A few things to consider, for any language, then a couple of things specific to JS.

1. Inside if statements etc are they in the correct order. i.e. starting with mostly likely to occur descending to least likely to occur.
2. Are you doing any calculations that aren't required.
3. Is your code overly abstracted?
4. Can you offload any of the work? Specifically for JS this would take the form of web workers.
5. Does the code utilise too many 3rd party libraries?
6. For the code that does require a 3rd part library - is that code optimised as well or is it just thrown together?

Specifics for javascript:
Check out this comment I gave for a very similar question: hashnode.com/post/what-is-difference-cjy15gkwy002…

You can also manually check by comparing performance.now() values either side of a section of code execution. Then refactor some parts and then try again and iterate.

Maneesh,

I think, you are asking more on code optimization than customisation (title says, customisation).

In my opinion, to optimize, we need to first understand the scope of optimization and where/what to optimize. There are tools available to help us.

• One that I like the Browser Performance tool. For example with Chrome we get this, developers.google.com/web/tools/chrome-devtools/e…
• Also look into the lighthouse capabilities provided by Chrome. It shows the optimization scope for overall app.

Apart from the tools indicating the optimization scope, following standard coding practices and design principles would also direct us towards it.