Consider using const and let about apm-agent-nodejs HOT 7 CLOSED

cc @Qard

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I've been using const/let in all my personal code for quite awhile now.

The nice thing about const/let is that you can't rebind them in keyword form, so you don't get confusing errors when you accidentally do something like this:

var thing = 1
if (condition) {
  var thing = 2
  useInternalThing(thing)
}
useExternalThing(thing)

If the var was let or const, it'd throw an error complaining about rebinding. With var it'll just happily replace the existing reference.

My personal style is to make everything const until proven it needs to be mutable. Linters are generally pretty good about detecting that and complaining when your code tries to mutate a const.

The performance issues are largely conditional, the main one being that reassigning a let in a loop body used to deopt. (NOTE: loop body, not loop condition statements, so that does not include the incrementer component.)

FYI: A simple search and replace wouldn't work as I know there are at least a few cases like if (condition) { var foo = 'bar' } that would no longer get hoisted out of the block.

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@Qard thanks :) Out of curiosity, do you remember when the let loop body reassignment issue was fixed? Is that still the case in Node 6 for instance?

I've also started using const and let now in my personal code... it took me quite a while 😅

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If I recall correctly, it was fixed in early 6.x releases. I don't remember the exact version, but not 6.0.0.

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We should probably wait with this until we drop support for 4.x.

Would be interesting to get some numbers on this as well: https://twitter.com/stephenbelanger/status/952988762421706753

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Note to self: If we make a PR with these changes, we should benchmark it against master to see if there's any performance gains or degradations

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We agreed to start using let and const on new code and refactors. We won't do a major refactor of all the code.

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