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No Unsafe Negation

lint/rules/no-unsafe-negation.md

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Disallows the usage of negation operator ! as the left operand of relational operators.

! operators appearing in the left operand of the following operators will sometimes cause an unexpected behavior because of the operator precedence:

  • in operator
  • instanceof operator

For example, when developers write a code like !key in someObject, most likely they want it to behave just like !(key in someObject), but actually it behaves like (!key) in someObject. This lint rule warns such usage of ! operator so it will be less confusing.

Invalid:

<!-- deno-fmt-ignore -->
typescript
if (!key in object) {}
if (!foo instanceof Foo) {}

Valid:

typescript
if (!(key in object)) {}
if (!(foo instanceof Foo)) {}
if ((!key) in object) {}
if ((!foo) instanceof Foo) {}