The JavaScript exception "x is (not)y" occurs when there was anunexpected type. Oftentimes, unexpectedundefined ornullvalues.

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

TypeError.

What went wrong?

There was an unexpected type. This occurs oftentimes withundefined ornull values.

Also, certain methods, such asObject.create() orSymbol.keyFor(), require a specific type, that must be provided.

Examples

Invalid cases

You cannot invoke a method on anundefined ornull variable.

const foo = undefined;foo.substring(1); // TypeError: foo is undefinedconst foo2 = null;foo2.substring(1); // TypeError: foo2 is null

Certain methods might require a specific type.

const foo = {};Symbol.keyFor(foo); // TypeError: foo is not a symbolconst foo2 = "bar";Object.create(foo2); // TypeError: "foo2" is not an object or null

Fixing the issue

To fix null pointer toundefined ornull values, you can test if the value isundefined ornull first.

if (foo !== undefined && foo !== null) {  // Now we know that foo is defined, we are good to go.}

Or, if you are confident thatfoo will not be anotherfalsy value like"" or0, or if filtering those cases out is not an issue, you can simply test for its truthiness.

if (foo) {  // Now we know that foo is truthy, it will necessarily not be null/undefined.}

See also

• undefined
• null