How to speed up Javascript builds on hosted VSTS agents

Synopsis

npm install can take somewhere between 8-10 minutes when there are a lot
of packages to be installed. I suspect npm install is CPU bound and network bound.

You can get around this by checking in your node_modules directory. However
this clutters up your source repository, and causes merge difficulty when node_modules
are added or removed.

Solution

Create another repository and commit your node_modules, then use git submodule
to link these two repositories together.

Make sure you are using a 64bit version of nodejs.exe:
```
 $ node -p process.arch
 x64
```
Create a secondary git repository For example https://github.com/teyc/vsts-npm-demo-submodule
Copy the package.json and package-lock.json from the primary repository
npm install to restore packages

In the primary repository, add a submodule reference

git submodule add https://github.com/teyc/vsts-npm-demo-submodule.git vsts-npm-demo-submodule

In VSTS, enable submodule to be checked out
Replace the npm install step with a Powershell step to move the directory
```
Move-Item vsts-npm-demo-submodule/node_modules .
```

Potential issues

node-sass

node-sass can be very stubborn even when package-lock.json is used. It uses a different
version of binding.node depending on CPU architecture and version of nodejs that is running
on the build server.

You may have to commit every version of node_modules/node-sass/vendor/win32-x64-??/binding.node in order for your build
to work. See: https://github.com/sass/node-sass/releases

Tips and tricks

Save a draft copy of the VSTS build definition, and work off there to avoid
disrupting normal builds
Disable irrelevant build steps while you are testing
Git submodules refer to a particular commit. If you check in new node modules
you must update the git submodule hash in your primary repository.
```
git submodule init vsts-npm-demo-submodule
git submodule update --remote
git add vsts-npm-demo-submodule
git commit -m 'Update node_modules'
```