Does var inside an if block leak to the outer scope?
Yes. var 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.
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More FAQs in Hoisting in Loops and Conditionals in JavaScript
Because var is function-scoped. There is one i shared by all iterations. By the time the setTimeout callbacks run, the loop has finished and i equals the final value, so all callbacks log that value.
Because let is block-scoped and the spec gives each iteration a fresh binding. Each closure captures its own copy of i, so callbacks log 0, 1, 2 instead of the final value.
Use let instead of var (each iteration gets a fresh binding), or wrap the body in an IIFE that captures the current value of i as a parameter.
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