v8 JavaScript performance implications of const, let, and var?

Question

Regardless of functional differences, does using the new keywords \'let\' and \'const\' have any generalized or specific impact on performance relative to \'var\'?

A

Answer 1

T.J. Crowder's answer is so excellent.

Here is an addition of: "When would I get the most bang for my buck on editing existing var declarations to const ?"

I've found that the most performance boost had to do with "exported" functions.

So if file A, B, R, and Z are calling on a "utility" function in file U that is commonly used through your app, then switching that utility function over to "const" and the parent file reference to a const can eak out some improved performance. It seemed for me that it wasn't measurably faster, but the overall memory consumption was reduced by about 1-3% for my grossly monolithic Frankenstein-ed app. Which if you're spending bags of cash on the cloud or your baremetal server, could be a good reason to spend 30 minutes to comb through and update some of those var declarations to const.

I realize that if you read into how const, var, and let work under the covers you probably already concluded the above... but in case you "glanced" over it :D.

From what I remember of the benchmarking on node v8.12.0 when I was making the update, my app went from idle consumption of ~240MB RAM to ~233MB RAM.