Why does let x = 1; { console.log(x); let x = 2; } throw ReferenceError in JavaScript?
Because the inner let x is hoisted (shadowing the outer) and is in the TDZ. It does not fall back to the outer x. Accessing it before the inner declaration line throws ReferenceError.
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More FAQs in Common Block Scope Bugs in JavaScript
TDZ access: using a let or const variable before its declaration line throws ReferenceError. This is intentional (to catch bugs), so the fix is to declare before use.
Because all cases in a switch share one block scope (unless wrapped in braces). A let in two cases is a re-declaration in the same scope. Fix by wrapping each case in its own block with braces.
Because i++ reassigns i, and const does not allow reassignment. Use let for classic for loop counters. const works in for...of and for...in because each iteration is a fresh binding.
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