This page lists some of the more advanced ways in which you can model types, it works in tandem with the Utility Types doc which includes types which are included in TypeScript and available globally.
\nUnion types are useful for modeling situations when values can overlap in the types they can take on.\nWhat happens when we need to know specifically whether we have a Fish?\nA common idiom in JavaScript to differentiate between two possible values is to check for the presence of a member.\nAs we mentioned, you can only access members that are guaranteed to be in all the constituents of a union type.
\ntsletTrypet =getSmallPet ();\n\n// You can use the 'in' operator to check\nif (\"swim\" inpet ) {\npet .swim ();\n}\n// However, you cannot use property access\nif (pet .) {\nProperty 'fly' does not exist on type 'Fish | Bird'.\n Property 'fly' does not exist on type 'Fish'.2339Property 'fly' does not exist on type 'Fish | Bird'.\n Property 'fly' does not exist on type 'Fish'. fly pet . \nProperty 'fly' does not exist on type 'Fish | Bird'.\n Property 'fly' does not exist on type 'Fish'.2339Property 'fly' does not exist on type 'Fish | Bird'.\n Property 'fly' does not exist on type 'Fish'.}fly ();
To get the same code working via property accessors, we’ll need to use a type assertion:
\n\ntsletTrypet =getSmallPet ();\nletfishPet =pet asFish ;\nletbirdPet =pet asBird ;\n\nif (fishPet .swim ) {\nfishPet .swim ();\n} else if (birdPet .fly ) {\nbirdPet .fly ();\n}
This isn’t the sort of code you would want in your codebase however.
\nIt would be much better if once we performed the check, we could know the type of pet within each branch.
It just so happens that TypeScript has something called a type guard.\nA type guard is some expression that performs a runtime check that guarantees the type in some scope.
\nTo define a type guard, we simply need to define a function whose return type is a type predicate:
\n\ntsfunctionTryisFish (pet :Fish |Bird ):pet isFish {\n return (pet asFish ).swim !==undefined ;\n}
pet is Fish is our type predicate in this example.\nA predicate takes the form parameterName is Type, where parameterName must be the name of a parameter from the current function signature.
Any time isFish is called with some variable, TypeScript will narrow that variable to that specific type if the original type is compatible.
\nts// Both calls to 'swim' and 'fly' are now okay.\nletTrypet =getSmallPet ();\n\nif (isFish (pet )) {\npet .swim ();\n} else {\npet .fly ();\n}
Notice that TypeScript not only knows that pet is a Fish in the if branch;\nit also knows that in the else branch, you don’t have a Fish, so you must have a Bird.
in operatorThe in operator also acts as a narrowing expression for types.
For a n in x expression, where n is a string literal or string literal type and x is a union type, the “true” branch narrows to types which have an optional or required property n, and the “false” branch narrows to types which have an optional or missing property n.
\ntsfunctionTrymove (pet :Fish |Bird ) {\n if (\"swim\" inpet ) {\n returnpet .swim ();\n }\n returnpet .fly ();\n}
typeof type guardsLet’s go back and write the code for a version of padLeft which uses union types.\nWe could write it with type predicates as follows:
\ntsfunctionTryisNumber (x : any):x is number {\n return typeofx === \"number\";\n}\n\nfunctionisString (x : any):x is string {\n return typeofx === \"string\";\n}\n\nfunctionpadLeft (value : string,padding : string | number) {\n if (isNumber (padding )) {\n returnArray (padding + 1).join (\" \") +value ;\n }\n if (isString (padding )) {\n returnpadding +value ;\n }\n throw newError (`Expected string or number, got '${padding }'.`);\n}
However, having to define a function to figure out if a type is a primitive is kind of a pain.\nLuckily, you don’t need to abstract typeof x === \"number\" into its own function because TypeScript will recognize it as a type guard on its own.\nThat means we could just write these checks inline.
