The change to the install script to require TypeScript works fine when
installing from npm (because on npm the `lib` directory with the
compiled code already exists) but doesn't if you install from a GitHub
URL. By default it seems npm uses the `files` list when you install from
GitHub which means it's missing a bunch of files that we need to
compile.
Additionally by default when installing from a GitHub URL npm doesn't
install the dependencies which is an issue for us when we need to
compile TypeScript.
The fix is to create a `prepare` script that runs TypeScript if
required. From the npm docs [1]:
> `prepare`: Run both BEFORE the package is packed and published, on
> local npm install without any arguments, and when installing git
> dependencies
And from the npm docs on install [2], it confirms that if a package has
a `prepare` script it is run when installing from GitHub:
> As with regular git dependencies, dependencies and devDependencies
> will be installed if the package has a prepare script, before the
> package is done installing.
Despite having the `prepare` script we still need the TypeScript check
in `install.js` to satisfy the 3rd scenario below where we need to force
a compile:
* If I'm a user installing `puppeteer@X` from npm, the module is
published with the `lib/` directory of compiled code, so I'm set.
* If I'm a user installing Puppeteer from GitHub, the `prepare` script
will run TypeScript for me so I'm set.
* If I'm a developer working on Puppeteer, the `prepare` script also
runs but _after_ `npm install` which means `install.js` fails as it
requires `./lib/helper.js`. So in `install.js` we call
`compileTypeScriptIfRequired` to catch this case.
[1]: https://docs.npmjs.com/misc/scripts
[2]: https://docs.npmjs.com/cli/install
Co-authored-by: Mathias Bynens <mathias@qiwi.be>
Fixes#5660.
