Allow ecmaVersion: latest

[nzakas]

As of v8.0.0, Acorn now allows ecmaVersion: latest, which automatically sets ecmaVersion to the latest available version implemented.

Can we also allow this for Espree?

[aladdin-add]

to clarify: ecmaVersion: "latest" is as the same as ecmaVersion: require("espree").latestEcmaVersion. not passing the "latest" to acorn options?

[nzakas]

No, Acorn supports “latest” so we can pass that directly through from Espree.

[mysticatea]

How about making the latest as default?

I'm not sure if ES5 is the most popular environment now. Plus, eslint-plugin-es or something like provides more readable errors instead of "unexpected token" if people want to restrict to an old environment.

[aladdin-add]

How about making the latest as default?

I'm not sure if ES5 is the most popular environment now. Plus, eslint-plugin-es or something like provides more readable errors instead of "unexpected token" if people want to restrict to an old environment.

As of now, I don't think we can go that far. maybe we can reconsider it after the "latest" option get landed.

[aladdin-add]

No, Acorn supports “latest” so we can pass that directly through from Espree.

• espree has exported a latestEcmaVersion;
• acorn option "latest" is 1e8 https://github.com/acornjs/acorn/blob/43c989c9898b1a8c70eeae0f4728f3255825b05a/acorn/src/options.js#L103-L104

it could cause potential confusing if the two values are not equal.

[nzakas]

@aladdin-add what problems are you thinking of? The value inside Acorn doesn’t have any effect on ESLint.

I think changing the default to latest makes sense. This should not affect ESLint because we are manually setting ecmaVersion in ESLint itself.

[aladdin-add]

an example: the latest espree's latestEcmaVersion is 2021, but passing {"ecmaVersion": "latest"} will also allow es2022 features.

[nzakas]

@aladdin-add Acorn doesn't implement experimental features, so I'm not sure if that's true. Can you give an example where Acorn is supporting something after 2021?

[aladdin-add]

class fields - it has been supported in acorn v8.2.0, but not for espree yet (#486 has not been merged & released).

[nzakas]

Ah right. Then we should update latestEcmaVersion too before the next Espree release.

[mysticatea]

Yes. #486 updates latestEcmaVersion (tests/lib/supported-ecmaversions.js). It comes from the last value of supportedEcmaVersions. Therefore, we cannot support new ecma versions unless latestEcmaVersion update.

[nzakas]

Initial implementation here: #499

Note: this issue isn't yet accepted, so I just sent the PR as a draft for now.

[mdjermanovic]

No, Acorn supports “latest” so we can pass that directly through from Espree.

Just to double-check, as it isn't clear from the discussion - we decided to pass Espree's latestEcmaVersion (number) instead of "latest" to Acorn?

[mdjermanovic]

I think changing the default to latest makes sense. This should not affect ESLint because we are manually setting ecmaVersion in ESLint itself.

If user doesn't specify parserOptions.ecmaVersion in their ESLint config, ESLint will pass ecmaVersion: undefined to Espree, so in that case Espree's default applies. This change would be then a breaking change for ESLint users (default for ecmaVersion changes from 5 to the latest).

[mdjermanovic]

Do we intend to allow specifying ecmaVersion: "latest" in ESLint config? I think that's the idea in rfcs#79 - 3. Remove the usage of eslint & espree.

A problem with ecmaVersion: "latest" is that eslint-scope also expects ecmaVersion. ESLint calls eslintScope.analyze with parserOptions.ecmaVersion from the configuration (or 5 if it wasn't specified).

https://github.com/eslint/eslint/blob/master/lib/linter/linter.js#L607-L620

If we pass "latest" from ESLint, scope will be analyzed as if it was ecmaVersion: 3.

[aladdin-add]

to avoid break eslint, sounds we should explicitly normalize the ecmaVersion to 5 in eslint (if not specified).

as for "latest", we have 2 choices:

• convert "latest" to espree.latestEcmaVersion, and passing it to eslintScope.analyze
• pass "latest" to eslintScope.analyze (it need updates to allow the value)

[nzakas]

Yes, the idea is to allow "latest" in ESLint configs eventually, but that doesn't have to be done along with this change in Espree. We can always address that later and just make sure that ESLint defaults to ecmaVersion: 5 when no other setting is present. We don't have to allow "latest" right away in ESLint (I'm pretty sure it will be a validation error right now).

The value for ecmaVersion in eslint-scope is only to enable let/const block-scope evaluations. It just checks if ecmaVersion > 5. As @aladdin-add pointed out, we can always normalize "latest" in ESLint to espree.latestEcmaVersion to avoid that.

I don't think any of this should block enabling "latest" for Espree, though.

[aladdin-add]

to allow "latest" in ESLint configs, this is not a must. I have made a PR to implement it: eslint/eslint#14622

[mdjermanovic]

I support both "latest" and changing the default to latest (assuming that we will update ESLint to keep ecmaVersion: 5 as ESLint's default) 👍