\ntsfunctionTrypadLeft (value : string,padding : string | number) {\n if (typeofpadding === \"number\") {\n returnArray (padding + 1).join (\" \") +value ;\n }\n if (typeofpadding === \"string\") {\n returnpadding +value ;\n }\n throw newError (`Expected string or number, got '${padding }'.`);\n}
These typeof type guards are recognized in two different forms: typeof v === \"typename\" and typeof v !== \"typename\", where \"typename\" must be \"number\", \"string\", \"boolean\", or \"symbol\".\nWhile TypeScript won’t stop you from comparing to other strings, the language won’t recognize those expressions as type guards.
instanceof type guardsIf you’ve read about typeof type guards and are familiar with the instanceof operator in JavaScript, you probably have some idea of what this section is about.
instanceof type guards are a way of narrowing types using their constructor function.\nFor instance, let’s borrow our industrial strength string-padder example from earlier:
\ntsinterfaceTryPadder {\ngetPaddingString (): string;\n}\n\nclassSpaceRepeatingPadder implementsPadder {\n constructor(privatenumSpaces : number) {}\ngetPaddingString () {\n returnArray (this.numSpaces + 1).join (\" \");\n }\n}\n\nclassStringPadder implementsPadder {\n constructor(privatevalue : string) {}\ngetPaddingString () {\n return this.value ;\n }\n}\n\nfunctiongetRandomPadder () {\n returnMath .random () < 0.5\n ? newSpaceRepeatingPadder (4)\n : newStringPadder (\" \");\n}\n\nletpadder :Padder =getRandomPadder ();\n// ^ = let padder: Padder\n\nif (padder instanceofSpaceRepeatingPadder ) {\npadder ;\n // ^?\n}\nif (padder instanceofStringPadder ) {\npadder ;\n // ^?\n}
The right side of the instanceof needs to be a constructor function, and TypeScript will narrow down to:
prototype property if its type is not anyin that order.
\nTypeScript has two special types, null and undefined, that have the values null and undefined respectively.\nWe mentioned these briefly in the Basic Types section.
By default, the type checker considers null and undefined assignable to anything.\nEffectively, null and undefined are valid values of every type.\nThat means it’s not possible to stop them from being assigned to any type, even when you would like to prevent it.\nThe inventor of null, Tony Hoare, calls this his “billion dollar mistake”.
The --strictNullChecks flag fixes this: when you declare a variable, it doesn’t automatically include null or undefined.\nYou can include them explicitly using a union type:
\ntsletTryexamapleString = \"foo\";\n= null;\nType 'null' is not assignable to type 'string'.2322Type 'null' is not assignable to type 'string'.\nlet examapleString stringOrNull : string | null = \"bar\";\nstringOrNull = null;\n\n= stringOrNull undefined ;\nType 'undefined' is not assignable to type 'string | null'.2322Type 'undefined' is not assignable to type 'string | null'.
Note that TypeScript treats null and undefined differently in order to match JavaScript semantics.\nstring | null is a different type than string | undefined and string | undefined | null.
From TypeScript 3.7 and onwards, you can use optional chaining to simplify working with nullable types.
\nWith --strictNullChecks, an optional parameter automatically adds | undefined:
\ntsfunctionTryf (x : number,y ?: number) {\n returnx + (y || 0);\n}\n\nf (1, 2);\nf (1);\nf (1,undefined );\nf (1,null );\nArgument of type 'null' is not assignable to parameter of type 'number | undefined'.2345Argument of type 'null' is not assignable to parameter of type 'number | undefined'.
The same is true for optional properties:
\n\ntsclassTryC {\na : number;\nb ?: number;\n}\n\nletc = newC ();\n\nc .a = 12;\nc .a =undefined ;\nType 'undefined' is not assignable to type 'number'.2322Type 'undefined' is not assignable to type 'number'.c .b = 13;\nc .b =undefined ;\nc .b = null;\nType 'null' is not assignable to type 'number | undefined'.2322Type 'null' is not assignable to type 'number | undefined'.
Since nullable types are implemented with a union, you need to use a type guard to get rid of the null.\nFortunately, this is the same code you’d write in JavaScript:
\ntsfunctionTryf (stringOrNull : string | null): string {\n if (stringOrNull === null) {\n return \"default\";\n } else {\n returnstringOrNull ;\n }\n}
The null elimination is pretty obvious here, but you can use terser operators too:
\ntsfunctionTryf (stringOrNull : string | null): string {\n returnstringOrNull || \"default\";\n}
In cases where the compiler can’t eliminate null or undefined, you can use the type assertion operator to manually remove them.\nThe syntax is postfix !: identifier! removes null and undefined from the type of identifier:
\ntsinterfaceTryUserAccount {\nid : number;\nuser =getUser (\"admin\");\n. user id ;\nObject is possibly 'undefined'.2532Object is possibly 'undefined'.\nif (user ) {\nuser .length ;\nObject is possibly 'undefined'.2532Object is possibly 'undefined'.}\n\n// Instead if you are sure that these objects or fields exist, the\n// postfix ! lets you short circuit the nullability\nuser !.length ;
Type aliases create a new name for a type.\nType aliases are sometimes similar to interfaces, but can name primitives, unions, tuples, and any other types that you’d otherwise have to write by hand.
