• If either operand of a operator is nullable, and neither operand is of type any or string.
• If either operand of a <, >, <=, >=, or in operator is nullable.
• If the operand of a , -, ~, ++, or — unary operator is nullable.An operand is considered nullable if the type of the operand is null or undefined or a union type that includes null or undefined. Note that the union type case only only occurs in —strictNullChecks mode because and undefined disappear from unions in classic type checking mode.