12.1 Ambient Declarations

Ambient declarations are written using the declare keyword and can declare variables, functions, classes, enums, namespaces, or modules.

  AmbientDeclaration:   declare AmbientVariableDeclaration   declare AmbientFunctionDeclaration   declare AmbientClassDeclaration   declare AmbientEnumDeclaration   declare AmbientNamespaceDeclaration

12.1.1 Ambient Variable Declarations

An ambient variable declaration introduces a variable in the containing declaration space.

  AmbientVariableDeclaration:   var AmbientBindingList ;   let AmbientBindingList ;   const AmbientBindingList ;

  AmbientBindingList:   AmbientBinding   AmbientBindingList , AmbientBinding

  AmbientBinding:   BindingIdentifier TypeAnnotation_opt

An ambient variable declaration may optionally include a type annotation. If no type annotation is present, the variable is assumed to have type Any.

An ambient variable declaration does not permit an initializer expression to be present.

12.1.2 Ambient Function Declarations

An ambient function declaration introduces a function in the containing declaration space.

  AmbientFunctionDeclaration:   function BindingIdentifier CallSignature ;

Ambient functions may be overloaded by specifying multiple ambient function declarations with the same name, but it is an error to declare multiple overloads that are considered identical (section 3.11.2) or differ only in their return types.

Ambient function declarations cannot specify a function bodies and do not permit default parameter values.

12.1.3 Ambient Class Declarations

An ambient class declaration declares a class type and a constructor function in the containing declaration space.

  AmbientClassDeclaration:   class BindingIdentifier TypeParameters_opt ClassHeritage { AmbientClassBody }

  AmbientClassBody:   AmbientClassBodyElements_opt

  AmbientClassBodyElements:   AmbientClassBodyElement   AmbientClassBodyElements AmbientClassBodyElement

  AmbientClassBodyElement:   AmbientConstructorDeclaration   AmbientPropertyMemberDeclaration   IndexSignature

  AmbientConstructorDeclaration:   constructor ( ParameterList_opt ) ;

  AmbientPropertyMemberDeclaration:   AccessibilityModifier_opt static_opt PropertyName TypeAnnotation_opt ;   AccessibilityModifier_opt static_opt PropertyName CallSignature ;

12.1.4 Ambient Enum Declarations

An ambient enum is grammatically equivalent to a non-ambient enum declaration.

  AmbientEnumDeclaration:   EnumDeclaration

Ambient enum declarations differ from non-ambient enum declarations in two ways:

• In ambient enum declarations, all values specified in enum member declarations must be classified as constant enum expressions.
• In ambient enum declarations that specify no const modifier, enum member declarations that omit a value are considered computed members (as opposed to having auto-incremented values assigned).

Ambient enum declarations are otherwise processed in the same manner as non-ambient enum declarations.

12.1.5 Ambient Namespace Declarations

An ambient namespace declaration declares a namespace.

  AmbientNamespaceDeclaration:   namespace IdentifierPath { AmbientNamespaceBody }

  AmbientNamespaceBody:   AmbientNamespaceElements_opt

  AmbientNamespaceElements:   AmbientNamespaceElement   AmbientNamespaceElements AmbientNamespaceElement

  AmbientNamespaceElement:   export_opt AmbientVariableDeclaration   export_opt AmbientLexicalDeclaration   export_opt AmbientFunctionDeclaration   export_opt AmbientClassDeclaration   export_opt InterfaceDeclaration   export_opt AmbientEnumDeclaration   export_opt AmbientNamespaceDeclaration   export_opt ImportAliasDeclaration

Except for ImportAliasDeclarations, AmbientNamespaceElements always declare exported entities regardless of whether they include the optional export modifier.