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)

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.

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

"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.

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

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

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.

const foo = { baz: "bar" };
"bar" in foo; // false

"PI" in Math; // true
"pi" in Math; // false

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.

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

• in