feat: add meta.defaultOptions

docs/src/extend/custom-rules.md

@@ -60,6 +60,8 @@ The source file for a rule exports an object with the following properties. Both
 
 * `schema`: (`object | array | false`) Specifies the [options](#options-schemas) so ESLint can prevent invalid [rule configurations](../use/configure/rules). Mandatory when the rule has options.
 
+* `defaultOptions`: (`array`) Specifies [default options](#option-defaults) for the rule. If present, any user-provided options in their config will be merged on top of them recursively.
+
 * `deprecated`: (`boolean`) Indicates whether the rule has been deprecated.  You may omit the `deprecated` property if the rule has not been deprecated.
 
 * `replacedBy`: (`array`) In the case of a deprecated rule, specify replacement rule(s).
@@ -796,6 +798,51 @@ module.exports = {
 
 To learn more about JSON Schema, we recommend looking at some examples on the [JSON Schema website](https://json-schema.org/learn/), or reading the free [Understanding JSON Schema](https://json-schema.org/understanding-json-schema/) ebook.
 
+#### Option Defaults
+
+Rules may specify a `meta.defaultOptions` array of default values for any options.
+When the rule is enabled in a user configuration, ESLint will recursively merge any user-provided option elements on top of the default elements.
+
+For example, given the following defaults:
+
+```js
+export default {
+    meta: {
+        defaultOptions: [{
+            alias: "basic",
+        }],
+        schema: {
+            type: "object",
+            properties: {
+                alias: {
+                    type: "string"
+                }
+            },
+            additionalProperties: false
+        }
+    },
+    create(context) {
+        const [{ alias }] = context.options;
+
+        return { /* ... */ };
+    }
+}
+```
+
+The rule would have a runtime `alias` value of `"basic"` unless the user configuration specifies a different value, such as with `["error", { alias: "complex" }]`.
+
+Each element of the options array is merged according to the following rules:
+
+* Any missing value or explicit user-provided `undefined` will fall back to a default option
+* User-provided arrays and primitive values other than `undefined` override a default option
+* User-provided objects will merge into a default option object and replace a non-object default otherwise
+
+Option defaults will also be validated against the rule's `meta.schema`.
+
+**Note:** ESLint internally uses [Ajv](https://ajv.js.org) for schema validation with its [`useDefaults` option](https://ajv.js.org/guide/modifying-data.html#assigning-defaults) enabled.
+Both user-provided and `meta.defaultOptions` options will override any defaults specified in a rule's schema.
+ESLint may disable Ajv's `useDefaults` in a future major version.
+
 ### Accessing Shebangs
 
 [Shebangs (#!)](https://en.wikipedia.org/wiki/Shebang_(Unix)) are represented by the unique tokens of type `"Shebang"`. They are treated as comments and can be accessed by the methods outlined in the [Accessing Comments](#accessing-comments) section, such as `sourceCode.getAllComments()`.

lib/config/rule-validator.js

@@ -10,6 +10,7 @@
 //-----------------------------------------------------------------------------
 
 const ajvImport = require("../shared/ajv");
+const { deepMergeArrays } = require("../shared/deep-merge-arrays");
 const ajv = ajvImport();
 const {
     parseRuleId,
@@ -164,7 +165,10 @@ class RuleValidator {
 
             if (validateRule) {
 
-                validateRule(ruleOptions.slice(1));
+                const slicedOptions = ruleOptions.slice(1);
+                const mergedOptions = deepMergeArrays(rule.meta && rule.meta.defaultOptions, slicedOptions);
+
+                validateRule(mergedOptions);
 
                 if (validateRule.errors) {
                     throw new Error(`Key "rules": Key "${ruleId}": ${
@@ -173,6 +177,10 @@ class RuleValidator {
                         ).join("")
                     }`);
                 }
+
+                if (mergedOptions.length) {
+                    config.rules[ruleId] = [ruleOptions[0], ...mergedOptions];
+                }
             }
         }
     }

