unsafeNegations

Reports negating the left operand of in or instanceof relations.

✅ This rule is included in the ts untyped presets.

Negating the left-hand side of in or instanceof changes the meaning due to operator precedence. Use !(left in right) or !(left instanceof Right) to negate the relation as intended.

Examples

❌ Incorrect

if ((!key) in object) {
Error ts(2322)  ― Type 'boolean' is not assignable to type 'string | number | symbol'.
}

if ((!object) instanceof Constructor) {
Error ts(2358)  ― The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
}

if ((!key) in object) {
Error ts(2322)  ― Type 'boolean' is not assignable to type 'string | number | symbol'.
}

✅ Correct

if (!(key in object)) {
}

if (!(object instanceof Constructor)) {
}

if (key in object) {
}
if (object instanceof Constructor) {
}

Options

This rule is not configurable.

When Not To Use It

If your codebase never negates in or instanceof relations, this rule may provide limited value. However, keeping it enabled prevents subtle precedence mistakes that change control-flow conditions.

Further Reading

• MDN: in operator
• MDN: instanceof operator
• MDN: Operator precedence

Equivalents in Other Linters

• Biome: noUnsafeNegation
• Deno: no-unsafe-negation
• ESLint: no-unsafe-negation
• Oxlint: eslint/no-unsafe-negation