1. Safe TypeScript Aseem Rastogi University of Maryland, College Park
End of Internship Talk
Joint work with: Nikhil Swamy (RiSE, Internship mentor),
Cédric Fournet (MSRC),
Gavin Bierman (Oracle),
Panagiotis Vekris (UCSD)

2. TypeScript Gradually typed superset of JavaScript (www. typescriptlang
TypeScript Gradually typed superset of JavaScript (
"Strong tools for large applications"
"Static checking, symbol-based navigation, statement completion, code refactoring"
"TypeScript offers classes, modules, and interfaces to help you build robust components"
"Compiled into simple JavaScript"
Compared to JavaScript, this is a great leap forward!

3. But Typing JavaScript is Hard ! For all its dynamic idioms
TypeScript (like Dart and Closure) gives up soundness, intentionally
Types are uniformly erased when compiling to JavaScript
E.g. casts are unchecked
Unsound type erasure is beneficial
Lightweight codegen (highly readable JavaScript output)
Performance identical to plain JavaScript
Types don’t get in the way of good programmers
But it also has its disadvantages
TypeScript components are not robust

4. (Un-)Robustness of TypeScript Components
Type safety violation
Client:
Client:
function client(Iterator<number> it) {
it["index"] = true; }
function client(it) {
it["index"] = true; }
Provider:
Provider:
interface Iterator<A> { next(): A }
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x); //client:Iterator<number> => void
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x);
TypeScript
JavaScript
TypeScript compiler

5. (Un-)Robustness of TypeScript Components
Abstraction violation
Client:
Client:
function client(Iterator<number> it) {
(<any> it).index = -1; }
function client(it) {
it.index = -1; }
Provider:
Provider:
interface Iterator<A> { next(): A }
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x); //client:Iterator<number> => void
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x);
TypeScript
JavaScript
TypeScript compiler

6. Safe TypeScript
Sound and efficient gradual typing is possible for idiomatic TypeScript
Sound typing is beneficial
Finds type errors early
Found and fixed 478 type errors in TypeScript compiler,
1 functional correctness bug in NavierStokes, a heavily tested Octane benchmark
Provably robust components
But it also has its cost
A runtime performance penalty
Ranging from 6% to 3x on 118,000 lines of code in 8 applications (details follow)
Need to understand subtle corners of JS semantics and our type system

7. TypeScript type inference
TypeScript Workflow
function f(x:any):number {
return x.f; }
function f(x):number {
app.ts
tsc app.ts
TypeScript type inference
TypeScript parsing
JavaScript emitting
function f(x) {
app.js
Syntactic errors
Static diagnostic
basic type errors

8. Safe TypeScript Workflow Fully integrated into TypeScript v0. 9
Safe TypeScript Workflow Fully integrated into TypeScript v0.9.5 as an optional compiler flag
function f(x:any):number {
return x.f; }
function f(x):number {
app.ts
tsc --safe app.ts
TypeScript type inference
TypeScript parsing
Safe TypeScript type checking & instrumentation
return RT.check(RT.Num,
RT.read(x, "f"));
JavaScript emitting
function f(x) {
app.js
inconsistent subtyping
implicit downcast from any
variables not in scope
unsafe use of this
projecting methods as fields
Static diagnostics
Syntactic errors
Static diagnostic
basic type errors

9. Highlights of The Type System
Object-oriented, with a mixture of structural and nominal types
Nominal types provide a sound model of JavaScript's semantics of classes
In contrast: TypeScript is purely structural
Types are compiled to provide precise run-time type information (RTTI)
Allows the runtime system to enforce invariants with dynamic checks
In contrast: RTTI in TypeScript is only what is available in JavaScript
Selective type-erasure for performance and robustness
The type system ensures that erasure does not compromise safety
In contrast: TypeScript uniformly erases all types
By example …

10. Nominal Classes and Structural Interfaces
interface PointI {
x:number;
y:number }
class PointC {
constructor(public x:number,
public y:number) { }
function f(p:PointC) {
assert(p instanceof PointC); }

