TypeScript Language Specification v1.5

4.18 The Comma Operator
The comma operator permits the operands to be of any type and produces a result that is of the same
type as the second operand.
4.19 Contextually Typed Expressions
Type checking of an expression is improved in several contexts by factoring in the type of the destination
of the value computed by the expression. In such situations, the expression is said to be contextually
typed by the type of the destination. An expression is contextually typed in the following circumstances:
In a variable, parameter, binding property, binding element, or member declaration, an initializer
expression is contextually typed by
o the type given in the declaration's type annotation, if any, or otherwise
o for a parameter, the contextual type for the declaration (section 4.9.3), if any, or otherwise
o the type implied by the binding pattern in the declaration (section 5.1.3), if any.
In the body of a function declaration, function expression, arrow function, method declaration, or
get accessor declaration that has a return type annotation, return expressions are contextually
typed by the type given in the return type annotation.
In the body of a function expression or arrow function that has no return type annotation, if the
function expression or arrow function is contextually typed by a function type with exactly one call
signature, and if that call signature is non-generic, return expressions are contextually typed by
the return type of that call signature.
In the body of a constructor declaration, return expressions are contextually typed by the
containing class type.
In the body of a get accessor with no return type annotation, if a matching set accessor exists and
that set accessor has a parameter type annotation, return expressions are contextually typed by
the type given in the set accessor's parameter type annotation.
In a typed function call, argument expressions are contextually typed by their corresponding
parameter types.
In a contextually typed object literal, each property value expression is contextually typed by
o the type of the property with a matching name in the contextual type, if any, or otherwise
o for a numerically named property, the numeric index type of the contextual type, if any,
or otherwise
o the string index type of the contextual type, if any.
In a contextually typed array literal expression containing no spread elements, an element
expression at index N is contextually typed by
o the type of the property with the numeric name N in the contextual type, if any, or
otherwise
o the numeric index type of the contextual type, if any.
In a contextually typed array literal expression containing one or more spread elements, an
element expression at index N is contextually typed by the numeric index type of the contextual
type, if any.
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