TypeScript Language Specification v1.5

3.4 Union Types
Union types represent values that may have one of several disjoint representations. A value of a union
type A | B is a value that is either of type A or type B. Union types are written using union type literals
(section 3.7.6).
A union type encompasses an unordered set of unrelated types (that is, types that aren't subtypes of each
other). The following rules govern union types:
A | B is equivalent to A if B is a subtype of A.
A | B is equivalent to B | A.
AB | C is equivalent to A | BC, where AB is A | B and BC is B | C.
Union types are reduced to the smallest possible set of constituent types using these rules.
Union types have the following subtype relationships:
A union type U is a subtype of a type T if each type in U is a subtype of T.
A type T is a subtype of a union type U if T is a subtype of any type in U.
Similarly, union types have the following assignability relationships:
A union type U is assignable to a type T if each type in U is assignable to T.
A type T is assignable to a union type U if T is assignable to any type in U.
The || and conditional operators (section 4.15.7 and 4.16) may produce values of union types, and array
literals (section 4.6) may produce array values that have union types as their element types.
Type guards (section 4.20) may be used to narrow a union type to a more specific type. In particular, type
guards are useful for narrowing union type values to a non-union type values.
In the example
var x: string | number;
var test: boolean;
x = "hello";
// Ok
x = 42;
// Ok
x = test;
// Error, boolean not assignable
x = test ? 5 : "five"; // Ok
x = test ? 0 : false; // Error, number | boolean not asssignable
it is possible to assign 'x' a value of type string, number, or the union type string | number, but not any
other type. To access a value in 'x', a type guard can be used to first narrow the type of 'x' to either string
or number:
var n = typeof x === "string" ? x.length : x; // Type of n is number
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