11. Nominal Classes and Structural Interfaces
interface PointI {
x:number;
y:number }
class PointC {
constructor(public x:number,
public y:number) { }
TypeScript output: leads to runtime error in f
function f(p:PointC) {
assert(p instanceof PointC); }
Safe TypeScript: Static Type Error
{x:number;y:number} is not a subtype of PointC
f({x:0, y:0});

12. Nominal Classes and Structural Interfaces
interface PointI {
x:number;
y:number }
class PointC {
constructor(public x:number,
public y:number) { }
function f(p:PointC) {
assert(p instanceof PointC); }
f(new PointC(0, 0));
Safe TypeScript: OK

13. Nominal Classes and Structural Interfaces
interface PointI {
x:number;
y:number }
class PointC {
constructor(public x:number,
public y:number) { }
function f(p:PointI) {
return p.x + p.y; }
Safe TypeScript: OK
PointC is a subtype of PointI
f(new PointC(0, 0));

14. Highlights of The Type System
Object-oriented, with a mixture of structural and nominal types
Nominal types provide a sound model of JavaScript's semantics of classes
In contrast: TypeScript is purely structural
Types are compiled to provide precise run-time type information (RTTI)
Allows the runtime system to enforce invariants with dynamic checks
In contrast: RTTI in TypeScript is only what is available in JavaScript
Selective type-erasure for performance and robustness
The type system ensures that erasure does not compromise safety
In contrast: TypeScript uniformly erases all types

15. Tag Objects with RTTI to Lock Invariants
function f(p:any) {
p.x = "boom"; }
TypeScript output: leads to runtime error in g
function g(p:PointI) {
f(p);
assert(typeof p.x === "number"); }

16. Tag Objects with RTTI to Lock Invariants
shallowTag for structural objects
function f(p:any) {
p.x = "boom"; }
function f(p) { … } //coming up !
function g(p:PointI) {
f(p);
assert(typeof p.x === "number"); }
function g(p) {
f(shallowTag(p, PointI)); … }
Safe TypeScript: Adds RTTI to objects to lock their type
shallowTag(x,t) = x.tag := combine(x.tag,t); x

17. Instrumentation of any Code
function f(p:any) {
p.x = "boom"; }
function f(p) {
write(p, "x", "boom"); }
// fails
function g(p:PointI) {
f(p);
assert(typeof p.x === "number"); }
function g(p) {
f(shallowTag(p, PointI)); … }
Safe TypeScript: Enforces type invariants in any code
write(o,f,v) =
let t = o.rtti;
o[f] = check(v, t[f]);

18. Tag Objects with RTTI to Lock Invariants
No tagging for class instances
function f(p:any) {
p.x = "boom"; }
function g(p:PointC) {
f(p);
assert(typeof p.x === "number"); }
function g(p) {
f(p); // no tagging … }
No tagging for class instances
Class instances have primitive RTTI (prototype chain)

19. Runtime Checked Downcasts
function f(p:PointI) {
assert(typeof p.x === "number"); }
TypeScript output: leads to runtime error in f
function g(p:any) {
f(<PointI> p); }
g({x:"boom",y:0});

20. Runtime Checked Downcasts
Check fields invariants for structural types
function f(p:PointI) {
assert(typeof p.x === "number"); }
function f(p) { … }
function g(p) {
f(check(p, PointI)); } …
function g(p:any) {
f(<PointI> p); }
g({x:"boom",y:0});
Safe TypeScript: Checks downcasts at runtime
check(o, PointI) =
if typeof o.x === “number”
&& typeof o.y === “number”
then o.rtti := PointI; o
else die

21. Runtime Checked Downcasts
Simple instanceof check for class instances
function f(p:PointC) { … }
function f(p) { … }
function g(p:any) {
f(<PointC> p); }
g({x:"boom",y:0});
function g(p) {
f(check(p, PointC)); } …
check(o, PointC) =
if o instanceof PointC
then o
else die
Fast instanceof check for class instances

