New: Array Config

designs/2019-array-config/README.md

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+- Start Date: 2019-05-12
+- RFC PR:
+- Authors: Toru Nagashima ([@mysticatea](https://github.com/mysticatea))
+
+# Array Config
+
+## Summary
+
+This proposal makes `.eslintrc` files allowing an array as the top-level config. This is a syntax sugar of `extends` property and `overrides` property to make configuration simpler. Also, this form is closer to ESLint internal structure about configuration (`ConfigArray`).
+
+## Motivation
+
+The current config file mixes all of parent configs, child configs, and config content. A property (`extends`) is the parent config. Another property (`overrides`) is child configs. And the other properties is the config content. This means that we can write the parent configs after the child configs as opposing to intuitive.
+
+The array notation makes this relationship simpler: "a later element in the array has precedence over an earlier element in the array."
+
+Also, if people used multiple kinds of files, it's more intuitive than `overrides` property.
+
+## Detailed Design
+
+Currently, config files don't allow an array at top-level.
+
+This RFC adds the support of the array. Each array element can be:
+
+1. a string that `extends` property supports.
+1. an object that is the same as the current config files.
+1. an object that is the same as the elements of `overrides` property.
+1. an array that is the same as top-level's. (nested)
+
+The config array factory loads strings as `extends` and normalizes the others recursively.
+
+<table><td>
+💡 <b>Example</b>:
+<pre lang="js">
+module.exports = {
+    extends: [
+        "eslint:recommended",
+        "plugin:node/recommended"
+    ],
+    rules: {
+        eqeqeq: "error"
+    },
+    overrides: [
+        {
+            files: "*.ts",
+            extends: "plugin:@typescript-eslint/recommended"
+        }
+    ]
+}
+</pre>
+is rewritable to:
+<pre lang="js">
+module.exports = [
+    "eslint:recommended",
+    "plugin:node/recommended",
+    {
+        rules: {
+            eqeqeq: "error"
+        }
+    },
+    {
+        files: "*.ts",
+        extends: "plugin:@typescript-eslint/recommended"
+    }
+]
+</pre>
+<code>extends</code> property will be still useful to apply shareable configs to limited files. (<code>extends</code> in <code>overrides</code> was implemented in <a href="https://github.com/eslint/eslint/pull/11554">eslint/eslint#11554</a>.)
+</td></table>
+
+## Documentation
+
+This should be described in the "Configuring ESLint" page.
+
+## Drawbacks
+
+This is mostly syntax sugar, so the worth of this enhancement is smaller.
+
+## Backwards Compatibility Analysis
+
+If people depend on the behavior that ESLint throws an error if they give an array as configuration, this enhancement breaks that. However, I believe that we don't need to worry.
+
+## Alternatives
+
+-
+
+## Open Questions
+
+-
+
+## Frequently Asked Questions
+
+-
+
+## Related Discussions
+
+- [#9]: https://github.com/eslint/rfcs/pull/9
+- [#13]: https://github.com/eslint/rfcs/pull/13