What is the difference between undefined and not defined in JavaScript?

In JavaScript, not defined are two different concepts related to variable handling. Here's the difference:

1. undefined

• A variable is undefined when it has been declared but has not been assigned a value.
• It is a primitive value that JavaScript assigns to declared but uninitialized variables.

• Example:

  let x;
  console.log(x); // Output: undefined (declared but not assigned)

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• A function that doesn’t explicitly return a value also returns undefined.

2. not defined (ReferenceError)

• A variable is not defined when it has not been declared at all in the current scope.
• Trying to access it throws a ReferenceError.

• Example:

  >

  console.log(y); // ReferenceError: y is not defined (never declared)

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Key Differences:

Example Comparing Both:

console.log(a); // ReferenceError: a is not defined (never declared)
let b;
console.log(b); // undefined (declared but no value assigned)

Fixing undefined vs not defined:

• undefined: Assign a value to the variable.

  let x = 10; // Now x is defined

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• not defined: Declare the variable before using it.

  let y; // Now y exists (even if undefined)
  console.log(y); // undefined (but no ReferenceError)

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Summary:

• undefined → Variable exists but has no value.
• not defined → Variable doesn’t exist (throws an error).

Would you like a practical example to clarify further? 😊