22. Highlights of The Type System
Object-oriented, with a mixture of structural and nominal types
Nominal types provide a sound model of JavaScript's semantics of classes
In contrast: TypeScript is purely structural
Types are compiled to provide precise run-time type information (RTTI)
Allows the runtime system to enforce invariants with dynamic checks
In contrast: RTTI in TypeScript is only what is available in JavaScript
Selective type-erasure for performance and robustness
The type system ensures that erasure does not compromise safety
In contrast: TypeScript uniformly erases all types

23. Safe TypeScript adds RTTI Tags On-demand
interface 3dPointI extends PointI {
z:number; }
function f(r:any) { ... }
function g(q:PointI) {
f(q);
function h(p:3dPointI) {
g(p);
function main(p:3dPointI) {
h(p);
function f(r) { … }
function g(q) {
f(shallowTag(q, PointI)); }
function h(p) {
g(shallowTag(p, {z:number}); // diff tagging }
function main(p) { h(p); } // no tagging
Safe TypeScript adds minimum RTTI to ensure safety
shallowTag(x, t) = x.rtti := combine(x.rtti, t); x

24. Programmer-controlled Type Erasure
A new operator on types: "Erased t" A value of type Erased t is known to be a t statically, and at runtime it may not have RTTI Erased types are erased from the JavaScript output

25. Programmer Controlled Type Erasure
interface PointI extends Erased {
x:number;
y:number }
function f(r) { ... }
function g(q) {
f(q); }
function h(p) {
g(p);
 static type error
Cannot pass erased types to any context
interface 3dPointI extends PointI {
z:number; }
function f(r:any) { ... }
function g(q:PointI) {
f(q);
function h(p:3dPointI) {
g(p);
 compiles as is
No tagging despite loss in precision
Recall that previously it was:
g(shallowTag(p, {z:number}))
Safe TypeScript: Erased types must only be used statically

26. Revisiting Robust Components
Client:
Robustness provided by Safe TypeScript
type soundness theorem
Several useful corollaries:
-- RTTI tags are always consistent
-- RTTI tags evolve in subtyping hierarchy
function client(Iterator<number> it) {
it["index"] = true; }
//runtime error
Provider:
interface Iterator<A> { next(): A }
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x); //client:Iterator<number> => void
Full formalization and proofs in technical report:

27. Revisiting Robust Components Provider’s perspective
Client:
function client(Iterator<number> it) {
(<any> it).index = -1; }
Provider:
interface Iterator<A> { next(): A }
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x); //client:Iterator<number> => void

28. Revisiting Robust Components Provider’s perspective
Client:
function client(Iterator<number> it) {
(<any> it).index = -1; }
Stronger abstraction using erased types
Safe TypeScript provides an abstraction theorem for erased types
//static error
Provider:
interface Iterator<A> extends Erased { next(): A }
var x = { state:[..], index:0,
next() { return state[index++]; } };
client(x); //client:Iterator<number> => void
Full formalization and proofs in technical report:

29. Much more … Arrays (with mutability controls) Dictionaries Inheritance
Overloading
Generics with bounded polymorphism
Optional fields/arguments/variadic functions
Auto-boxing
Primitive prototype hierarchy
Closed vs. open records
Nominal interfaces
Enums …
All these features allow us to handle practical TypeScript developments …

30. Experience with SafeTypeScript Bootstrapping Safe TypeScript compiler (implemented in TypeScript v0.9.5)
90,000 lines of code (80,000 lines of TypeScript compiler)
Heavily class based, most of the code is carefully type annotated
Static errors
478 in total
98 uses of bi-variant array subtyping
130 uses of covariant method argument subtyping
128 cases of variable scoping issues
52 cases of projecting a method (leading to potential unsound use of this parameter) …
Runtime errors
26 failed downcasts
5 in our own code !
15% runtime overhead of type safety

