what is undefined vs defined

1. Undefined

• A variable is declared but not assigned any value.

• JavaScript automatically assigns the value undefined to such variables.

• It means: “The variable exists, but there is no value yet.”

Example:

var a;
console.log(a); // undefined

Here, a is declared, so it exists in memory, but it has no value assigned → undefined.

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2. Not Defined

• A variable is not declared at all in the scope.

• Trying to access it will cause a ReferenceError.

• It means: “The variable does not exist in memory.”

Example:

console.log(b); // ReferenceError: b is not defined

Here, b was never declared.

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3. Key Difference

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4. Example with Both

var x;
console.log(x); // undefined (declared but no value)

console.log(y); // ReferenceError: y is not defined

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5. Relation to Hoisting

• Variables declared with var are hoisted as undefined (not error).

• Variables declared with let/const are hoisted but remain in Temporal Dead Zone (TDZ), so accessing before declaration → ReferenceError (similar to "not defined").

Example:

console.log(a); // undefined (because of var hoisting)
var a = 10;

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 20;