TypeScript Language Specification v1.5

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Otherwise, no type is associated with the binding property and an error occurs. The type T associated with a binding element is determined as follows: Let S be the type associated with the immediately containing destructuring variable declaration, binding property, or binding element. If S is the Any type: o If the binding element specifies an initializer expression, T is the type of that initializer expression. o Otherwise, T is the Any type. If S is not an array-like type (section 3.3.2), no type is associated with the binding property and an error occurs. If the binding element is a rest element, T is an array type with an element type E, where E is the type of the numeric index signature of S. Otherwise, if S is a tuple-like type (section 3.3.3): o Let N be the zero-based index of the binding element in the array binding pattern. o If S has a property with the numerical name N, T is the type of that property. o Otherwise, no type is associated with the binding element and an error occurs. Otherwise, if S has a numeric index signature, T is the type of the numeric index signature. Otherwise, no type is associated with the binding element and an error occurs. When a destructuring variable declaration, binding property, or binding element specifies an initializer expression, the type of the initializer expression is required to be assignable to the widened form of the type associated with the destructuring variable declaration, binding property, or binding element. When the output target is ECMAScript 6 or higher, except for removing the optional type annotation, destructuring variable declarations remain unchanged in the emitted JavaScript code. When the output target is ECMAScript 3 or 5, destructuring variable declarations are rewritten to simple variable declarations. For example, an object destructuring declaration of the form var { x, p: y, q: z = false } = getSomeObject(); is rewritten to the simple variable declarations var _a = getSomeObject(), x = _a.x, y = _a.p, _b = _a.q, z = _b === void 0 ? false : _b; The '_a' and '_b' temporary variables exist to ensure the assigned expression is evaluated only once, and the expression 'void 0' simply denotes the JavaScript value 'undefined'. 90
Similarly, an array destructuring declaration of the form var [x, y, z = 10] = getSomeArray(); is rewritten to the simple variable declarations var _a = getSomeArray(), x = _a[0], y = _a[1], _b = _a[2], z = _b === void 0 ? 10 : _b; Combining both forms of destructuring, the example var { x, p: [y, z = 10] = getSomeArray() } = getSomeObject(); is rewritten to var _a = getSomeObject(), x = _a.x, _b = _a.p, _c = _b === void 0 ? getSomeArray() : _b, y = _c[0], _d = _c[1], z = _d === void 0 ? 10 : _d; 5.1.3 Implied Type A variable, parameter, binding property, or binding element declaration that specifies a binding pattern has an implied type which is determined as follows: If the declaration specifies an object binding pattern, the implied type is an object type with a set of properties corresponding to the specified binding property declarations. The type of each property is the type implied by its binding property declaration, and a property is optional when its binding property declaration specifies an initializer expression. If the declaration specifies an array binding pattern without a rest element, the implied type is a tuple type with elements corresponding to the specified binding element declarations. The type of each element is the type implied by its binding element declaration. If the declaration specifies an array binding pattern with a rest element, the implied type is an array type with an element type of Any. The implied type of a binding property or binding element declaration is the type of the declaration's initializer expression, if any, or otherwise the implied type of the binding pattern specified in the declaration, if any, or otherwise 91
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TypeScript Language Specification V
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Table of Contents 1 Introduction ..
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4.9.1 Standard Function Expressions
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9.1 Enum Declarations .............
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1 Introduction JavaScript applicati
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1.1 Ambient Declarations An ambient
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call '$' as a function. The behavio
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function getX(p: Point) { return p.
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This TypeScript class declaration c
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Section 9 describes how programmers
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Interface definitions, like the one
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TypeScript modules provide a mechan
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2 Basic Concepts The remainder of t
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Each enum declaration has a declara
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When an identifier is resolved as a
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which infers the type of 'x' to be
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The only possible values for the Vo
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interface Array { length: number; [
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3.4 Union Types Union types represe
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3.5 Type Parameters A type paramete
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3.6 Named Types Classes, interfaces
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3.7.1 Predefined Types The any, num
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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) {
Page 92 and 93: function processValue(value: number
Page 95 and 96: 5 Statements This chapter describes
Page 97: BindingProperty: Identifier Initial
Page 101 and 102: 5.5 Continue Statements A 'continue
Page 103 and 104: 6 Functions TypeScript extends Java
Page 105 and 106: function f(x: number) { if (x
Page 107 and 108: When the output target is ECMAScrip
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Page 112 and 113: An interface has the members specif
Page 114 and 115: interface Document { createElement(
Page 117 and 118: 8 Classes TypeScript supports class
Page 119 and 120: The following constraints must be s
Page 121 and 122: Private and protected accessibility
Page 123 and 124: interface A { x: number; f: () => v
Page 125 and 126: ConstructorOverloads: ConstructorOv
Page 127 and 128: BaseClass.apply(this, arguments); a
Page 129 and 130: Initializer expressions for instanc
Page 131 and 132: var x = A.create(); // new A() var
Page 133 and 134: MemberVariableAssignments is a sequ
Page 135 and 136: In addition, the '__extends' functi
Page 137 and 138: 9 Enums An enum type is a distinct
Page 139 and 140: An identifier or property access th
Page 141: var Color; (function (Color) { Colo
Page 144 and 145: If the declaration of 'M' above had
Page 146 and 147: 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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