TypeScript Language Specification v1.5

For purposes of property access and function calls, the apparent members (section 3.10.1) of a union type
U are those that are present in every one of its constituent types, with types that are unions of the
respective apparent members in the constituent types. The following example illustrates the merging of
member types that occurs when union types are created from object types.
interface A {
a: string;
b: number;
}
interface B {
a: number;
b: number;
c: number;
}
var x: A | B;
var a = x.a; // a has type string | number
var b = x.b; // b has type number
var c = x.c; // Error, no property c in union type
Note that 'x.a' has a union type because the type of 'a' is different in 'A' and 'B', whereas 'x.b' simply has
type number because that is the type of 'b' in both 'A' and 'B'. Also note that there is no property 'x.c'
because only 'A' has a property 'c'.
3.4.1 Contextual Union Types
When used as a contextual type (section 4.19), a union type U has those members that are present in any
of its constituent types, with types that are unions of the respective members in the constituent types.
Specifically:
Let S be the set of types in U that has a property P. If S is not empty, U has a property P of a union
type of the types of P from each type in S.
Let S be the set of types in U that have call signatures. If S is not empty and the sets of call
signatures of the types in S are identical ignoring return types, U has the same set of call
signatures, but with return types that are unions of the return types of the respective call
signatures from each type in S.
Let S be the set of types in U that have construct signatures. If S is not empty and the sets of
construct signatures of the types in S are identical ignoring return types, U has the same set of
construct signatures, but with return types that are unions of the return types of the respective
construct signatures from each type in S.
Let S be the set of types in U that has a string index signature. If S is not empty, U has a string
index signature of a union type of the types of the string index signatures from each type in S.
Let S be the set of types in U that has a numeric index signature. If S is not empty, U has a
numeric index signature of a union type of the types of the numeric index signatures from each
type in S.
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