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: Cannot read properties of undefined (reading 'x') (V8-based) TypeError: "x" is undefined (Firefox) TypeError: "undefined" is not an object (Firefox) TypeError: undefined is not an object (evaluating 'obj.x') (Safari) TypeError: "x" is not a symbol (V8-based & Firefox) TypeError: Symbol.keyFor requires that the first argument be a symbol (Safari)
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
js
// undefined and null cases on which the substring method won't work
const foo = undefined;
foo.substring(1); // TypeError: foo is undefined
const foo = null;
foo.substring(1); // TypeError: foo is null
// Certain methods might require a specific type
const foo = {};
Symbol.keyFor(foo); // TypeError: foo is not a symbol
const 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 test if the value is undefined
or null
first.
js
if (foo !== undefined && foo !== null) {
// Now we know that foo is defined, we are good to go.
}
Or, if you are confident that foo
will not be another falsy value like ""
or 0
, or if filtering those cases out is not an issue, you can simply test for its truthiness.
js
if (foo) {
// Now we know that foo is truthy, it will necessarily not be null/undefined.
}