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let and const are two relatively new types of variable declarations in JavaScript.
let is similar to var in some respects, but allows users to avoid some of the
common "gotchas" that users run into in JavaScript. const is an augmentation of let in
that it prevents re-assignment to a variable.
let prevents variables from being used before their declaration.
var = bad
let = good
let = good
https://www.typescriptlang.org/docs/handbook/variable-declarations.html