\n\ntstypeTrySecond = number;\n\nlettimeInSecond : number = 10;\nlettime :Second = 10;
Aliasing doesn’t actually create a new type - it creates a new name to refer to that type.\nAliasing a primitive is not terribly useful, though it can be used as a form of documentation.
\nJust like interfaces, type aliases can also be generic - we can just add type parameters and use them on the right side of the alias declaration:
\n\ntstype Container<T> = { value: T };
We can also have a type alias refer to itself in a property:
\n\ntstype Tree<T> = {\n value: T;\n left?: Tree<T>;\n right?: Tree<T>;\n};
Together with intersection types, we can make some pretty mind-bending types:
\n\ntstypeTryLinkedList <Type > =Type & {next :LinkedList <Type > };\n\ninterfacePerson {\nname : string;\n}\n\nletpeople =getDriversLicenseQueue ();\npeople .name ;\npeople .next .name ;\npeople .next .next .name ;\npeople .next .next .next .name ;\n// ^ = (property) next: LinkedList
As we mentioned, type aliases can act sort of like interfaces; however, there are some subtle differences.
\nAlmost all features of an interface are available in type, the key distinction is that a type cannot be re-opened to add new properties vs a interface which is always extendable.
Interface | \n Type | \n
|---|---|
| \n Extending an interface \n | \n \n Extending a type via intersections \n | \n
| \n Adding new fields to an existing interface \n | \n \n A type cannot be changed after being created \n | \n
Because an interface more closely maps how JavaScript object work by being open to extension, we recommend using an interface over a type alias when possible.
\nOn the other hand, if you can’t express some shape with an interface and you need to use a union or tuple type, type aliases are usually the way to go.
\nAs mentioned in our section on enums, enum members have types when every member is literal-initialized.
\nMuch of the time when we talk about “singleton types”, we’re referring to both enum member types as well as numeric/string literal types, though many users will use “singleton types” and “literal types” interchangeably.
\nthis typesA polymorphic this type represents a type that is the subtype of the containing class or interface.\nThis is called F-bounded polymorphism, a lot of people know it as the fluent API pattern.\nThis makes hierarchical fluent interfaces much easier to express, for example.\nTake a simple calculator that returns this after each operation:
\ntsclassTryBasicCalculator {\n public constructor(protectedvalue : number = 0) {}\n publiccurrentValue (): number {\n return this.value ;\n }\n publicadd (operand : number): this {\n this.value +=operand ;\n return this;\n }\n publicmultiply (operand : number): this {\n this.value *=operand ;\n return this;\n }\n // ... other operations go here ...\n}\n\nletv = newBasicCalculator (2).multiply (5).add (1).currentValue ();
Since the class uses this types, you can extend it and the new class can use the old methods with no changes.
\ntsclassTryScientificCalculator extendsBasicCalculator {\n public constructor(value = 0) {\n super(value );\n }\n publicsin () {\n this.value =Math .sin (this.value );\n return this;\n }\n // ... other operations go here ...\n}\n\nletv = newScientificCalculator (2).multiply (5).sin ().add (1).currentValue ();
Without this types, ScientificCalculator would not have been able to extend BasicCalculator and keep the fluent interface.\nmultiply would have returned BasicCalculator, which doesn’t have the sin method.\nHowever, with this types, multiply returns this, which is ScientificCalculator here.
