Conditional Types

TypeScript 2.8 introduces conditional types which add the ability to express non-uniform type mappings. A conditional type selects one of two possible types based on a condition expressed as a type relationship test:

T extends U ? X : Y

The type above means when T is assignable to U the type is X, otherwise the type is Y.

A conditional type T extends U ? X : Y is either resolved to X or Y, or deferred because the condition depends on one or more type variables. Whether to resolve or defer is determined as follows:

  • First, given types T' and U' that are instantiations of T and U where all occurrences of type parameters are replaced with any, if T' is not assignable to U', the conditional type is resolved to Y. Intuitively, if the most permissive instantiation of T is not assignable to the most permissive instantiation of U, we know that no instantiation will be and we can just resolve to Y.
  • Next, for each type variable introduced by an infer (more later) declaration within U collect a set of candidate types by inferring from T to U (using the same inference algorithm as type inference for generic functions). For a given infer type variable V, if any candidates were inferred from co-variant positions, the type inferred for V is a union of those candidates. Otherwise, if any candidates were inferred from contra-variant positions, the type inferred for V is an intersection of those candidates. Otherwise, the type inferred for V is never.
  • Then, given a type T'' that is an instantiation of T where all infer type variables are replaced with the types inferred in the previous step, if T'' is definitely assignable to U, the conditional type is resolved to X. The definitely assignable relation is the same as the regular assignable relation, except that type variable constraints are not considered. Intuitively, when a type is definitely assignable to another type, we know that it will be assignable for all instantiations of those types.
  • Otherwise, the condition depends on one or more type variables and the conditional type is deferred.
type TypeName<T> =
    T extends string ? "string" :
    T extends number ? "number" :
    T extends boolean ? "boolean" :
    T extends undefined ? "undefined" :
    T extends Function ? "function" :

type T0 = TypeName<string>;  // "string"
type T1 = TypeName<"a">;  // "string"
type T2 = TypeName<true>;  // "boolean"
type T3 = TypeName<() => void>;  // "function"
type T4 = TypeName<string[]>;  // "object"

Distributive conditional types

Conditional types in which the checked type is a naked type parameter are called distributive conditional types. Distributive conditional types are automatically distributed over union types during instantiation. For example, an instantiation of T extends U ? X : Y with the type argument A | B | C for T is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y).

type T10 = TypeName<string | (() => void)>;  // "string" | "function"
type T12 = TypeName<string | string[] | undefined>;  // "string" | "object" | "undefined"
type T11 = TypeName<string[] | number[]>;  // "object"

In instantiations of a distributive conditional type T extends U ? X : Y, references to T within the conditional type are resolved to individual constituents of the union type (i.e. T refers to the individual constituents after the conditional type is distributed over the union type). Furthermore, references to T within X have an additional type parameter constraint U (i.e. T is considered assignable to U within X).

type BoxedValue<T> = { value: T };
type BoxedArray<T> = { array: T[] };
type Boxed<T> = T extends any[] ? BoxedArray<T[number]> : BoxedValue<T>;

type T20 = Boxed<string>;  // BoxedValue<string>;
type T21 = Boxed<number[]>;  // BoxedArray<number>;
type T22 = Boxed<string | number[]>;  // BoxedValue<string> | BoxedArray<number>;

Notice that T has the additional constraint any[] within the true branch of Boxed<T> and it is therefore possible to refer to the element type of the array as T[number]. Also, notice how the conditional type is distributed over the union type in the last example.

The distributive property of conditional types can conveniently be used to filter union types:

type Diff<T, U> = T extends U ? never : T;  // Remove types from T that are assignable to U
type Filter<T, U> = T extends U ? T : never;  // Remove types from T that are not assignable to U

type T30 = Diff<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "b" | "d"
type T31 = Filter<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "a" | "c"
type T32 = Diff<string | number | (() => void), Function>;  // string | number
type T33 = Filter<string | number | (() => void), Function>;  // () => void

type NonNullable<T> = Diff<T, null | undefined>;  // Remove null and undefined from T

type T34 = NonNullable<string | number | undefined>;  // string | number
type T35 = NonNullable<string | string[] | null | undefined>;  // string | string[]

function f1<T>(x: T, y: NonNullable<T>) {
    x = y;  // Ok
    y = x;  // Error

function f2<T extends string | undefined>(x: T, y: NonNullable<T>) {
    x = y;  // Ok
    y = x;  // Error
    let s1: string = x;  // Error
    let s2: string = y;  // Ok

Conditional types are particularly useful when combined with mapped types:

type FunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? K : never }[keyof T];
type FunctionProperties<T> = Pick<T, FunctionPropertyNames<T>>;

