Improve typing of && operator with --strictNullChecks #8949
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This PR improves the types produced by the
&&
operator in strict null checking mode. Given operandsa
andb
of types A and B, the operationa && b
produces a value of type A' | B where A' is the type formed by removing all but the possibly-falsy types from A. The possibly-falsy types arestring
,number
,boolean
,void
,undefined
, andnull
. This change in particular accounts for the situations where the left operand is""
,0
, orfalse
which previously weren't reflected in the resulting types.Some examples:
Fixes #8892.