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

The JavaScript exception "right-hand side of 'in' should be an object" occurs when the in operator was used to search in strings, or in numbers, or other primitive types. It can only be used to check if a property is in an object.

In this article

Message

TypeError: Cannot use 'in' operator to search for 'x' in 'y' (V8-based & Firefox) TypeError: right-hand side of 'in' should be an object, got null (Firefox) TypeError: "y" is not an Object. (evaluating '"x" in "y"') (Safari)

Error type

TypeError

What went wrong?

The in operator can only be used to check if a property is in an object. You can't search in strings, or in numbers, or other primitive types.

Examples

Searching in strings

Unlike in other programming languages (e.g., Python), you can't search in strings using the in operator.

js
"Hello" in "Hello World"; // TypeError: cannot use 'in' operator to search for 'Hello' in 'Hello World'

Instead you will need to use String.prototype.includes(), for example.

js
"Hello World".includes("Hello"); // true

The operand can't be null or undefined

Make sure the object you are inspecting isn't actually null or undefined.

js
const foo = null; "bar" in foo; // TypeError: cannot use 'in' operator to search for 'bar' in 'foo' (Chrome) // TypeError: right-hand side of 'in' should be an object, got null (Firefox)

The in operator always expects an object.

js
const foo = { baz: "bar" }; "bar" in foo; // false "PI" in Math; // true "pi" in Math; // false

Searching in arrays

Be careful when using the in operator to search in Array objects. The in operator checks the index number, not the value at that index.

js
const trees = ["redwood", "bay", "cedar", "oak", "maple"]; 3 in trees; // true "oak" in trees; // false

See also

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  1. JavaScript
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    50. SyntaxError: invalid identity escape in regular expression
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    52. SyntaxError: invalid property name in regular expression
    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
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    60. SyntaxError: missing ) after argument list
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    107. TypeError: can't delete non-configurable array element
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    110. TypeError: can't set prototype: it would cause a prototype chain cycle
    111. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    112. TypeError: class constructors must be invoked with 'new'
    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
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    117. TypeError: invalid 'instanceof' operand 'x'
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    120. TypeError: Iterator/AsyncIterator constructor can't be used directly
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  14. Misc
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    4. Iteration protocols
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    8. Deprecated features

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