Defer windowWidth() to improve perf for non-help usage

[nolanlawson]

Hi 👋, I'm a big fan of yargs, so after talking with @lrlna about #521, I wanted to try my hand at improving the overall runtime of require('yargs').

I started by profiling with the Chrome dev tools using node-nightly and the --inspect option, where I noticed that windowWidth() is a very expensive function:

Since windowWidth() only has to be calculated when we need the column width, e.g. when the user has typed --help or -h, I figured we could lazily evaluate it. That's what this PR does.

Here is a very small benchmark to demonstrate the perf improvement:

time for i in {1..500}; do node -e 'require("./index")'; done

Running in node 5.12.0 on a 2013 Macbook Air, I got the following results (real time, averaged):

• Before: 122.14ms
• After: 108.746ms

Since that includes the cost of the Node process itself, we can also use performance-now to measure just the runtime of require('yargs'):

for ((i=0;i<500;i++)); do node -e 'var now = require("performance-now"); var s = now(); require("./index"); var e = now() - s; console.log(e)'; done | awk '{ sum+=$1 } END { print sum/NR }'

This gives:

• Before: 41.3002ms
• After: 28.5307ms

So in rough terms require('yargs') should be about ~30% faster with this PR, with an average of ~12ms shaved off each invocation of require('yargs'). (Obviously it doesn't make any difference when doing -h or --help, but my hunch is that that's not the average use case.)

Please let me know if I missed something in the style/coverage/etc. or if this PR is unfeasible for some other reason I didn't notice. Hope this helps!

[maxrimue]

This. is. awesome.
Thank you so much @nolanlawson 🎉

[nexdrew]

I would initialize this to false, for clarity.

[nolanlawson]

agreed, good catch!

[nexdrew]

Besides my nit-picky inline comments, this LGTM! Thanks @nolanlawson!

[nexdrew]

Me likey. 😺 Thanks again, @nolanlawson, this is great!

[bcoe]

@nolanlawson thank you very much for digging into this.

[lrlna]

woot!! Thanks so much @nolanlawson ✨ ✨

[bcoe]

@nolanlawson @lrlna want to give this a spin?

npm cache clear; yargs@6.0.0-alpha.1?

These performance changes go out with the next release (would love the extra set of eyes).

[lrlna]

@bcoe i am using that on a refactor of one of my projects right now; it's all good so far and I'll keep an eye :D