [ ListExpression  0  ListExpression  1, AssignmentExpression  ] do Validate [ ListExpression  1 ](cxt, env); Validate [ AssignmentExpression  ](cxt, env) end proc; proc Eval [ ListExpression  ] (env: Environment, phase: Phase): ObjOrRef [ ListExpression   AssignmentExpression  ] do return Eval [ AssignmentExpression  ](env, phase); [ ListExpression  0  ListExpression  1, AssignmentExpression  ] do ra: ObjOrRef  Eval [ ListExpression  1 ](env, phase); readReference(ra, phase); rb: ObjOrRef  Eval [ AssignmentExpression  ](env, phase); return readReference(rb, phase) end proc;

77 Outline Background Motivation Construct the JavaScript 2.0 language Process Conclusion

78 Availability Open source (NPL, GPL, or LGPL) JavaScript 1.5 in C or Java JavaScript 2.0 in C++ Compact, stand-alone sources Embed or run as a command line tool

79 More Information Waldemar Horwat