What is the fastest way to loop thorugh a long array

One way (but tricky) is using parallel functions, see: How to run async JavaScript functions in sequence or parallel[^]
This way you could use several functions that only process a 'chunk' of the array.

Also see: which-type-of-loop-is-fastest-in-javascript-ec834a0f21b9[^]

Quote:

What is the fastest way to loop thorugh a long array

First of all, a list of 121 is not long, long usually start after 100,000.
Whatever the flavor of the loop (for, while ...), by itself, it is fast.
What make a loop slow is what you do inside the loop.

The problem is that you told nothing of interest, we have no idea of what is the game or how you did the loop.

Before thinking to multithreading, you need to think about optimization.
Program optimization - Wikipedia[^]

Consider the response, above, that ultimately references you towards code optimization.

There are a few things you can do (no code, so these are rather general) to speed up processing.

0 - Do any calculations and value setting that are not modified within the loop outside of the loop. No need to repeatedly set a constant to the same value.
1 - if you're calculation values from a limited set (such as powers of 2, or something like that, use a lookup table instead of the calculations. If you can just use an index into a table of pre-calculated values you can speed things up by saving, potentially, an immense number of calculations. See, also, item 0
2 - if your loop has internal nested loops, see if you can "unroll them" into straight series of calculations. This is most feasible if you have a fixed number of elements for any the inner loops.
3 - If you have internal functions, if they're very simple then just plug in the code, instead. Calling a function and returning is a lot of overhead.

These are just a few and they don't all apply all the time. Use good judgement.