Type Checking JavaScript

Type Checking JavaScript Pascal-Louis Perez (pascal@kaching.com) - Closure Compiler

What is Type Checking? • A particular kind of code analysis • Looks at the ﬂow of values • Groups values as types • Abstract interpretation to propagate types • Can be dynamic or static

Dynamic Type Checking Java Object[] objects = new Integer[1]; objects[0] = new Double(4.3); Object o = new Integer(6); Date d = (Date) o; JavaScript new 3 null.getName();

Static Type Checking 3 7 a + b 10

Static Type Checking 3 7 ‘a’ 7 a + b a + b 10 ‘a7’

Static Type Checking 3 7 ‘a’ 7 3 null a + b a + b a + b 10 ‘a7’ 3

Static Type Checking 3 7 ‘a’ 7 3 null ‘3’ null a + b a + b a + b a + b 10 ‘a7’ 3 ‘3null’

Static Type Checking 3 7 ‘a’ 7 3 null ‘3’ null a + b a + b a + b a + b ... 10 ‘a7’ 3 ‘3null’

Static Type Checking 3 7 ‘a’ 7 3 null ‘3’ null number number a + b a + b a + b a + b ... a + b 10 ‘a7’ 3 ‘3null’ number

Motivating Example: Correctness function(opt_i) { /** @type number */ var index = opt_i === null ? 0 : opt_i; ... } • Meaningless or useless operations (such as null == null, undefined.foo) • Wrong number of arguments for methods • ...

Motivating Example: Optimizations • Function Inlining • Dead Code • Constant Folding ... Foo.prototype.get() = function() { return 5 } Bar.prototype.get() = function() { return 2 } ... v.get();

JavaScript Types null boolean number Object string undefined

JavaScript Types null Error boolean number Object string undefined

JavaScript Types null Error SyntaxError boolean number Object string undefined

JavaScript Types null Error SyntaxError boolean number Object string (boolean,string) undefined

Optionality and Nullability • JavaScript has two unit types null and undefined • Thanks to union types, we can make the type system understand optionality and nullability • Optionality: optional type T = (T, undefined) • Nullability: nullable type T = (T, null)

Closure Compiler assumptions • Functions and methods are not changed at runtime • Protoypes are not changed at runtime, i.e. the type hierarchy is ﬁxed • Functions are called statically or not (but not both) • eval is used in a non disruptive way • no use of with

Closure Compiler assumptions • Functions and methods are not changed at runtime • Protoypes are not changed at runtime, i.e. the type hierarchy is ﬁxed • Functions are called statically or not (but not both) • eval is used in a non disruptive way • no use of with clean programming

Variables /** @type number */ var n = 4; /** @type {Array.<string>} */ var a = [‘hello’, ‘Zurich’];

Properties /** ... */ function Foo() { /** @type {string?} this.bar = null; } /** @type {boolean|undefined} */ Foo.prototype.default = true;

Enums /** @enum */ var options = { GOOD: 0, BAD: 1 } /** @enum {string} */ var airports = { SFO: ‘San Francisco’, ... } /** @type {airports} */ var local = airports.SFO;

Functions /** * @param {String} s * @return string */ function cast(s) { return String(s); }

Constructors /** * ... * @constructor */ function MyObject(...) { ... } new MyObject();

This /** * @this Foo */ function handler(...) { ... this.bar ... }

Type Reﬁnement • Unlike other language, the type of a variable depends on the program point /** @type {string?} */ var s = ...; if (s) { ... } else { ... }

Type Reﬁnement • Unlike other language, the type of a variable depends on the program point /** @type {string?} */ var s = ...; if (s) { ... s : string } else { ... }

Type Reﬁnement • Unlike other language, the type of a variable depends on the program point /** @type {string?} */ var s = ...; if (s) { ... s : string } else { ... s : (sting,null) }

Type Reﬁnement • Semantic based • variable == null, variable === undefined, variable < 7, etc. • Pattern based • goog.isDef(variable), goog.isNull(variable), goog.isDefAndNotNull(variable)

Type Inference • Find the type of an expression without requiring any annotation var goog = {}; goog.FOO = 5; /** @type number */ goog.BAR = goog.FOO + 12;

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Type Checking JavaScript

by Pascal-Louis Perez on Nov 14, 2009

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Type Checking JavaScript — Presentation Transcript