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JavaScript used the in operator on a non-object value

TypeError: cannot use 'in' operator to search for 'x' in 'y'

JavaScript rejected the in operator because the right-hand side is not an object that can be searched for properties.

TypeError: cannot use 'in' operator to search for 'x' in 'y'operators

Observed message

TypeError: cannot use 'in' operator to search for 'x' in 'y'

Why it happens

  • The right-hand side of in is null or undefined.
  • The code uses in with a primitive where an object was expected.
  • Data shape assumptions broke before the membership check.

How to fix it

  1. Check that the right-hand side is an object before using in.
  2. Guard against null and undefined explicitly.
  3. Use alternative checks when working with arrays, strings, or other primitives.

Official reference

MDN: TypeError: cannot use in operator to search for x in y

Related errors

SyntaxError: cannot use ?? unparenthesized within || and && expressionsJavaScript mixed ?? with || or && without parenthesesReferenceError: can't access lexical declaration before initializationJavaScript accessed a let or const binding before initializationReferenceError: assignment to undeclared variableJavaScript assigned to an undeclared variableTypeError: BigInt value can't be serialized in JSONJavaScript BigInt value cannot be serialized in JSON

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