Update: add a fixer for certain statically-verifiable eqeqeq cases (#…

…7389)

* Update: add a fixer for certain statically-verifiable `eqeqeq` cases

* Add note to docs to clarify which cases can be fixed

docs/rules/eqeqeq.md

@@ -1,5 +1,7 @@
 # Require === and !== (eqeqeq)
 
+(fixable) The `--fix` option on the [command line](../user-guide/command-line-interface#fix) automatically fixes problems reported by this rule.
+
 It is considered good practice to use the type-safe equality operators `===` and `!==` instead of their regular counterparts `==` and `!=`.
 
 The reason for this is that `==` and `!=` do type coercion which follows the rather obscure [Abstract Equality Comparison Algorithm](http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3).
@@ -27,6 +29,8 @@ if ("" == text) { }
 if (obj.getStuff() != undefined) { }
 ```
 
+The `--fix` option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a `typeof` expression, or if both operands are literals with the same type.
+
 ## Options
 
 ### always

lib/rules/eqeqeq.js

@@ -47,7 +47,9 @@ module.exports = {
                     additionalItems: false
                 }
             ]
-        }
+        },
+
+        fixable: "code"
     },
 
     create(context) {
@@ -118,16 +120,26 @@ module.exports = {
         /**
          * Reports a message for this rule.
          * @param {ASTNode} node The binary expression node that was checked
-         * @param {string} message The message to report
+         * @param {string} expectedOperator The operator that was expected (either '==', '!=', '===', or '!==')
          * @returns {void}
          * @private
          */
-        function report(node, message) {
+        function report(node, expectedOperator) {
             context.report({
                 node,
                 loc: getOperatorLocation(node),
-                message,
-                data: { op: node.operator.charAt(0) }
+                message: "Expected '{{expectedOperator}}' and instead saw '{{actualOperator}}'.",
+                data: {expectedOperator, actualOperator: node.operator},
+                fix(fixer) {
+
+                    // If the comparison is a `typeof` comparison or both sides are literals with the same type, then it's safe to fix.
+                    if (isTypeOfBinary(node) || areLiteralsAndSameType(node)) {
+                        const operatorToken = sourceCode.getTokensBetween(node.left, node.right).find(token => token.value === node.operator);
+
+                        return fixer.replaceText(operatorToken, expectedOperator);
+                    }
+                    return null;
+                }
             });
         }
 
@@ -137,7 +149,7 @@ module.exports = {
 
                 if (node.operator !== "==" && node.operator !== "!=") {
                     if (enforceInverseRuleForNull && isNull) {
-                        report(node, "Expected '{{op}}=' and instead saw '{{op}}=='.");
+                        report(node, node.operator.slice(0, -1));
                     }
                     return;
                 }
@@ -151,7 +163,7 @@ module.exports = {
                     return;
                 }
 
-                report(node, "Expected '{{op}}==' and instead saw '{{op}}='.");
+                report(node, `${node.operator}=`);
             }
         };
 

tests/lib/rules/eqeqeq.js

@@ -47,33 +47,33 @@ ruleTester.run("eqeqeq", rule, {
     invalid: [
         { code: "a == b", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "a != b", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "typeof a == 'number'", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "typeof a == 'number'", options: ["always"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "'string' != typeof a", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "true == true", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "2 == 3", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "2 == 3", options: ["always"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "'hello' != 'world'", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "'hello' != 'world'", options: ["always"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "typeof a == 'number'", output: "typeof a === 'number'", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "typeof a == 'number'", output: "typeof a === 'number'", options: ["always"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "'string' != typeof a", output: "'string' !== typeof a", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "true == true", output: "true === true", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "2 == 3", output: "2 === 3", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "2 == 3", output: "2 === 3", options: ["always"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "'hello' != 'world'", output: "'hello' !== 'world'", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "'hello' != 'world'", output: "'hello' !== 'world'", options: ["always"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
         { code: "a == null", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "a == null", options: ["always"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "null != a", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
         { code: "true == 1", options: ["smart"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "0 != '1'", options: ["smart"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
         { code: "'wee' == /wee/", options: ["smart"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "typeof a == 'number'", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "'string' != typeof a", options: ["allow-null"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "'hello' != 'world'", options: ["allow-null"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "2 == 3", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "true == true", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "typeof a == 'number'", output: "typeof a === 'number'", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "'string' != typeof a", output: "'string' !== typeof a", options: ["allow-null"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "'hello' != 'world'", output: "'hello' !== 'world'", options: ["allow-null"], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "2 == 3", output: "2 === 3", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "true == true", output: "true === true", options: ["allow-null"], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "true == null", options: ["always", {null: "always"}], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
         { code: "true != null", options: ["always", {null: "always"}], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
-        { code: "null == null", options: ["always", {null: "always"}], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
-        { code: "null != null", options: ["always", {null: "always"}], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
+        { code: "null == null", output: "null === null", options: ["always", {null: "always"}], errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression"}] },
+        { code: "null != null", output: "null !== null", options: ["always", {null: "always"}], errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression"}] },
         { code: "true === null", options: ["always", {null: "never"}], errors: [{ message: "Expected '==' and instead saw '==='.", type: "BinaryExpression"}] },
         { code: "true !== null", options: ["always", {null: "never"}], errors: [{ message: "Expected '!=' and instead saw '!=='.", type: "BinaryExpression"}] },
-        { code: "null === null", options: ["always", {null: "never"}], errors: [{ message: "Expected '==' and instead saw '==='.", type: "BinaryExpression"}] },
-        { code: "null !== null", options: ["always", {null: "never"}], errors: [{ message: "Expected '!=' and instead saw '!=='.", type: "BinaryExpression"}] },
+        { code: "null === null", output: "null == null", options: ["always", {null: "never"}], errors: [{ message: "Expected '==' and instead saw '==='.", type: "BinaryExpression"}] },
+        { code: "null !== null", output: "null != null", options: ["always", {null: "never"}], errors: [{ message: "Expected '!=' and instead saw '!=='.", type: "BinaryExpression"}] },
         { code: "a\n==\nb", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression", line: 2 }] },
         { code: "(a) == b", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression", line: 1 }] },
         { code: "(a) != b", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression", line: 1 }] },
@@ -83,5 +83,7 @@ ruleTester.run("eqeqeq", rule, {
         { code: "(a) != (b)", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression", line: 1 }] },
         { code: "(a == b) == (c)", errors: [{ message: "Expected '===' and instead saw '=='.", type: "BinaryExpression", line: 1 }, { message: "Expected '===' and instead saw '=='.", type: "BinaryExpression", line: 1 }] },
         { code: "(a != b) != (c)", errors: [{ message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression", line: 1 }, { message: "Expected '!==' and instead saw '!='.", type: "BinaryExpression", line: 1 }] }
-    ]
+
+    // If no output is provided, assert that the output is the same as the original code.
+    ].map(invalidCase => Object.assign({output: invalidCase.code}, invalidCase))
 });