lib/linter/linter.js

@@ -45,6 +45,7 @@ const { FlatConfigArray } = require("../config/flat-config-array");
 const { RuleValidator } = require("../config/rule-validator");
 const { assertIsRuleOptions, assertIsRuleSeverity } = require("../config/flat-config-schema");
 const { normalizeSeverityToString } = require("../shared/severity");
+const { deepMergeArrays } = require("../shared/deep-merge-arrays");
 const debug = require("debug")("eslint:linter");
 const MAX_AUTOFIX_PASSES = 10;
 const DEFAULT_PARSER_NAME = "espree";
@@ -778,14 +779,31 @@ function stripUnicodeBOM(text) {
 /**
  * Get the options for a rule (not including severity), if any
  * @param {Array|number} ruleConfig rule configuration
+ * @param {Object|undefined} defaultOptions rule.meta.defaultOptions
  * @returns {Array} of rule options, empty Array if none
  */
-function getRuleOptions(ruleConfig) {
+function getRuleOptions(ruleConfig, defaultOptions) {
     if (Array.isArray(ruleConfig)) {
-        return ruleConfig.slice(1);
+        return deepMergeArrays(defaultOptions, ruleConfig.slice(1));
     }
-    return [];
+    return defaultOptions ?? [];
+}
+
+/**
+ * Get the options for a rule's inline comment, including severity
+ * @param {string} ruleId Rule name being configured.
+ * @param {Array|number} ruleValue rule severity and options, if any
+ * @param {Object|undefined} defaultOptions rule.meta.defaultOptions
+ * @returns {Array} of rule options, empty Array if none
+ */
+function getRuleOptionsInline(ruleId, ruleValue, defaultOptions) {
+    assertIsRuleOptions(ruleId, ruleValue);
+
+    const [ruleSeverity, ...ruleConfig] = Array.isArray(ruleValue) ? ruleValue : [ruleValue];
 
+    assertIsRuleSeverity(ruleId, ruleSeverity);
+
+    return [ruleSeverity, ...deepMergeArrays(defaultOptions, ruleConfig)];
 }
 
 /**
@@ -1006,7 +1024,7 @@ function runRules(sourceCode, configuredRules, ruleMapper, parserName, languageO
                 Object.create(sharedTraversalContext),
                 {
                     id: ruleId,
-                    options: getRuleOptions(configuredRules[ruleId]),
+                    options: getRuleOptions(configuredRules[ruleId], rule.meta?.defaultOptions),
                     report(...args) {
 
                         /*
@@ -1713,10 +1731,7 @@ class Linter {
 
                         try {
 
-                            const ruleOptions = Array.isArray(ruleValue) ? ruleValue : [ruleValue];
-
-                            assertIsRuleOptions(ruleId, ruleValue);
-                            assertIsRuleSeverity(ruleId, ruleOptions[0]);
+                            const ruleOptions = getRuleOptionsInline(ruleId, ruleValue, rule.meta?.defaultOptions);
 
                             ruleValidator.validate({
                                 plugins: config.plugins,

lib/rules/accessor-pairs.js

@@ -139,6 +139,12 @@ module.exports = {
     meta: {
         type: "suggestion",
 
+        defaultOptions: [{
+            enforceForClassMembers: true,
+            getWithoutSet: false,
+            setWithoutGet: true
+        }],
+
         docs: {
             description: "Enforce getter and setter pairs in objects and classes",
             recommended: false,
@@ -149,16 +155,13 @@ module.exports = {
             type: "object",
             properties: {
                 getWithoutSet: {
-                    type: "boolean",
-                    default: false
+                    type: "boolean"
                 },
                 setWithoutGet: {
-                    type: "boolean",
-                    default: true
+                    type: "boolean"
                 },
                 enforceForClassMembers: {
-                    type: "boolean",
-                    default: true
+                    type: "boolean"
                 }
             },
             additionalProperties: false
@@ -174,7 +177,7 @@ module.exports = {
         }
     },
     create(context) {
-        const config = context.options[0] || {};
+        const [config] = context.options;
         const checkGetWithoutSet = config.getWithoutSet === true;
         const checkSetWithoutGet = config.setWithoutGet !== false;
         const enforceForClassMembers = config.enforceForClassMembers !== false;

lib/rules/array-bracket-spacing.js

@@ -18,6 +18,8 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: ["never"],
+
         docs: {
             description: "Enforce consistent spacing inside array brackets",
             recommended: false,

lib/rules/array-callback-return.js

@@ -215,6 +215,12 @@ module.exports = {
     meta: {
         type: "problem",
 
+        defaultOptions: [{
+            allowImplicit: false,
+            checkForEach: false,
+            allowVoid: false
+        }],
+
         docs: {
             description: "Enforce `return` statements in callbacks of array methods",
             recommended: false,
@@ -229,16 +235,13 @@ module.exports = {
                 type: "object",
                 properties: {
                     allowImplicit: {
-                        type: "boolean",
-                        default: false
+                        type: "boolean"
                     },
                     checkForEach: {
-                        type: "boolean",
-                        default: false
+                        type: "boolean"
                     },
                     allowVoid: {
-                        type: "boolean",
-                        default: false
+                        type: "boolean"
                     }
                 },
                 additionalProperties: false
@@ -256,8 +259,7 @@ module.exports = {
     },
 
