Work out use of underscore in JS in this repo and document in front-end standards

[anselmbradford]

Moved from https://github.com/cfpb/consumerfinance.gov/issues/919

My understanding of the underscore convention is:

Use an underscore to designate a variable or function in the export scope (the nesting level where module.exports is called) as private, except if it's used to refer to an imported class or third-party naming convention (jquery, lodash, etc.).

That last bit is where it becomes unclear. Do imported packages that are not using a convention or are not a class get an underscore or not?

Pro: Makes it consistent with how other variables are used. Con: Things like var handlebars are not clear on their convention.

No underscore

Pro: Makes all imported modules consistently not use underscores. A variable without an underscore is either: an imported module, a function parameter, or a local variable. Cons: Not consistent with how other variables that don't get exported are defined in the export level scope.

Example file:

var $ = require( 'jquery' ); // No underscore needed b/c it uses a well-established convention name.
var Widget = require( 'Widget' ); // No underscore needed b/c it's a class instantiated with `new`.

var pkg = require( 'pkg' ); // No underscore needed b/c it's a third-party package to this module, name follows filename.

// or

var _pkg = require( 'pkg' ); // Underscore needed b/c it's a third-party package that doesn't have an established convention and it's not being exported.

var _globalVar = true; // Underscore needed b/c it's in the module.exports scope.

// Underscore needed for `_helper` b/c it's in the module.exports scope,
// no underscored needed for `thing` b/c it's in a local scope.
function _helper( thing ) {
    // private api implementation.
   var localVar = new Widget(); // No underscore needed b/c it's in the scope of _helper.
}

function Example() { // No underscore, b/c it's exported publicly.
    // public api implementation.
   _helper( 'stuff' );
}

module.exports = Example;

Which convention seems best to you?