Other way round. They're implementing a language of the LISP family, which tend to use let this way. LISP languages originated around 1960. The use of let then

Other way round. They're implementing a language of the LISP family, which tend to use let this way. LISP languages originated around 1960. The use of let then caught on in other languages, including OCaml and Haskell, and eventually JavaScript. (It's possible some language other than LISP was the first to use let, but my suspicion is it was LISP.) https://en.wikipedia.org/wiki/Lisp_(programming_language) https://en.wikipedia.org/wiki/Lisp_(programming_language)