Using the keyword `let` to declare constants is *not* a general convention. The keyword `let` is used that way in Swift, but in Javascript it denotes a regular variable (just one with a special scope).

As for where the use of `let` to declare variables come from, it's a very old tradition for mathematical proofs:

[![A proof in group theory using lots of "let"s][1]][1]


  [1]: https://i.sstatic.net/YivXC.gif