The answer
It is the message in a ReferenceError. It means the variable x does not exist in any accessible scope: it was never declared, is misspelled, is out of scope, or was not imported.
JavaScript: not defined vs ReferenceError vs TypeError
What does 'x is not defined' mean in JavaScript?
More FAQs in JavaScript: not defined vs ReferenceError vs TypeError
ReferenceError is thrown when a variable does not exist or is in the TDZ. TypeError is thrown when a value is the wrong type for the operation, like calling a non-function or reading a property of null or undefined.
Because the var is hoisted with undefined, so the variable exists (no ReferenceError). But undefined() throws TypeError: x is not a function, because the value is not callable.
An error thrown during parsing, before any code runs. Malformed code like const x = ; causes it. The entire script or module does not execute when there is a SyntaxError.
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