JavaScript Scoping, Closures, and Polyfills

Problem Explain the difference between var and let, what a closure is, and what a polyfill does.

Be ready to discuss

• var: function-scoped, hoisted and initialised to undefined, redeclarable in the same scope, and attached to the global object at top level.
• let/const: block-scoped, hoisted into the temporal dead zone so access before declaration throws a ReferenceError, and not redeclarable in the same scope.
• The classic loop bug: var i in a for loop shares one binding across all iterations, while let i creates a fresh binding per iteration - and how the pre-ES6 fix was an IIFE.
• Closures: a function keeps a reference to its enclosing lexical environment even after the outer function returns, because the scope chain lives on as long as the inner function does.
• What closures are used for: data privacy and encapsulation, factory functions, module pattern, memoization, and partial application/currying.
• Closure pitfalls: unintended memory retention when a long-lived closure captures a large object.
• Polyfills: runtime code that implements a standard API missing from an older environment (e.g. Array.prototype.includes, Promise, Object.assign).
• Polyfill vs transpilation: Babel rewrites syntax, polyfills supply missing built-ins - and why feature detection should gate a polyfill rather than UA sniffing.