The answer
Namespace pollution (variables linger after the block), bugs (accidentally reading or overwriting a finished block's variable), the classic closure loop bug, and accidental globals (var at the top level becomes a property of window).
var Leaking From Blocks and Loops in JavaScript
What problems does var leaking from blocks cause in JavaScript?
More FAQs in var Leaking From Blocks and Loops in JavaScript
Because var is function-scoped, not block-scoped. It ignores block boundaries (except function bodies). So a var inside an if is hoisted to the enclosing function or global scope. let and const are block-scoped and do not leak.
Because var is function-scoped. The loop variable exists in the enclosing function or global scope, not in the loop block. After the loop, the variable is still accessible. let is block-scoped and stays in the loop.
let is block-scoped and creates a fresh binding per iteration. Each callback closes over its own copy of i. With var, all callbacks share one i, which is the final value by the time they run.
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