TypeScript Language Specification v1.5

5 Statements
This chapter describes the static type checking TypeScript provides for JavaScript statements. TypeScript
itself does not introduce any new statement constructs.
5.1 Variable Statements
Variable statements are extended to include optional type annotations and ECMAScript 6 destructuring
declarations.
VariableDeclaration: ( Modified )
SimpleVariableDeclaration
DestructuringVariableDeclaration
A variable declaration is either a simple variable declaration or a destructuring variable declaration.
5.1.1 Simple Variable Declarations
A simple variable declaration introduces a single named variable and optionally assigns it an initial
value.
SimpleVariableDeclaration:
Identifier TypeAnnotation opt Initialiser opt
TypeAnnotation:
: Type
The type T of a variable introduced by a simple variable declaration is determined as follows:
If the declaration includes a type annotation, T is that type.
Otherwise, if the declaration includes an initializer expression, T is the widened form (section 3.11)
of the type of the initializer expression.
Otherwise, T is the Any type.
When a variable declaration specifies both a type annotation and an initializer expression, the type of the
initializer expression is required to be assignable to (section 3.10.4) the type given in the type annotation.
Multiple declarations for the same variable name in the same declaration space are permitted, provided
that each declaration associates the same type with the variable.
When a variable declaration has a type annotation, it is an error for that type annotation to use the
typeof operator to reference the variable being declared.
Below are some examples of simple variable declarations and their associated types.
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