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];
type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;

interface Part {
    id: number;
    name: string;
    subparts: Part[];
    updatePart(newName: string): void;

type T40 = FunctionPropertyNames<Part>;  // "updatePart"
type T41 = NonFunctionPropertyNames<Part>;  // "id" | "name" | "subparts"
type T42 = FunctionProperties<Part>;  // { updatePart(newName: string): void }
type T43 = NonFunctionProperties<Part>;  // { id: number, name: string, subparts: Part[] }

Similar to union and intersection types, conditional types are not permitted to reference themselves recursively. For example the following is an error.

type ElementType<T> = T extends any[] ? ElementType<T[number]> : T;  // Error

Type inference in conditional types

Within the extends clause of a conditional type, it is now possible to have infer declarations that introduce a type variable to be inferred. Such inferred type variables may be referenced in the true branch of the conditional type. It is possible to have multiple infer locations for the same type variable.

For example, the following extracts the return type of a function type:

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : any;

Conditional types can be nested to form a sequence of pattern matches that are evaluated in order:

type Unpacked<T> =
    T extends (infer U)[] ? U :
    T extends (...args: any[]) => infer U ? U :
    T extends Promise<infer U> ? U :

type T0 = Unpacked<string>;  // string
type T1 = Unpacked<string[]>;  // string
type T2 = Unpacked<() => string>;  // string
type T3 = Unpacked<Promise<string>>;  // string
type T4 = Unpacked<Promise<string>[]>;  // Promise<string>
type T5 = Unpacked<Unpacked<Promise<string>[]>>;  // string

The following example demonstrates how multiple candidates for the same type variable in co-variant positions causes a union type to be inferred:

type Foo<T> = T extends { a: infer U, b: infer U } ? U : never;
type T10 = Foo<{ a: string, b: string }>;  // string
type T11 = Foo<{ a: string, b: number }>;  // string | number

Likewise, multiple candidates for the same type variable in contra-variant positions causes an intersection type to be inferred:

type Bar<T> = T extends { a: (x: infer U) => void, b: (x: infer U) => void } ? U : never;
type T20 = Bar<{ a: (x: string) => void, b: (x: string) => void }>;  // string
type T21 = Bar<{ a: (x: string) => void, b: (x: number) => void }>;  // string & number

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

declare function foo(x: string): number;
declare function foo(x: number): string;
declare function foo(x: string | number): string | number;
type T30 = ReturnType<typeof foo>;  // string | number

It is not possible to use infer declarations in constraint clauses for regular type parameters:

type ReturnType<T extends (...args: any[]) => infer R> = R;  // Error, not supported

However, much the same effect can be obtained by erasing the type variables in the constraint and instead specifying a conditional type:

type AnyFunction = (...args: any[]) => any;
type ReturnType<T extends AnyFunction> = T extends (...args: any[]) => infer R ? R : any;

Predefined conditional types

TypeScript 2.8 adds several predefined conditional types to lib.d.ts:

  • Exclude<T, U> – Exclude from T those types that are assignable to U.
  • Extract<T, U> – Extract from T those types that are assignable to U.
  • NonNullable<T> – Exclude null and undefined from T.
  • ReturnType<T> – Obtain the return type of a function type.
  • InstanceType<T> – Obtain the instance type of a constructor function type.
type T00 = Exclude<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "b" | "d"
type T01 = Extract<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "a" | "c"

type T02 = Exclude<string | number | (() => void), Function>;  // string | number
type T03 = Extract<string | number | (() => void), Function>;  // () => void

type T04 = NonNullable<string | number | undefined>;  // string | number
type T05 = NonNullable<(() => string) | string[] | null | undefined>;  // (() => string) | string[]

function f1(s: string) {
    return { a: 1, b: s };

class C {
    x = 0;
    y = 0;

type T10 = ReturnType<() => string>;  // string
type T11 = ReturnType<(s: string) => void>;  // void
type T12 = ReturnType<(<T>() => T)>;  // {}
type T13 = ReturnType<(<T extends U, U extends number[]>() => T)>;  // number[]
type T14 = ReturnType<typeof f1>;  // { a: number, b: string }
type T15 = ReturnType<any>;  // any
type T16 = ReturnType<never>;  // any
type T17 = ReturnType<string>;  // Error
type T18 = ReturnType<Function>;  // Error

type T20 = InstanceType<typeof C>;  // C
type T21 = InstanceType<any>;  // any
type T22 = InstanceType<never>;  // any
type T23 = InstanceType<string>;  // Error
type T24 = InstanceType<Function>;  // Error

Note: The Exclude type is a proper implementation of the Diff type suggested here. We’ve used the name Exclude to avoid breaking existing code that defines a Diff, plus we feel that name better conveys the semantics of the type. We did not include the Omit<T, K> type because it is trivially written as Pick<T, Exclude<keyof T, K>>.

