Type guard narrows type any in a primitive type check #1621
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ahejlsberg
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Jan 8, 2015
Type guard narrows type any in a primitive type check
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Fixes #1595.
When a variable of type any appears in a
typeoftype guard that checks for one of the primtive types, the type of the variable is narrowed to that type. Type guards otherwise do not affect variables of type any.Rationale: Since narrowing a variable of type any potentially introduces errors (because the type checking becomes stronger), we can only do so when we know for sure that the variable is exactly of the type that was checked for (and couldn't for example have additional properties)--and the only case that is true is a
typeofcheck for a primitive type.