With index types, you can get the compiler to check code that uses dynamic property names.\nFor example, a common JavaScript pattern is to pick a subset of properties from an object:
\n\njsfunction pluck(o, propertyNames) {\n return propertyNames.map((n) => o[n]);\n}
Here’s how you would write and use this function in TypeScript, using the index type query and indexed access operators:
\n\ntsfunctionTrypluck <T ,K extends keyofT >(o :T ,propertyNames :K []):T [K ][] {\n returnpropertyNames .map ((n ) =>o [n ]);\n}\n\ninterfaceCar {\nmanufacturer : string;\nmodel : string;\nyear : number;\n}\n\nlettaxi :Car = {\nmanufacturer : \"Toyota\",\nmodel : \"Camry\",\nyear : 2014,\n};\n\n// Manufacturer and model are both of type string,\n// so we can pluck them both into a typed string array\nletmakeAndModel : string[] =pluck (taxi , [\"manufacturer\", \"model\"]);\n\n// If we try to pluck model and year, we get an\n// array of a union type: (string | number)[]\nletmodelYear =pluck (taxi , [\"model\", \"year\"]);
The compiler checks that manufacturer and model are actually properties on Car.\nThe example introduces a couple of new type operators.\nFirst is keyof T, the index type query operator.\nFor any type T, keyof T is the union of known, public property names of T.\nFor example:
\ntsletTrycarProps : keyofCar ;\n// ^ = let carProps: \"manufacturer\" | \"model\" | \"year\"
keyof Car is completely interchangeable with \"manufacturer\" | \"model\" | \"year\".\nThe difference is that if you add another property to Car, say ownersAddress: string, then keyof Car will automatically update to be \"manufacturer\" | \"model\" | \"year\" | \"ownersAddress\".\nAnd you can use keyof in generic contexts like pluck, where you can’t possibly know the property names ahead of time.\nThat means the compiler will check that you pass the right set of property names to pluck:
\nts// error, Type '\"unknown\"' is not assignable to type '\"manufacturer\" | \"model\" | \"year\"'\npluck(taxi, [\"year\", \"unknown\"]);
The second operator is T[K], the indexed access operator.\nHere, the type syntax reflects the expression syntax.\nThat means that taxi[\"name\"] has the type Car[\"name\"] — which in our example is just string.\nHowever, just like index type queries, you can use T[K] in a generic context, which is where its real power comes to life.\nYou just have to make sure that the type variable K extends keyof T.\nHere’s another example with a function named getProperty.
\ntsfunction getProperty<T, K extends keyof T>(o: T, propertyName: K): T[K] {\n return o[propertyName]; // o[propertyName] is of type T[K]\n}
In getProperty, o: T and propertyName: K, so that means o[propertyName]: T[K].\nOnce you return the T[K] result, the compiler will instantiate the actual type of the key, so the return type of getProperty will vary according to which property you request.
\ntsletTrymanufacturer : string =getProperty (taxi , \"manufacturer\");\nletyear : number =getProperty (taxi , \"year\");\n\nletunknown =getProperty (taxi ,"unknown" );\nArgument of type '\"unknown\"' is not assignable to parameter of type '\"manufacturer\" | \"model\" | \"year\"'.2345Argument of type '\"unknown\"' is not assignable to parameter of type '\"manufacturer\" | \"model\" | \"year\"'.
keyof and T[K] interact with index signatures. An index signature parameter type must be ‘string’ or ‘number’.\nIf you have a type with a string index signature, keyof T will be string | number\n(and not just string, since in JavaScript you can access an object property either\nby using strings (object[\"42\"]) or numbers (object[42])).\nAnd T[string] is just the type of the index signature:
\ntsinterfaceTryDictionary <T > {\n [key : string]:T ;\n}\nletkeys : keyofDictionary <number>;\n// ^ = let keys: string | number\nletvalue :Dictionary <number>[\"foo\"];\n// ^ = let value: number
If you have a type with a number index signature, keyof T will just be number.
\ntsinterfaceTryDictionary <T > {\n [key : number]:T ;\n}\n\nletkeys : keyofDictionary <number>;\n// ^ = let keys: number\nletnumberValue :Dictionary <number>[42];\n// ^ = let numberValue: number\nletvalue :Dictionary <number>["foo" ];\nProperty 'foo' does not exist on type 'Dictionary<number>'.2339Property 'foo' does not exist on type 'Dictionary<number>'.
