str.indexOf is not a function

The error message "str.indexOf is not a function" usually occurs when you try to use the `indexOf()` method on a variable that is not a string.

Here are some possible reasons for this error:

1. The variable `str` is not defined or is `undefined`.

2. The variable `str` is defined, but it is not a string. For example, it could be a number, boolean, or object.

3. The `indexOf()` method is not defined on the prototype of the object assigned to the variable `str`.

To fix this error, make sure that the variable `str` is defined and is a string, and that the `indexOf()` method is available on the string prototype. You can also use the `typeof` operator to check the type of the variable before using any string methods on it.