• 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.
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