Toggle Function in JavaScript
Short and sweet is best.
Your statement

&quot;The main benefit I see here is that anybody reading the code can skim to the last line and know the overall purpose&quot;

That makes no sense at all, you say all that is needed to understand the functions is
$scope.emailList = emailListUpdate;

Nobody jumping into someone else code will just skim, the only people that skim code are those that know the code.
You can make a few assumptions.

All that read your code are competent coders.
All that read your code have read the project specs.
Every line of code will be read by a coder new to the code.

Example
The best code is brief as possible without being a code golf entrant.
Notes

Why innerHTML, should it not be textContent????
This function is not a toggle. It is based on selection value.
The ternary expression is too long, break the line so it does need to be scrolled
The ternary has a syntax error. Misplaced ;
the ; on the last line after &quot;}&quot; is redundant.

Code, best option.
$scope.selectEmailUsers = event =&gt; {
  $scope.selection = event.target.textContent;
  $scope.emailList = $scope.selection === &quot;Select All&quot; ? 
       $scope.users.filter(user =&gt; !user.report.emailed); : [];
  //                                                    ^ remove syntax error
}; // &lt;&lt; remove the ;