A common task is to take an existing type and make each of its properties optional:
\n\ntsinterface PersonSubset {\n name?: string;\n age?: number;\n}
Or we might want a readonly version:
\n\ntsinterface PersonReadonly {\n readonly name: string;\n readonly age: number;\n}
This happens often enough in JavaScript that TypeScript provides a way to create new types based on old types — mapped types.\nIn a mapped type, the new type transforms each property in the old type in the same way.\nFor example, you can make all properties of a type readonly or optional.\nHere are a couple of examples:
\ntstypeTryReadonly <T > = {\n readonly [P in keyofT ]:T [P ];\n};\n\ntypePartial <T > = {\n [P in keyofT ]?:T [P ];\n};
And to use it:
\n\ntstypeTryPersonPartial =Partial <Person >;\n// ^ = type PersonPartial = {\n name?: string | undefined;\n age?: number | undefined;\n}\ntypeReadonlyPerson =Readonly <Person >;\n// ^ = type ReadonlyPerson = {\n readonly name: string;\n readonly age: number;\n}
Note that this syntax describes a type rather than a member.\nIf you want to add members, you can use an intersection type:
\n\nts// Use this:\ntypeTryPartialWithNewMember <T > = {\n [P in keyofT ]?:T [P ];\n} & {newMember : boolean }\n\n// This is an error!\ntypeWrongPartialWithNewMember <T > = {\n [P in keyofT ]?:T [P ];\nnewMember : \n'boolean' only refers to a type, but is being used as a value here.'}' expected.2693boolean ;
1005'boolean' only refers to a type, but is being used as a value here.'}' expected.} \nDeclaration or statement expected.1128Declaration or statement expected.
Let’s take a look at the simplest mapped type and its parts:
\n\ntstypeTryKeys = \"option1\" | \"option2\";\ntypeFlags = { [K inKeys ]: boolean };
The syntax resembles the syntax for index signatures with a for .. in inside.\nThere are three parts:
K, which gets bound to each property in turn.Keys, which contains the names of properties to iterate over.In this simple example, Keys is a hard-coded list of property names and the property type is always boolean, so this mapped type is equivalent to writing:
\ntstypeTryFlags = {\noption1 : boolean;\noption2 : boolean;\n};
Real applications, however, look like Readonly or Partial above.\nThey’re based on some existing type, and they transform the properties in some way.\nThat’s where keyof and indexed access types come in:
\ntstypeTryNullablePerson = { [P in keyofPerson ]:Person [P ] | null };\n// ^ = type NullablePerson = {\n name: string | null;\n age: number | null;\n}\ntypePartialPerson = { [P in keyofPerson ]?:Person [P ] };\n// ^ = type PartialPerson = {\n name?: string | undefined;\n age?: number | undefined;\n}
But it’s more useful to have a general version.
\n\ntstype Nullable<T> = { [P in keyof T]: T[P] | null };\ntype Partial<T> = { [P in keyof T]?: T[P] };
In these examples, the properties list is keyof T and the resulting type is some variant of T[P].\nThis is a good template for any general use of mapped types.\nThat’s because this kind of transformation is homomorphic, which means that the mapping applies only to properties of T and no others.\nThe compiler knows that it can copy all the existing property modifiers before adding any new ones.\nFor example, if Person.name was readonly, Partial<Person>.name would be readonly and optional.
Here’s one more example, in which T[P] is wrapped in a Proxy<T> class:
\ntstypeTryProxy <T > = {\nget ():T ;\nset (value :T ): void;\n};\n\ntypeProxify <T > = {\n [P in keyofT ]:Proxy <T [P ]>;\n};\n\nfunctionproxify <T >(o :T ):Proxify <T > {\n // ... wrap proxies ...\n}\n\nletprops = {rooms : 4 };\nletproxyProps =proxify (props );\n// ^ = let proxyProps: Proxify<{\n rooms: number;\n}>
Note that Readonly<T> and Partial<T> are so useful, they are included in TypeScript’s standard library along with Pick and Record:
\ntstype Pick<T, K extends keyof T> = {\n [P in K]: T[P];\n};\n\ntype Record<K extends keyof any, T> = {\n [P in K]: T;\n};
Readonly, Partial and Pick are homomorphic whereas Record is not.\nOne clue that Record is not homomorphic is that it doesn’t take an input type to copy properties from:
\ntstypeTryThreeStringProps =Record <\"prop1\" | \"prop2\" | \"prop3\", string>;
Non-homomorphic types are essentially creating new properties, so they can’t copy property modifiers from anywhere.
