Why should you care?

This explains why console.log(x) might print undefined with var but throw with let — a common interview and debugging topic.

See it live — copy this example

Paste into an HTML file or the browser console (F12). Use Run below when the live editor is available.

console.log(a); // undefined
var a = 5;

// console.log(b); // ReferenceError
let b = 10;

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What happened?

• var a is hoisted and starts as undefined until assignment.
• let b cannot be used before its declaration line.

Practice next

1. Run the var example and note undefined before assignment.
2. Try the let example and read ReferenceError.
3. Declare function hoisted() {} after a call — see it works.
4. Add a var b = 2 after console.log(b) in the same block.
5. Use a function declaration called before its definition in the file.

Remember

var is hoisted as undefined let/const cannot be used before declaration Declare variables before use

ScriptVerse legacy script

Maintainers refactor old var-heavy code where variables were used before declaration, causing subtle undefined bugs.

Outcome: Understanding hoisting explains mysterious undefined values in production logs.