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.
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
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