TypeScript Language Specification v1.5

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3.6 Named Types Classes, interfaces, enums, and type aliases are named types that are introduced through class declarations (section 8.1), interface declarations (section 7.1), enum declarations (9.1), and type alias declarations (section 3.9). Class and interface types may have type parameters and are then called generic types. Conversely, named types without type parameters are called non-generic types. Interface declarations only introduce named types, whereas class declarations introduce named types and constructor functions that create instances of implementations of those named types. The named types introduced by class and interface declarations have only minor differences (classes can't declare optional members and interfaces can't declare private or protected members) and are in most contexts interchangeable. In particular, class declarations with only public members introduce named types that function exactly like those created by interface declarations. Named types are referenced through type references (section 3.7.2) that specify a type name and, if applicable, the type arguments to be substituted for the type parameters of the named type. Named types are technically not types—only references to named types are. This distinction is particularly evident with generic types: Generic types are "templates" from which multiple actual types can be created by writing type references that supply type arguments to substitute in place of the generic type's type parameters. This substitution process is known as instantiating a generic type. Only once a generic type is instantiated does it denote an actual type. <strong>TypeScript</strong> has a structural type system, and therefore an instantiation of a generic type is indistinguishable from an equivalent manually written expansion. For example, given the declaration interface Pair { first: T1; second: T2; } the type reference Pair is indistinguishable from the type { first: string; second: Entity; } 3.6.1 Instance Types Each class and interface has an associated actual type known as the instance type. For a non-generic class or interface, the instance type is simply a type reference to the class or interface. For a generic class or interface, the instance type is an instantiation of the generic type where each of the type arguments is the corresponding type parameter. Since the instance type uses the type parameters it can be used only where the type parameters are in scope—that is, inside the declaration of the class or interface. Within the constructor and instance member functions of a class, the type of this is the instance type of the class. 34
The following example illustrates the concept of an instance type: class G { // Introduce type parameter T self: G; // Use T as type argument to form instance type f() { this.self = this; // self and this are both of type G } } 3.7 Specifying Types Types are specified either by referencing their keyword or name, or by writing object type literals, array type literals, tuple type literals, function type literals, constructor type literals, or type queries. Type: PrimaryOrUnionType FunctionType ConstructorType PrimaryOrUnionType: PrimaryType UnionType PrimaryType: ParenthesizedType PredefinedType TypeReference ObjectType ArrayType TupleType TypeQuery ParenthesizedType: ( Type ) Parentheses are required around union, function, or constructor types when they are used as array element types, and parentheses are required around function or constructor types in union types. For example: (string | number)[] ((x: string) => string) | ((x: number) => number) The different forms of type notations are described in the following sections. 35
Page 1 and 2: TypeScript Language Specification V
Page 3 and 4: Table of Contents 1 Introduction ..
Page 5 and 6: 4.9.1 Standard Function Expressions
Page 7 and 8: 9.1 Enum Declarations .............
Page 9 and 10: 1 Introduction JavaScript applicati
Page 11 and 12: 1.1 Ambient Declarations An ambient
Page 13 and 14: call '$' as a function. The behavio
Page 15 and 16: function getX(p: Point) { return p.
Page 17 and 18: This TypeScript class declaration c
Page 19 and 20: Section 9 describes how programmers
Page 21 and 22: Interface definitions, like the one
Page 23 and 24: TypeScript modules provide a mechan
Page 25 and 26: 2 Basic Concepts The remainder of t
Page 27 and 28: Each enum declaration has a declara
Page 29: When an identifier is resolved as a
Page 32 and 33: which infers the type of 'x' to be
Page 34 and 35: The only possible values for the Vo
Page 36 and 37: interface Array { length: number; [
Page 38 and 39: 3.4 Union Types Union types represe
Page 40 and 41: 3.5 Type Parameters A type paramete
Page 44 and 45: 3.7.1 Predefined Types The any, num
Page 46 and 47: TypeMember: PropertySignature CallS
Page 48 and 49: 3.7.8 Constructor Type Literals A c
Page 50 and 51: 3.8.2 Call Signatures A call signat
Page 52 and 53: RestParameter: ... Identifier TypeA
Page 54 and 55: IndexSignature: [ Identifier : stri
Page 56 and 57: Type aliases are referenced using t
Page 58 and 59: o o If each type in U has an appare
Page 60 and 61: o o o M is a non-specialized call o
Page 62 and 63: 3.10.5 Contextual Signature Instant
Page 64 and 65: interface List { data: T; next: Lis
Page 66 and 67: 4.3 Identifiers When an expression
Page 68 and 69: of the object literal includes a nu
Page 70 and 71: A super call invokes the constructo
Page 72 and 73: ( id ) => { ... } ( id ) => expr Th
Page 74 and 75: A dot notation property access of t
Page 76 and 77: If func is of type Any, or of an ob
Page 78 and 79: An example: signature are fixed, an
Page 80 and 81: This rule means that the call to 'f
Page 82 and 83: 4.14.4 The delete Operator The 'del
Page 84 and 85: Any Boolean Number String Other Any
Page 86 and 87: where ??= is one of the compound as
Page 88 and 89: 4.18 The Comma Operator The comma o
Page 90 and 91: function foo(x: number | string) {
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function processValue(value: number
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5 Statements This chapter describes
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BindingProperty: Identifier Initial
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Similarly, an array destructuring d
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5.5 Continue Statements A 'continue
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6 Functions TypeScript extends Java
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function f(x: number) { if (x
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When the output target is ECMAScrip
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101
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An interface has the members specif
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interface Document { createElement(
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8 Classes TypeScript supports class
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The following constraints must be s
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Private and protected accessibility
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interface A { x: number; f: () => v
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ConstructorOverloads: ConstructorOv
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BaseClass.apply(this, arguments); a
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Initializer expressions for instanc
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var x = A.create(); // new A() var
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MemberVariableAssignments is a sequ
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In addition, the '__extends' functi
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9 Enums An enum type is a distinct
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An identifier or property access th
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var Color; (function (Color) { Colo
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If the declaration of 'M' above had
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module A { export interface X { s:
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module outer { var local = 1; expor
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where ModuleName is the name of the
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DeclarationElement: ExportAssignmen
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that source file is considered an e
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Unlike a non-instantiated internal
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import g = require("./geometry"); v
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12.1.3 Ambient Class Declarations A
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An ExternalImportDeclaration in an
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PrimaryType: ParenthesizedType Pred
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ParameterList: RequiredParameterLis
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UnaryExpression: ( Modified ) … <
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ClassExtendsClause: extends ClassTy
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ModuleBody: ModuleElements opt Modu
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AmbientClassBodyElements: AmbientCl
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TypeScript Language Specification v1.5

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