Allow setting ecmaVersion to latest

What version are you using? 3.9.1

Problem I want to solve I want to be able to always use the latest available ecmaVersion

Proposed solution Allow 'latest' as a config option for ecmaVersion, aliasing whatever the latest version the included acorn supports. Kinda like https://babeljs.io/docs/plugins/preset-latest/

[sindresorhus]

@ilyavolodin I always want the latest syntax. Browsers have versionless JS. I don’t see why ESLint shouldn’t at least have the ability to opt into such. IMHO latest should have been the default.

This seems like a micro-optimization that requires a lot of thought and effort to solve the issue that takes less than 1 second to fix by hands.

Micro-optimization to you, better UX for many others. Easiest solution would just be to hard-code a list of supported ECMAScript versions. It only needs updating once a year and then you’re in control of what latest means for ESLint.

1 second * ESLint users * configs, adds up.

[kaicataldo]

I’m still very much against tying this to Espree’s release cycle, even with a disclaimer, for a few reasons:

• I feel like just having this option encourages users to use it. It sounds very appealing to set the version once and never have to worry about it again. However, there is always a delay between when Espree lands a new ecmaVersion and when ESLint fully supports the new features, because rules need to be updated for the new syntax. This means that there’s a very high probability that ESLint is broken for those users for that duration. Example: see the following changes that had to be made to ensure async/await worked for all of ESLint’s core rules. You could argue that the user doesn’t have to use the new features - but then what’s the point of the option? I’m not a fan of knowingly giving users an option that has a high probability of breaking their build.
• Tying latest to Espree’s release cycle means that two users (or even two builds by the same person) running the same version of ESLint could potentially have been linted with the parser set to two different ecmaVersions. How can one have confidence in a quality assurance check if it’s not deterministic?

The “eslint:all” configuration is different because it only changes on new releases of ESLint. To be clear: I’m not against the inclusion of this option, but I do think it should be tied to ESLint’s release cycle.