\nNow that you know how to wrap the properties of a type, the next thing you’ll want to do is unwrap them.\nFortunately, that’s pretty easy:
\n\ntsfunctionTryunproxify <T >(t :Proxify <T >):T {\n letresult = {} asT ;\n for (constk int ) {\nresult [k ] =t [k ].get ();\n }\n returnresult ;\n}\n\nletoriginalProps =unproxify (proxyProps );\n// ^ = let originalProps: {\n rooms: number;\n}
Note that this unwrapping inference only works on homomorphic mapped types.\nIf the mapped type is not homomorphic you’ll have to give an explicit type parameter to your unwrapping function.
\nA conditional type selects one of two possible types based on a condition expressed as a type relationship test:
\n\ntsT 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.\nWhen T or U contains type variables, whether to resolve to X or Y, or to defer, is determined by whether or not the type system has enough information to conclude that T is always assignable to U.
As an example of some types that are immediately resolved, we can take a look at the following example:
\n\ntsdeclare functionTryf <T extends boolean>(x :T ):T extends true ? string : number;\n\n// Type is 'string | number'\nletx =f (Math .random () < 0.5);\n// ^ = let x: string | number
Another example would be the TypeName type alias, which uses nested conditional types:
\ntstypeTryTypeName <T > =T extends string\n ? \"string\"\n :T extends number\n ? \"number\"\n :T extends boolean\n ? \"boolean\"\n :T extends undefined\n ? \"undefined\"\n :T extendsFunction \n ? \"function\"\n : \"object\";\n\ntypeT0 =TypeName <string>;\n// ^ = type T0 = \"string\"\ntypeT1 =TypeName <\"a\">;\n// ^ = type T1 = \"string\"\ntypeT2 =TypeName <true>;\n// ^ = type T2 = \"boolean\"\ntypeT3 =TypeName <() => void>;\n// ^ = type T3 = \"function\"\ntypeT4 =TypeName <string[]>;\n// ^ = type T4 = \"object\"
But as an example of a place where conditional types are deferred - where they stick around instead of picking a branch - would be in the following:
\n\ntsinterfaceTryFoo {\npropA : boolean;\npropB : boolean;\n}\n\ndeclare functionf <T >(x :T ):T extendsFoo ? string : number;\n\nfunctionfoo <U >(x :U ) {\n // Has type 'U extends Foo ? string : number'\n leta =f (x );\n\n // This assignment is allowed though!\n letb : string | number =a ;\n}
In the above, the variable a has a conditional type that hasn’t yet chosen a branch.\nWhen another piece of code ends up calling foo, it will substitute in U with some other type, and TypeScript will re-evaluate the conditional type, deciding whether it can actually pick a branch.
In the meantime, we can assign a conditional type to any other target type as long as each branch of the conditional is assignable to that target.\nSo in our example above we were able to assign U extends Foo ? string : number to string | number since no matter what the conditional evaluates to, it’s known to be either string or number.
Conditional types in which the checked type is a naked type parameter are called distributive conditional types.\nDistributive conditional types are automatically distributed over union types during instantiation.\nFor 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).
\ntstypeTryT5 =TypeName <string | (() => void)>;\n// ^ = type T5 = \"string\" | \"function\"\ntypeT6 =TypeName <string | string[] | undefined>;\n// ^ = type T6 = \"string\" | \"undefined\" | \"object\"\ntypeT7 =TypeName <string[] | number[]>;\n// ^ = type T7 = \"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).\nFurthermore, references to T within X have an additional type parameter constraint U (i.e. T is considered assignable to U within X).