31. Experience with SafeTypeScript Compiling TypeScript v1.1
18,000 lines of code
Heavily interface based
Static errors
81 in total
Mainly variable scoping and array subtyping
3x runtime overhead of type safety
High overhead because of more structural types
Have not optimized Safe TypeScript runtime for structural types

32. Experience with SafeTypeScript Octane benchmarks
10,000 lines of code
Static errors
Found 1 variable scoping bug in heavily tested NavierStokes
High runtime overhead when no annotations
2.4x (Splay) to 72x (Crypto), Average: 22x
Performance recovers once we add type annotations
Average overhead: 6.5%

33. Demo ( Examples on Online Playground: )

34. Limitations and Work in Progress
eval and friends
Adversarial typing of unsafe constructs – Swamy et. al. POPL'14
Implementation limitations:
Does not support external modules
Current implementation is in TypeScript v0.9.5 that has evolved to v1.1
Ongoing discussion about integrating Safe TypeScript in TypeScript v1.1

35. Safe TypeScript Download:
Sound and efficient gradual type system for TypeScript
Download:
5a5a0c38cc57/
Submitted POPL'15 paper:
Technical report (with full formalization and proofs):
Online playground:

36. Structural types distinguish fields from methods
Handling this soundly
class Line {
constructor(public p1:Point,
public p2:Point){}
public moveUp() {
this.p1.y++; this.p2.y++; }
function g(l:{moveUp:() => void}) {
var f = l.moveUp;
f();
function h(p:Point) {
g(new Line(p, p));
Compiles without warnings in TypeScript
Classes are convertible with their structure
Line <: {moveUp() :void; //method
p1:Point; //field
p2:Point} //field
window.p1 is undefined, so p1.y crashes
SafeTypeScript
Line does not contain a field called moveUp
Only a method called moveUp

37. Structural types distinguish fields from methods
Handling this soundly
class Line {
constructor(public p1:Point,
public p2:Point){}
public moveUp() {
this.p1.y++; this.p2.y++; }
function g(l:{moveUp(): void}) {
var f = l.moveUp;
f();
function h(p:Point) {
g(new Line(p, p));
Compiles without warnings in TypeScript
Classes are convertible with their structure
Line <: {moveUp() :void; //method
p1:Point; //field
p2:Point} //field
SafeTypeScript
Cannot project a method

38. Structural types distinguish fields from methods
Handling this soundly
class Line {
constructor(public p1:Point,
public p2:Point){}
public moveUp() {
this.p1.y++; this.p2.y++; }
function g(l:{moveUp(): void}) {
l.moveUp();
function h(p:Point) {
g(new Line(p, p));
g({moveUp() { this.p1.y++; },
p1:p,
p2:p});
Compiles without warnings in TypeScript
Classes are convertible with their structure
Line <: {moveUp() :void; //method
p1:Point; //field
p2:Point} //field
SafeTypeScript: Ok!
g({moveUp : () => {this.p1.y++;},
p1:p,
p2:p})
SafeTypeScript: A function is not a method
SafeTypeScript: Object literal with a method

39. Field Addition and Deletion Dynamically typed unless it becomes static
function f(p:PointI) {
p["z"] = 0;
delete p.z; }
function f(p: PointI) {
write(p, "z", 0);
delete(p, "z"); }
SafeTypeScript: Both write and delete succeed at runtime

40. Field Addition and Deletion Dynamically typed unless it becomes static
function g(p:3dPointI) { … }
function f(p:PointI) {
p["z"] = 0;
g(<3dPointI> p);
delete p.z; }
function g(p:3dPointI) { … }
function f(p:PointI) {
write(p, "z", 0);
g(check(p, 3dPointI));
delete(p, "z"); }
SafeTypeScript: write succeeds at runtime
SafeTypeScript: check succeeds at runtime
SafeTypeScript: delete fails at runtime – violates invariant of g