no-unsafe-negation

NOTE: this rule is part of the recommended rule set.

Enable full set in deno.json:

{
  "lint": {
    "rules": {
      "tags": ["recommended"]
    }
  }
}

Enable full set using the Deno CLI:

deno lint --rules-tags=recommended

This rule can be explictly included to or excluded from the rules present in the current tag by adding it to the include or exclude array in deno.json:

{
  "lint": {
    "rules": {
      "include": ["no-unsafe-negation"],
      "exclude": ["no-unsafe-negation"]
    }
  }
}

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:

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

Valid:

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

no-unsafe-negation

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