Prefer const over let

When defining variables, it is best to use const wherever possible. The const keyword signals to the next developer that the variable will not be reassigned. This reduces the amount of context that the developer needs to keep track of.

Below is a contrived example only using the let keyword to declare and define variables:

let myFunction = () => {
  let a = 1
  let b = 2
  let c = 3
  let d = 4
  let r

  a = a + b + c + d

  r = `The result is ${a}`

  return r
}

Here is the above example using const and let where appropriate:

const myFunction = () => {
  let a = 1
  const b = 2
  const c = 3
  const d = 4
  let r

  a = a + b + c + d

  r = `The result is ${a}`

  return r
}

In the second example, the const keyword frees up mental capacity allowing developers to understand the code more quickly and to think about opportunities to improve the code.