In JavaScript there are 3 different kinds of variables, global variables(using var), constants(const, and local variableslet). All of the act a little differently from each other. Allow me to explain what they do.
Global variables, made using var, are as they sound, global variables. Once defined, they can be used, edited, or redefined anywhere.
Constants, defined with const, are constant, they cannot change, at all.
Local variables, defined with let, are variables that can only be used in the place that it is defined, like a function, and it's children.
Now that we has that established, let's talk about how local variables got their name.
Turns out "let" is a mathematical term.
The "let" expression may also be defined in mathematics, where it associates a Boolean condition with a restricted scope. Quote from Wikipedia "Let expression" as of
It was first used in programming in early languages like Basic.
In case you're wondering, the main source is an answer on Stack Overflow: "Why was the name 'let' chosen for block-scoped variable declarations in JavaScript?", answer by exebook, edited by Pikamander2; link.