• 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.
Vladimir Matveev
Sheetal Nandi
Josh Kalderimis
Daniel Rosenwasser
Yui T
Anders Hejlsberg
Jason Freeman
CyrusNajmabadi