• A type guard of the form expr1 || expr2
o when true, narrows the type of x to T1 | T2, where T1 is the type of x narrowed by expr1 when true, and T2 is the type of x narrowed by expr1 when false and then by expr2 when true, or
o when false, narrows the type of x by expr1 when false and then by expr2 when false.
• A type guard of the form expr1 && expr2
o when true, narrows the type of x by expr1 when true and then by expr2 when true, or
o when false, narrows the type of x to T1 | T2, where T1 is the type of x narrowed by expr1 when false, and T2 is the type of x narrowed by expr1 when true and then by expr2 when false.
• A type guard of the form typeof x === s, where s is a string literal with the value ‘string’, ‘number’, or ‘boolean’,
o when true, narrows the type of x to the given primitive type, or
o when false, removes the primitive type from the type of x.
• A type guard of the form typeof x === s, where s is a string literal with any value but ‘string’, ‘number’, or ‘boolean’,
o when true, removes the primitive types string, number, and boolean from the type of x, or
o when false, has no effect on the type of x.
• A type guard of the form typeof x !== s, where s is a string literal,
o when true, narrows the type of x by typeof x === s when false, or
o when false, narrows the type of x by typeof x === s when true.
• A type guard of the form x instanceof C, where C is of a subtype of the global type ‘Function’ and C has a property named ‘prototype’
o when true, narrows the type of x to the type of the ‘prototype’ property in C provided it is a subtype of the type of x, or
o when false, has no effect on the type of x.
• In the right operand of a || operation, the type of a variable or parameter is narrowed by any type guard in the left operand when false, provided the right operand contains no assignments to the variable or parameter.
• In the right operand of a && operation, the type of a variable or parameter is narrowed by any type guard in the left operand when true, provided the right operand contains no assignments to the variable or parameter.
• In the true expression of a conditional expression, the type of a variable or parameter is narrowed by any type guard in the condition when true, provided the expression contains no assignments to the variable or parameter.
• In the false expression of a conditional expression, the type of a variable or parameter is narrowed by any type guard in the condition when false, provided the expression contains no assignments to the variable or parameter.
The type of a variable or parameter is narrowed in the following situations:
• In the true branch statement of an ‘if’ statement, the type of a variable or parameter is narrowed by any type guard in the ‘if’ condition when true, provided the if statement contains no assignments to the variable or parameter.
• In the false branch statement of an ‘if’ statement, the type of a variable or parameter is narrowed by any type guard in the ‘if’ condition when false, provided the if statement contains no assignments to the variable or parameter.
if f is a contextually typed function expression (section 4.9.3), the inferred return type is the union type (section 3.3.4) of the types of the return statement expressions in the function body, ignoring return statements with no expressions.
Otherwise, the inferred return type is the first of the types of the return statement expressions in the function body that is a supertype (section 3.8.3) of each of the others, ignoring return statements with no expressions. A compile-time error occurs if no return statement expression has a type that is a supertype of each of the others.
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.
A union type literal is written as a sequence of types separated by vertical bars.
UnionType:
PrimaryOrUnionType | PrimaryType
A union typle literal references a union type.
When function or constructor types are included in union types they must be enclosed in parentheses
• If each type in U has a string index signature, U has a string index signature of a union type of the types of the string index signatures from each type in U.
• If each type in U has a numeric index signature, U has a numeric index signature of a union type of the types of the numeric index signatures from each type in U.
If each type in U has construct signatures and the sets of construct signatures 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 U.
If each type in U has call signatures and the sets of call signatures 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 U.
• 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.
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.
