TypeScript Language Specification v1.5

If the || expression is contextually typed (section 4.19), the operands are contextually typed by the same
type. Otherwise, the left operand is not contextually typed and the right operand is contextually typed by
the type of the left operand.
The type of the result is the union type of the two operand types.
Any Boolean Number String Other
Any Any Any Any Any Any
Boolean Any Boolean N | B S | B B | O
Number Any N | B Number S | N N | O
String Any S | B S | N String S | O
Other Any B | O N | O S | O Other
4.16 The Conditional Operator
In a conditional expression of the form
test ? expr1 : expr2
the test expression may be of any type.
If the conditional expression is contextually typed (section 4.19), expr1 and expr2 are contextually typed by
the same type. Otherwise, expr1 and expr2 are not contextually typed.
The type of the result is the union type of the types of expr1 and expr2.
4.17 Assignment Operators
An assignment of the form
v = expr
requires v to be classified as a reference (section 4.1) or as an assignment pattern (section 4.17.1). The expr
expression is contextually typed (section 4.19) by the type of v, and the type of expr must be assignable to
(section 3.10.4) the type of v, or otherwise a compile-time error occurs. The result is a value with the type
of expr.
A compound assignment of the form
v ??= expr
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