Improved control over mapped type modifiers

Mapped types support adding a readonly or ? modifier to a mapped property, but they did not provide support the ability to remove modifiers. This matters in homomorphic mapped types which by default preserve the modifiers of the underlying type.

TypeScript 2.8 adds the ability for a mapped type to either add or remove a particular modifier. Specifically, a readonly or ? property modifier in a mapped type can now be prefixed with either + or - to indicate that the modifier should be added or removed.


type MutableRequired<T> = { -readonly [P in keyof T]-?: T[P] };  // Remove readonly and ?
type ReadonlyPartial<T> = { +readonly [P in keyof T]+?: T[P] };  // Add readonly and ?

A modifier with no + or - prefix is the same as a modifier with a + prefix. So, the ReadonlyPartial<T> type above corresponds to

type ReadonlyPartial<T> = { readonly [P in keyof T]?: T[P] };  // Add readonly and ?

Using this ability, lib.d.ts now has a new Required<T> type. This type strips ? modifiers from all properties of T, thus making all properties required.

type Required<T> = { [P in keyof T]-?: T[P] };

Note that in --strictNullChecks mode, when a homomorphic mapped type removes a ? modifier from a property in the underlying type it also removes undefined from the type of that property:

type Foo = { a?: string };  // Same as { a?: string | undefined }
type Bar = Required<Foo>;  // Same as { a: string }

Improved keyof with intersection types

With TypeScript 2.8 keyof applied to an intersection type is transformed to a union of keyof applied to each intersection constituent. In other words, types of the form keyof (A & B) are transformed to be keyof A | keyof B. This change should address inconsistencies with inference from keyof expressions.

type A = { a: string };
type B = { b: string };

type T1 = keyof (A & B);  // "a" | "b"
type T2<T> = keyof (T & B);  // keyof T | "b"
type T3<U> = keyof (A & U);  // "a" | keyof U
type T4<T, U> = keyof (T & U);  // keyof T | keyof U
type T5 = T2<A>;  // "a" | "b"
type T6 = T3<B>;  // "a" | "b"
type T7 = T4<A, B>;  // "a" | "b"

Better handling for namespace patterns in .js files

TypeScript 2.8 adds support for understanding more namespace patterns in .js files. Empty object literals declarations on top level, just like functions and classes, are now recognized as as namespace declarations in JavaScript.

var ns = {};     // recognized as a declaration for a namespace `ns`
ns.constant = 1; // recognized as a declaration for var `constant`

Assignments at the top-level should behave the same way; in other words, a var or const declaration is not required.

app = {}; // does NOT need to be `var app = {}`
app.C = class {
app.f = function() {
app.prop = 1;

IIFEs as namespace declarations

An IIFE returning a function, class or empty object literal, is also recognized as a namespace:

var C = (function () {
  function C(n) {
    this.p = n;
  return C;
C.staticProperty = 1;

Defaulted declarations

“Defaulted declarations” allow initializers that reference the declared name in the left side of a logical or:

my = || {}; = || {};

Prototype assignment

You can assign an object literal directly to the prototype property. Individual prototype assignments still work too:

var C = function (p) {
  this.p = p;
C.prototype = {
  m() {
C.prototype.q = function(r) {
  return this.p === r;

Nested and merged declarations

Nesting works to any level now, and merges correctly across files. Previously neither was the case.

var app = || {};
app.C = class { };

Per-file JSX factories

TypeScript 2.8 adds support for a per-file configurable JSX factory name using @jsx dom paragma. JSX factory can be configured for a compilation using --jsxFactory (default is React.createElement). With TypeScript 2.8 you can override this on a per-file-basis by adding a comment to the beginning of the file.

/** @jsx dom */
import { dom } from "./renderer"


var renderer_1 = require("./renderer");
renderer_1.dom("h", null);

Locally scoped JSX namespaces

JSX type checking is driven by definitions in a JSX namespace, for instance JSX.Element for the type of a JSX element, and JSX.IntrinsicElements for built-in elements. Before TypeScript 2.8 the JSX namespace was expected to be in the global namespace, and thus only allowing one to be defined in a project. Starting with TypeScript 2.8 the JSX namespace will be looked under the jsxNamespace (e.g. React) allowing for multiple jsx factories in one compilation. For backward compatibility the global JSX namespace is used as a fallback if none was defined on the factory function. Combined with the per-file @jsx pragma, each file can have a different JSX factory.

New --emitDeclarationsOnly

--emitDeclarationsOnly allows for only generating declaration files; .js/.jsx output generation will be skipped with this flag. The flag is useful when the .js output generation is handled by a different transpiler like Babel.