no-unsafe-negation.md

Pattern: Negation of left operand in relational expression

Issue: -

Description

Negating the left operand of in or instanceof operators is usually a mistake. Due to operator precedence, !key in object is evaluated as (!key) in object, when the intent was likely !(key in object). Similar issues occur with instanceof.

Examples

Example of incorrect code:

if (!key in object) {
  // Evaluated as (!key) in object
}

if (!obj instanceof Class) {
  // Evaluated as (!obj) instanceof Class
}

if (!a in b && !c in d) {
  // Multiple unsafe negations
}

const result = !number in range;

Example of correct code:

if (!(key in object)) {
  // Properly negates membership test
}

if (!(obj instanceof Class)) {
  // Properly negates instance check
}

const result = !(number in range);

// Or rewrite without negation
if (obj instanceof Class === false) {
  // Alternative to negation
}