\ntstypeTryBoxedValue <T > = {value :T };\ntypeBoxedArray <T > = {array :T [] };\ntypeBoxed <T > =T extends any[] ?BoxedArray <T [number]> :BoxedValue <T >;\n\ntypeT1 =Boxed <string>;\n// ^ = type T1 = {\n value: string;\n}\ntypeT2 =Boxed <number[]>;\n// ^ = type T2 = {\n array: number[];\n}\ntypeT3 =Boxed <string | number[]>;\n// ^ = type T3 = BoxedValue| BoxedArray
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:
\n\nts// Remove types from T that are assignable to U\ntypeTryDiff <T ,U > =T extendsU ? never :T ;\n// Remove types from T that are not assignable to U\ntypeFilter <T ,U > =T extendsU ?T : never;\n\ntypeT1 =Diff <\"a\" | \"b\" | \"c\" | \"d\", \"a\" | \"c\" | \"f\">;\n// ^ = type T1 = \"b\" | \"d\"\ntypeT2 =Filter <\"a\" | \"b\" | \"c\" | \"d\", \"a\" | \"c\" | \"f\">; // \"a\" | \"c\"\n// ^ = type T2 = \"a\" | \"c\"\ntypeT3 =Diff <string | number | (() => void),Function >; // string | number\n// ^ = type T3 = string | number\ntypeT4 =Filter <string | number | (() => void),Function >; // () => void\n// ^ = type T4 = () => void\n\n// Remove null and undefined from T\ntypeNotNullable <T > =Diff <T , null | undefined>;\n\ntypeT5 =NotNullable <string | number | undefined>;\n// ^ = type T5 = string | number\ntypeT6 =NotNullable <string | string[] | null | undefined>;\n// ^ = type T6 = string | string[]\n\nfunctionf1 <T >(x :T ,y :NotNullable <T >) {\nx =y ;\n= y x ;\nType 'T' is not assignable to type 'Diff<T, null | undefined>'.2322Type 'T' is not assignable to type 'Diff<T, null | undefined>'.}\n\nfunctionf2 <T extends string | undefined>(x :T ,y :NotNullable <T >) {\nx =y ;\n= y x ;\nType 'T' is not assignable to type 'Diff<T, null | undefined>'.\n Type 'string | undefined' is not assignable to type 'Diff<T, null | undefined>'.\n Type 'undefined' is not assignable to type 'Diff<T, null | undefined>'.2322Type 'T' is not assignable to type 'Diff<T, null | undefined>'.\n Type 'string | undefined' is not assignable to type 'Diff<T, null | undefined>'.\n Type 'undefined' is not assignable to type 'Diff<T, null | undefined>'. let: string = s1 x ;\nType 'T' is not assignable to type 'string'.\n Type 'string | undefined' is not assignable to type 'string'.\n Type 'undefined' is not assignable to type 'string'.2322Type 'T' is not assignable to type 'string'.\n Type 'string | undefined' is not assignable to type 'string'.\n Type 'undefined' is not assignable to type 'string'. lets2 : string =y ;\n}
Conditional types are particularly useful when combined with mapped types:
\n\ntstypeTryFunctionPropertyNames <T > = {\n [K in keyofT ]:T [K ] extendsFunction ?K : never;\n}[keyofT ];\ntypeFunctionProperties <T > =Pick <T ,FunctionPropertyNames <T >>;\n\ntypeNonFunctionPropertyNames <T > = {\n [K in keyofT ]:T [K ] extendsFunction ? never :K ;\n}[keyofT ];\ntypeNonFunctionProperties <T > =Pick <T ,NonFunctionPropertyNames <T >>;\n\ninterfacePart {\nid : number;\nname : string;\nsubparts :Part [];\nupdatePart (newName : string): void;\n}\n\ntypeT1 =FunctionPropertyNames <Part >;\n// ^ = type T1 = \"updatePart\"\ntypeT2 =NonFunctionPropertyNames <Part >;\n// ^ = type T2 = \"id\" | \"name\" | \"subparts\"\ntypeT3 =FunctionProperties <Part >;\n// ^ = type T3 = {\n updatePart: (newName: string) => void;\n}\ntypeT4 =NonFunctionProperties <Part >;\n// ^ = type T4 = {\n id: number;\n name: string;\n subparts: Part[];\n}
Similar to union and intersection types, conditional types are not permitted to reference themselves recursively.\nFor example the following is an error.
\n\ntstypeTry// Error\nType alias 'ElementType' circularly references itself.Type 'ElementType' is not generic.2456 ElementType <T > =T extends any[] ?ElementType <T [number]> :T ;
2315Type alias 'ElementType' circularly references itself.Type 'ElementType' is not generic.
Within the extends clause of a conditional type, it is now possible to have infer declarations that introduce a type variable to be inferred.\nSuch inferred type variables may be referenced in the true branch of the conditional type.\nIt is possible to have multiple infer locations for the same type variable.
