Add option to the `prefer-template` rule to only warn on concat with variables

~/dev
❯ eslint --version
v2.13.1

~/dev
❯ echo "var key = 'fooBar';var foo = '--' + key.replace(/[A-Z]/g, '-$&').toLowerCase();" | eslint --stdin --rule='prefer-template:2'

<text>
  1:30  error  Unexpected string concatenation  prefer-template

✖ 1 problem (1 error, 0 warnings)

Problem

The following:

var key = 'fooBar';
var foo = '--' + key.replace(/[A-Z]/g, '-$&').toLowerCase();

Would be very unreadable in a template string:

var key = 'fooBar';
var foo = `--${key.replace(/[A-Z]/g, '-$&').toLowerCase()}`;

Request

I would like an option to only have template literals enforced when concatenating a string with variables, like:

var key = 'fooBar';
var foo = '--' + key;

⬇️

var key = 'fooBar';
var foo = `--${key}`;

[platinumazure]

@sindresorhus I’m not really convinced by the new example-- seems to me that this is quite readable:

var foo = `${chalk.red.bold('I')} like ${chalk.blue.bold('unicorns')} and ${chalk.yellow.bold('ponies')}`;

Seems to me it’s honestly really case by case (because I agree your first example can be pretty hard to read). I just feel like assuming “variables” should be the only thing that should go in templates might be overly simplistic, certainly subjective (e.g., why not MemberExpressions, or maybe CallExpressions with no arguments?). I honestly still think at this point that your best bet is to use // eslint-disable-line prefer-template in those cases where you think a template is less readable.

[platinumazure]

Between @vitorbal’s suggestion of using an intermediate variable, the existence of inline config comments, and our new rule change guideline “Is the rule clearly incomplete without the requested enhancement?”, I’m 👎 on this proposal.