Chore: Test on Node.js 15

[btmills]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain: Add Node.js 15 to the build matrix

What changes did you make? (Give an overview)

Running npm install in the repository with Node.js 15 currently requires the --legacy-peer-deps requirement because npm is resolving eslint-plugin-eslint-plugin's eslint peer dependency version as undefined.

Is there anything you'd like reviewers to focus on?

We can figure out later if that's an issue on our end or npm's, but in the meantime this will let us run CI on Node.js 15. I'm hoping this is a temporary situation so didn't update the contributing documentation to mention the flag.

[btmills]

Replacing the file: devDependencies with the new npm v7 workspaces feature resolves (heh) the peer resolution issues. With this diff (and an upgrade to eslint-plugin-jsdoc@^25.0.0 with a bumped eslint@^7.0.0 peerdep), npm install with npm v7 works without --legacy-peer-deps. Unfortunately it's mutually exclusive: we can't keep the file: devDependencies for npm v6 and have workspaces for npm v7, so we won't be able to do this until we can require at least npm v7 for dev environments. --legacy-peer-deps it is for the time being. (Unless the eslint@undefined version ends up being an npm bug.)

diff --git package.json package.json
index 3a90c63da..7f6d578f3 100644
--- package.json
+++ package.json
@@ -98,10 +98,7 @@
     "dateformat": "^3.0.3",
     "ejs": "^3.0.2",
     "escape-string-regexp": "^3.0.0",
-    "eslint": "file:.",
-    "eslint-config-eslint": "file:packages/eslint-config-eslint",
     "eslint-plugin-eslint-plugin": "^2.2.1",
-    "eslint-plugin-internal-rules": "file:tools/internal-rules",
     "eslint-plugin-jsdoc": "^22.1.0",
     "eslint-plugin-node": "^11.1.0",
     "eslint-release": "^2.0.0",
@@ -145,5 +142,10 @@
   "license": "MIT",
   "engines": {
     "node": "^10.12.0 || >=12.0.0"
-  }
+  },
+  "workspaces": [
+    ".",
+    "packages/eslint-config-eslint",
+    "tools/internal-rules"
+  ]
 }

[mdjermanovic]

LGTM, thanks!

Running npm install in the repository with Node.js 15 currently requires the --legacy-peer-deps requirement because npm is resolving eslint-plugin-eslint-plugin's eslint peer dependency version as undefined.

Looks like a npm v7 bug to me.

[btmills]

Looks like a npm v7 bug to me.

Thanks for the double check on that. Opened npm/cli#2199 🤞