For example, the following extracts the return type of a function type:
\n\ntstypeTryReturnType <T > =T extends (...args : any[]) => inferR ?R : any;
Conditional types can be nested to form a sequence of pattern matches that are evaluated in order:
\n\ntstypeTryUnpacked <T > =T extends (inferU )[]\n ?U \n :T extends (...args : any[]) => inferU \n ?U \n :T extendsPromise <inferU >\n ?U \n :T ;\n\ntypeT0 =Unpacked <string>;\n// ^ = type T0 = string\ntypeT1 =Unpacked <string[]>;\n// ^ = type T1 = string\ntypeT2 =Unpacked <() => string>;\n// ^ = type T2 = string\ntypeT3 =Unpacked <Promise <string>>;\n// ^ = type T3 = string\ntypeT4 =Unpacked <Promise <string>[]>;\n// ^ = type T4 = Promise\ntype T5 =Unpacked <Unpacked <Promise <string>[]>>;\n// ^ = type T5 = string
The following example demonstrates how multiple candidates for the same type variable in co-variant positions causes a union type to be inferred:
\n\ntstypeTryFoo <T > =T extends {a : inferU ;b : inferU } ?U : never;\n\ntypeT1 =Foo <{a : string;b : string }>;\n// ^ = type T1 = string\ntypeT2 =Foo <{a : string;b : number }>;\n// ^ = type T2 = string | number
Likewise, multiple candidates for the same type variable in contra-variant positions causes an intersection type to be inferred:
\n\ntstypeTryBar <T > =T extends {a : (x : inferU ) => void;b : (x : inferU ) => void }\n ?U \n : never;\n\ntypeT1 =Bar <{a : (x : string) => void;b : (x : string) => void }>;\n// ^ = type T1 = string\ntypeT2 =Bar <{a : (x : string) => void;b : (x : number) => void }>;\n// ^ = type T2 = never
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).\nIt is not possible to perform overload resolution based on a list of argument types.
\n\ntsdeclare functionTryfoo (x : string): number;\ndeclare functionfoo (x : number): string;\ndeclare functionfoo (x : string | number): string | number;\n\ntypeT1 =ReturnType <typeoffoo >;\n// ^ = type T1 = string | number
It is not possible to use infer declarations in constraint clauses for regular type parameters:
\ntstypeTryReturnedType <T extends (...args : any[]) =>infer \n'infer' declarations are only permitted in the 'extends' clause of a conditional type.Cannot find name 'R'.1338R > =R ;
2304'infer' declarations are only permitted in the 'extends' clause of a conditional type.Cannot find name 'R'.
However, much the same effect can be obtained by erasing the type variables in the constraint and instead specifying a conditional type:
\n\ntstypeTryAnyFunction = (...args : any[]) => any;\ntypeReturnType <T extendsAnyFunction > =T extends (...args : any[]) => inferR \n ?R \n : any;
TypeScript adds several predefined conditional types, you can find the full list and examples in Utility Types.
","headings":[{"value":"Type Guards and Differentiating Types","depth":2},{"value":"User-Defined Type Guards","depth":2},{"value":"Using type predicates","depth":4},{"value":"Using the in operator","depth":3},{"value":"typeof type guards","depth":2},{"value":"instanceof type guards","depth":2},{"value":"Nullable types","depth":2},{"value":"Optional parameters and properties","depth":3},{"value":"Type guards and type assertions","depth":3},{"value":"Type Aliases","depth":2},{"value":"Interfaces vs. Type Aliases","depth":2},{"value":"Enum Member Types","depth":2},{"value":"Polymorphic this types","depth":2},{"value":"Index types","depth":2},{"value":"Index types and index signatures","depth":2},{"value":"Mapped types","depth":2},{"value":"Inference from mapped types","depth":2},{"value":"Conditional Types","depth":2},{"value":"Distributive conditional types","depth":2},{"value":"Example","depth":4},{"value":"Example","depth":4},{"value":"Example","depth":4},{"value":"Type inference in conditional types","depth":2},{"value":"Predefined conditional types","depth":2}],"frontmatter":{"permalink":"/docs/handbook/advanced-types.html","title":"Advanced Types","disable_toc":null,"oneline":"Advanced concepts around types in TypeScript"}},"prev":null,"next":null},"pageContext":{"id":"advanced-types","slug":"/docs/handbook/advanced-types.html","repoPath":"/packages/documentation/copy/en/reference/Advanced Types.md","lang":"en","modifiedTime":"2020-08-07T17:00:36.339Z"}}}