Speed improvements
TypeScript 3.5 introduces several optimizations around type-checking and incremental builds.
Type-checking speed-ups
TypeScript 3.5 contains certain optimizations over TypeScript 3.4 for type-checking more efficiently. These improvements are significantly more pronounced in editor scenarios where type-checking drives operations like code completion lists.
--incremental
improvements
TypeScript 3.5 improves on 3.4’s incremental
build mode, by saving information about how the state of the world was calculated - compiler settings, why files were looked up, where files were found, etc.
In scenarios involving hundreds of projects using TypeScript’s project references in --build
mode, we’ve found that the amount of time rebuilding can be reduced by as much as 68% compared to TypeScript 3.4!
For more details, you can see the pull requests to
The Omit
helper type
TypeScript 3.5 introduces the new Omit
helper type, which creates a new type with some properties dropped from the original.
ts
type Person = {name: string;age: number;location: string;};type QuantumPerson = Omit<Person, "location">;// equivalent totype QuantumPerson = {name: string;age: number;};
Here we were able to copy over all the properties of Person
except for location
using the Omit
helper.
For more details, see the pull request on GitHub to add Omit
, as well as the change to use Omit
for object rest.
Improved excess property checks in union types
In TypeScript 3.4 and earlier, certain excess properties were allowed in situations where they really shouldn’t have been.
For instance, TypeScript 3.4 permitted the incorrect name
property in the object literal even though its types don’t match between Point
and Label
.
ts
type Point = {x: number;y: number;};type Label = {name: string;};const thing: Point | Label = {x: 0,y: 0,name: true // uh-oh!};
Previously, a non-disciminated union wouldn’t have any excess property checking done on its members, and as a result, the incorrectly typed name
property slipped by.
In TypeScript 3.5, the type-checker at least verifies that all the provided properties belong to some union member and have the appropriate type, meaning that the sample above correctly issues an error.
Note that partial overlap is still permitted as long as the property types are valid.
ts
const pl: Point | Label = {x: 0,y: 0,name: "origin" // okay};
The --allowUmdGlobalAccess
flag
In TypeScript 3.5, you can now reference UMD global declarations like
export as namespace foo;
from anywhere - even modules - using the new allowUmdGlobalAccess
flag.
This mode adds flexibility for mixing and matching the way 3rd party libraries, where globals that libraries declare can always be consumed, even from within modules.
For more details, see the pull request on GitHub.
Smarter union type checking
In TypeScript 3.4 and prior, the following example would fail:
ts
type S = { done: boolean; value: number };type T = { done: false; value: number } | { done: true; value: number };declare let source: S;declare let target: T;target = source;
That’s because S
isn’t assignable to { done: false, value: number }
nor { done: true, value: number }
.
Why?
Because the done
property in S
isn’t specific enough - it’s boolean
whereas each constituent of T
has a done
property that’s specifically true
or false
.
That’s what we meant by each constituent type being checked in isolation: TypeScript doesn’t just union each property together and see if S
is assignable to that.
If it did, some bad code could get through like the following:
ts
interface Foo {kind: "foo";value: string;}interface Bar {kind: "bar";value: number;}function doSomething(x: Foo | Bar) {if (x.kind === "foo") {x.value.toLowerCase();}}// uh-oh - luckily TypeScript errors here!doSomething({kind: "foo",value: 123});
However, this was a bit overly strict for the original example.
If you figure out the precise type of any possible value of S
, you can actually see that it matches the types in T
exactly.
In TypeScript 3.5, when assigning to types with discriminant properties like in T
, the language actually will go further and decompose types like S
into a union of every possible inhabitant type.
In this case, since boolean
is a union of true
and false
, S
will be viewed as a union of { done: false, value: number }
and { done: true, value: number }
.
For more details, you can see the original pull request on GitHub.
Higher order type inference from generic constructors
In TypeScript 3.4, we improved inference for when generic functions that return functions like so:
ts
function compose<T, U, V>(f: (x: T) => U, g: (y: U) => V): (x: T) => V {return x => g(f(x));}
took other generic functions as arguments, like so:
ts
function arrayify<T>(x: T): T[] {return [x];}type Box<U> = { value: U };function boxify<U>(y: U): Box<U> {return { value: y };}let newFn = compose(arrayify, boxify);
Instead of a relatively useless type like (x: {}) => Box<{}[]>
, which older versions of the language would infer, TypeScript 3.4’s inference allows newFn
to be generic.
Its new type is <T>(x: T) => Box<T[]>
.
TypeScript 3.5 generalizes this behavior to work on constructor functions as well.
ts
class Box<T> {kind: "box";value: T;constructor(value: T) {this.value = value;}}class Bag<U> {kind: "bag";value: U;constructor(value: U) {this.value = value;}}function composeCtor<T, U, V>(F: new (x: T) => U,G: new (y: U) => V): (x: T) => V {return x => new G(new F(x));}let f = composeCtor(Box, Bag); // has type '<T>(x: T) => Bag<Box<T>>'let a = f(1024); // has type 'Bag<Box<number>>'
In addition to compositional patterns like the above, this new inference on generic constructors means that functions that operate on class components in certain UI libraries like React can more correctly operate on generic class components.
ts
type ComponentClass<P> = new (props: P) => Component<P>;declare class Component<P> {props: P;constructor(props: P);}declare function myHoc<P>(C: ComponentClass<P>): ComponentClass<P>;type NestedProps<T> = { foo: number; stuff: T };declare class GenericComponent<T> extends Component<NestedProps<T>> {}// type is 'new <T>(props: NestedProps<T>) => Component<NestedProps<T>>'const GenericComponent2 = myHoc(GenericComponent);
To learn more, check out the original pull request on GitHub.