Closures
Function closures are one of the most powerful, yet most misunderstood, concepts of JavaScript that are actually really simple to understand. A closure refers to a function along with its lexical environment. It is essentially what allows us to return a function A, from another function B, that remembers the local variables defined in B, even after B exits. The idea of closures is employed in nearly every other JavaScript program, hence, it's paramount for a JavaScript developer to know it really well.
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