fix: Additional checks for prefer-flat

rules/prefer-flat.js

@@ -1,3 +1,5 @@
+// eslint-disable-next-line eslint-comments/disable-enable-pair
+/* eslint-disable no-magic-numbers */
 /**
  * @author Martin Giger
  * @license MIT
@@ -7,10 +9,9 @@
 //TODO no works.
 
 const firstElement = (arr) => {
-        const [ el ] = arr;
-        return el;
-    },
-    SECOND = 1;
+    const [ el ] = arr;
+    return el;
+};
 
 module.exports = {
     meta: {
@@ -37,18 +38,46 @@ module.exports = {
                 });
             },
             'CallExpression[callee.type="MemberExpression"][callee.property.name="reduce"] > *:function > CallExpression[callee.type="MemberExpression"][callee.property.name="concat"]'(node) {
-                if(node.parent.parent.arguments.length > SECOND && node.parent.parent.arguments[SECOND].type === "ArrayExpression" &&
-                    firstElement(node.arguments).name === node.parent.params[SECOND].name &&
-                    node.callee.object.name === firstElement(node.parent.params).name) {
-                    context.report({
-                        node: node.parent.parent,
-                        message: "Use flat to flatten an array",
-                        fix(fixer) {
-                            const sourceCode = context.getSourceCode();
-                            return fixer.replaceText(node.parent.parent, `${sourceCode.getText(node.parent.parent.callee.object)}.flat()`);
-                        }
-                    });
+                const concatFunction = node,
+                    reduceFunction = node.parent.parent,
+                    reduceCallbackParams = node.parent.params;
+
+                // reduce function must have two args
+                if(reduceFunction.arguments.length !== 2) {
+                    return;
+                }
+
+                // reduce callback arguments must be 2 and all must be Identifier
+                if(reduceCallbackParams.length !== 2 || reduceCallbackParams[0].type !== 'Identifier' || reduceCallbackParams[1].type !== 'Identifier') {
+                    return;
+                }
+
+                // reduce function must have empty array as initial values
+                if(reduceFunction.arguments[1].type !== "ArrayExpression" || reduceFunction.arguments[1].elements.length !== 0) {
+                    return;
+                }
+
+                // concat function must have one argument which must be variable
+                if(concatFunction.arguments.length !== 1 || concatFunction.arguments[0].type !== 'Identifier') {
+                    return;
                 }
+
+                // concat callee object name must be same as reducer callback accumulator and
+                // concat argument must be same as reducer callback item
+                if(concatFunction.arguments[0].name !== reduceCallbackParams[1].name ||
+                    node.callee.object.name !== reduceCallbackParams[0].name
+                ) {
+                    return;
+                }
+
+                context.report({
+                    node: node.parent.parent,
+                    message: "Use flat to flatten an array",
+                    fix(fixer) {
+                        const sourceCode = context.getSourceCode();
+                        return fixer.replaceText(node.parent.parent, `${sourceCode.getText(node.parent.parent.callee.object)}.flat()`);
+                    }
+                });
             }
         };
     }

test/rules/prefer-flat.js

@@ -12,7 +12,10 @@ ruleTester.run('prefer-flat', rule, {
     valid: [
         'array.flat()',
         'array.reduce((p, n) => n.concat(p), [])',
-        'array.reduce((p, n) => n + p, 0)'
+        'array.reduce((p, n) => n + p, 0)',
+        'array.reduce((p, []) => p.concat({}), [])',
+        'array.reduce((p, n) => p.concat(n), [1])',
+        'array.reduce((p, n) => p.concat(n, n), [])'
     ],
     invalid: [
         {