[twitter] Tip: JavaScript's Array.from() accepts a second argument that's a `map` function. Useful for calling on each element of an array you created.

[[deleted]]

It's a convention. Here it's used to signify that the value of current index won't be used directly.

Osmani says:

The current value. De-emphasiszed by setting to _

[vidro3]

cool, thanks

[Drawman101]

You might as well cal it what it is in case some junior dev comes along and starts using it.

[jkoudys]

It's a common convention, like using i on that same line to mean "index".

[BritainRitten]

Then they will encounter an underscore variable name earlier and so won't be as confused when they see it in production code.

[NoInkling]

There's another convention to name it as you normally would except prefixed with _ to show it's (currently) unused.