Update: add ignoreNonDeclaration to no-multi-assign rule (fixes #12545)…

… (#14185)

* Update: add an option to ignore non declaration (refs #12545)

* Doc: add description of the option (refs #12545)

* Chore: reflect review comments (refs #12545)

* Chore: modify the document according to comments　(refs #12545)

docs/rules/no-multi-assign.md

@@ -43,6 +43,38 @@ let a = c;
 let b = c;
 ```
 
+## Options
+
+This rule has an object option:
+
+* `"ignoreNonDeclaration"`: When set to `true`, the rule allows chains that don't include initializing a variable in a declaration. Default is `false`.
+
+### ignoreNonDeclaration
+
+Examples of **correct** code for the `{ "ignoreNonDeclaration": true }` option:
+
+```js
+/*eslint no-multi-assign: ["error", { "ignoreNonDeclaration": true }]*/
+
+let a;
+let b;
+a = b = "baz";
+
+const x = {};
+const y = {};
+x.one = y.one = 1;
+```
+
+Examples of **incorrect** code for the `{ "ignoreNonDeclaration": true }` option:
+
+```js
+/*eslint no-multi-assign: ["error", { "ignoreNonDeclaration": true }]*/
+
+let a = b = "baz";
+
+const foo = bar = 1;
+```
+
 ## Related Rules
 
 * [max-statements-per-line](max-statements-per-line.md)

lib/rules/no-multi-assign.js

@@ -21,7 +21,16 @@ module.exports = {
             url: "https://eslint.org/docs/rules/no-multi-assign"
         },
 
-        schema: [],
+        schema: [{
+            type: "object",
+            properties: {
+                ignoreNonDeclaration: {
+                    type: "boolean",
+                    default: false
+                }
+            },
+            additionalProperties: false
+        }],
 
         messages: {
             unexpectedChain: "Unexpected chained assignment."
@@ -33,10 +42,14 @@ module.exports = {
         //--------------------------------------------------------------------------
         // Public
         //--------------------------------------------------------------------------
+        const options = context.options[0] || {
+            ignoreNonDeclaration: false
+        };
+        const targetParent = options.ignoreNonDeclaration ? ["VariableDeclarator"] : ["AssignmentExpression", "VariableDeclarator"];
 
         return {
             AssignmentExpression(node) {
-                if (["AssignmentExpression", "VariableDeclarator"].indexOf(node.parent.type) !== -1) {
+                if (targetParent.indexOf(node.parent.type) !== -1) {
                     context.report({
                         node,
                         messageId: "unexpectedChain"

tests/lib/rules/no-multi-assign.js

@@ -51,7 +51,9 @@ ruleTester.run("no-mutli-assign", rule, {
         { code: "for(let a = 0, b = 0;;){}", parserOptions: { ecmaVersion: 6 } },
         { code: "for(const a = 0, b = 0;;){}", parserOptions: { ecmaVersion: 6 } },
         { code: "export let a, b;", parserOptions: { ecmaVersion: 6, sourceType: "module" } },
-        { code: "export let a,\n b = 0;", parserOptions: { ecmaVersion: 6, sourceType: "module" } }
+        { code: "export let a,\n b = 0;", parserOptions: { ecmaVersion: 6, sourceType: "module" } },
+        { code: "const x = {};const y = {};x.one = y.one = 1;", options: [{ ignoreNonDeclaration: true }], parserOptions: { ecmaVersion: 6 } },
+        { code: "let a, b;a = b = 1", options: [{ ignoreNonDeclaration: true }], parserOptions: { ecmaVersion: 6 } }
     ],
 
     invalid: [
@@ -137,6 +139,39 @@ ruleTester.run("no-mutli-assign", rule, {
             errors: [
                 errorAt(1, 5, "AssignmentExpression")
             ]
+        },
+        {
+            code: "const x = {};\nconst y = x.one = 1;",
+            options: [{ ignoreNonDeclaration: true }],
+            parserOptions: { ecmaVersion: 6 },
+            errors: [
+                errorAt(2, 11, "AssignmentExpression")
+            ]
+
+        },
+        {
+            code: "let a, b;a = b = 1",
+            options: [{}],
+            parserOptions: { ecmaVersion: 6 },
+            errors: [
+                errorAt(1, 14, "AssignmentExpression")
+            ]
+        },
+        {
+            code: "let x, y;x = y = 'baz'",
+            options: [{ ignoreNonDeclaration: false }],
+            parserOptions: { ecmaVersion: 6 },
+            errors: [
+                errorAt(1, 14, "AssignmentExpression")
+            ]
+        },
+        {
+            code: "const a = b = 1",
+            options: [{ ignoreNonDeclaration: true }],
+            parserOptions: { ecmaVersion: 6 },
+            errors: [
+                errorAt(1, 11, "AssignmentExpression")
+            ]
         }
     ]
 });