     create(context) {
-
-        const options = context.options[0] || { allowImplicit: false, checkForEach: false, allowVoid: false };
+        const [options] = context.options;
         const sourceCode = context.sourceCode;
 
         let funcInfo = {

lib/rules/array-element-newline.js

@@ -19,6 +19,8 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: ["always"],
+
         docs: {
             description: "Enforce line breaks after each array element",
             recommended: false,

lib/rules/arrow-body-style.js

@@ -19,6 +19,8 @@ module.exports = {
     meta: {
         type: "suggestion",
 
+        defaultOptions: ["as-needed"],
+
         docs: {
             description: "Require braces around arrow function bodies",
             recommended: false,
@@ -71,7 +73,7 @@ module.exports = {
     create(context) {
         const options = context.options;
         const always = options[0] === "always";
-        const asNeeded = !options[0] || options[0] === "as-needed";
+        const asNeeded = options[0] === "as-needed";
         const never = options[0] === "never";
         const requireReturnForObjectLiteral = options[1] && options[1].requireReturnForObjectLiteral;
         const sourceCode = context.sourceCode;

lib/rules/arrow-parens.js

@@ -35,6 +35,8 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: ["always"],
+
         docs: {
             description: "Require parentheses around arrow function arguments",
             recommended: false,
@@ -51,8 +53,7 @@ module.exports = {
                 type: "object",
                 properties: {
                     requireForBlockBody: {
-                        type: "boolean",
-                        default: false
+                        type: "boolean"
                     }
                 },
                 additionalProperties: false

lib/rules/arrow-spacing.js

@@ -22,6 +22,11 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: [{
+            after: true,
+            before: true
+        }],
+
         docs: {
             description: "Enforce consistent spacing before and after the arrow in arrow functions",
             recommended: false,
@@ -35,12 +40,10 @@ module.exports = {
                 type: "object",
                 properties: {
                     before: {
-                        type: "boolean",
-                        default: true
+                        type: "boolean"
                     },
                     after: {
-                        type: "boolean",
-                        default: true
+                        type: "boolean"
                     }
                 },
                 additionalProperties: false
@@ -57,13 +60,7 @@ module.exports = {
     },
 
     create(context) {
-
-        // merge rules with default
-        const rule = Object.assign({}, context.options[0]);
-
-        rule.before = rule.before !== false;
-        rule.after = rule.after !== false;
-
+        const [options] = context.options;
         const sourceCode = context.sourceCode;
 
         /**
@@ -104,7 +101,7 @@ module.exports = {
             const tokens = getTokens(node);
             const countSpace = countSpaces(tokens);
 
-            if (rule.before) {
+            if (options.before) {
 
                 // should be space(s) before arrow
                 if (countSpace.before === 0) {
@@ -130,7 +127,7 @@ module.exports = {
                 }
             }
 
-            if (rule.after) {
+            if (options.after) {
 
                 // should be space(s) after arrow
                 if (countSpace.after === 0) {

lib/rules/block-spacing.js

@@ -19,6 +19,8 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: ["always"],
+
         docs: {
             description: "Disallow or enforce spaces inside of blocks after opening block and before closing block",
             recommended: false,

lib/rules/brace-style.js

@@ -19,6 +19,8 @@ module.exports = {
         replacedBy: [],
         type: "layout",
 
+        defaultOptions: ["1tbs", { allowSingleLine: false }],
+
         docs: {
             description: "Enforce consistent brace style for blocks",
             recommended: false,
@@ -33,8 +35,7 @@ module.exports = {
                 type: "object",
                 properties: {
                     allowSingleLine: {
-                        type: "boolean",
-                        default: false
+                        type: "boolean"
                     }
                 },
                 additionalProperties: false
@@ -54,8 +55,8 @@ module.exports = {
     },
 
     create(context) {
-        const style = context.options[0] || "1tbs",
-            params = context.options[1] || {},
+        const style = context.options[0],
+            params = context.options[1],
             sourceCode = context.sourceCode;
 
         //--------------------------------------------------------------------------