Update: Add no-magic-numbers 'ignoreDefaultValues' option

docs/rules/no-magic-numbers.md

@@ -128,6 +128,27 @@ a = data[4294967295]; // above the max array index
 a = data[1e500]; // same as data["Infinity"]
 ```
 
+### ignoreDefaultValues
+
+A boolean to specify if numbers used in default value assignments are considered okay. `false` by default.
+
+Examples of **correct** code for the `{ "ignoreDefaultValues": true }` option:
+
+```js
+/*eslint no-magic-numbers: ["error", { "ignoreDefaultValues": true }]*/
+
+const { tax = 0.25 } = accountancy;
+
+function mapParallel(concurrency = 3) { /***/ }
+```
+
+```js
+/*eslint no-magic-numbers: ["error", { "ignoreDefaultValues": true }]*/
+
+let head;
+[head = 100] = []
+```
+
 ### enforceConst
 
 A boolean to specify if we should check for the const keyword in variable declaration of numbers. `false` by default.

lib/rules/no-magic-numbers.js

@@ -61,6 +61,10 @@ module.exports = {
                 ignoreArrayIndexes: {
                     type: "boolean",
                     default: false
+                },
+                ignoreDefaultValues: {
+                    type: "boolean",
+                    default: false
                 }
             },
             additionalProperties: false
@@ -77,7 +81,8 @@ module.exports = {
             detectObjects = !!config.detectObjects,
             enforceConst = !!config.enforceConst,
             ignore = (config.ignore || []).map(normalizeIgnoreValue),
-            ignoreArrayIndexes = !!config.ignoreArrayIndexes;
+            ignoreArrayIndexes = !!config.ignoreArrayIndexes,
+            ignoreDefaultValues = !!config.ignoreDefaultValues;
 
         const okTypes = detectObjects ? [] : ["ObjectExpression", "Property", "AssignmentExpression"];
 
@@ -90,6 +95,17 @@ module.exports = {
             return ignore.indexOf(value) !== -1;
         }
 
+        /**
+         * Returns whether the number is a default value assignment.
+         * @param {ASTNode} fullNumberNode `Literal` or `UnaryExpression` full number node
+         * @returns {boolean} true if the number is a default value
+         */
+        function isDefaultValue(fullNumberNode) {
+            const parent = fullNumberNode.parent;
+
+            return parent.type === "AssignmentPattern" && parent.right === fullNumberNode;
+        }
+
         /**
          * Returns whether the given node is used as a radix within parseInt() or Number.parseInt()
          * @param {ASTNode} fullNumberNode `Literal` or `UnaryExpression` full number node
@@ -172,18 +188,19 @@ module.exports = {
                     raw = node.raw;
                 }
 
+                const parent = fullNumberNode.parent;
+
                 // Always allow radix arguments and JSX props
                 if (
                     isIgnoredValue(value) ||
+                    (ignoreDefaultValues && isDefaultValue(fullNumberNode)) ||
                     isParseIntRadix(fullNumberNode) ||
                     isJSXNumber(fullNumberNode) ||
                     (ignoreArrayIndexes && isArrayIndex(fullNumberNode, value))
                 ) {
                     return;
                 }
 
-                const parent = fullNumberNode.parent;
-
                 if (parent.type === "VariableDeclarator") {
                     if (enforceConst && parent.parent.kind !== "const") {
                         context.report({

tests/lib/rules/no-magic-numbers.js

@@ -214,6 +214,31 @@ ruleTester.run("no-magic-numbers", rule, {
             options: [{ ignore: ["-100n"] }],
             parserOptions: { ecmaVersion: 2020 }
         },
+        {
+            code: "const { param = 123 } = sourceObject;",
+            options: [{ ignoreDefaultValues: true }],
+            env: { es6: true }
+        },
+        {
+            code: "const func = (param = 123) => {}",
+            options: [{ ignoreDefaultValues: true }],
+            env: { es6: true }
+        },
+        {
+            code: "const func = ({ param = 123 }) => {}",
+            options: [{ ignoreDefaultValues: true }],
+            env: { es6: true }
+        },
+        {
+            code: "const [one = 1, two = 2] = []",
+            options: [{ ignoreDefaultValues: true }],
+            env: { es6: true }
+        },
+        {
+            code: "var one, two; [one = 1, two = 2] = []",
+            options: [{ ignoreDefaultValues: true }],
+            env: { es6: true }
+        },
 
         // Optional chaining
         {
@@ -738,6 +763,55 @@ ruleTester.run("no-magic-numbers", rule, {
             errors: [
                 { messageId: "noMagic", data: { raw: "100" }, line: 1 }
             ]
+        },
+        {
+            code: "const func = (param = 123) => {}",
+            options: [{ ignoreDefaultValues: false }],
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "123" }, line: 1 }
+            ]
+        },
+        {
+            code: "const { param = 123 } = sourceObject;",
+            options: [{}],
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "123" }, line: 1 }
+            ]
+        },
+        {
+            code: "const { param = 123 } = sourceObject;",
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "123" }, line: 1 }
+            ]
+        },
+        {
+            code: "const { param = 123 } = sourceObject;",
+            options: [{ ignoreDefaultValues: false }],
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "123" }, line: 1 }
+            ]
+        },
+        {
+            code: "const [one = 1, two = 2] = []",
+            options: [{ ignoreDefaultValues: false }],
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "1" }, line: 1 },
+                { messageId: "noMagic", data: { raw: "2" }, line: 1 }
+            ]
+        },
+        {
+            code: "var one, two; [one = 1, two = 2] = []",
+            options: [{ ignoreDefaultValues: false }],
+            env: { es6: true },
+            errors: [
+                { messageId: "noMagic", data: { raw: "1" }, line: 1 },
+                { messageId: "noMagic", data: { raw: "2" }, line: 1 }
+            ]
         }
     ]
 });