Message
TypeError: "x" is (not) "y" Examples: TypeError: "x" is undefined TypeError: "x" is null TypeError: "undefined" is not an object TypeError: "x" is not an object or null TypeError: "x" is not a symbol
Error type
What went wrong?
There was an unexpected type. This occurs oftentimes with undefined or null values.
Also, certain methods, such as Object.create() or Symbol.keyFor(), require a specific type, that must be provided.
Examples
Invalid cases
// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: foo is undefined
var foo = null;
foo.substring(1); // TypeError: foo is null
// Certain methods might require a specific type
var foo = {}
Symbol.keyFor(foo); // TypeError: foo is not a symbol
var foo = "bar"
Object.create(foo); // TypeError: "foo" is not an object or null
Fixing the issue
To fix null pointer to undefined or null values, you can use the typeof operator, for example.
if (typeof foo !== 'undefined') {
// Now we know that foo is defined, we are good to go.
}