TypeError: "x" is (not) "y"
The JavaScript exception "x is (not) y" occurs when there was an
unexpected type. Oftentimes, unexpected undefined
or null
values.
Message
TypeError: Unable to get property {x} of undefined or null reference (Edge)
TypeError: "x" is (not) "y" (Firefox)
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
values,
you can use the typeof operator,
for example.
if (foo !== undefined) {
// Now we know that foo is defined, we are good to go.
}
if (typeof foo !== 'undefined') {
// The same good idea, but don't use this implementation - it can bring problems
// because of the confusion between truly undefined and undeclared variables.
}