Fix: no-implicit-coercion string concat false positive (fixes #7057) (#7060)

Author: kaicataldo

lib/rules/no-implicit-coercion.js

@@ -5,6 +5,8 @@
 
 "use strict";
 
+const astUtils = require("../ast-utils");
+
 //------------------------------------------------------------------------------
 // Helpers
 //------------------------------------------------------------------------------
@@ -105,15 +107,35 @@ function getNonNumericOperand(node) {
     return null;
 }
 
+/**
+ * Checks whether a node is a string literal or not.
+ * @param {ASTNode} node The node to check.
+ * @returns {boolean} Whether or not the passed in node is a
+ * string literal or not.
+ */
+function isStringLiteral(node) {
+    return astUtils.isStringLiteral(node) && node.type !== "TemplateLiteral";
+}
+
+/**
+ * Checks whether a node is an empty string literal or not.
+ * @param {ASTNode} node The node to check.
+ * @returns {boolean} Whether or not the passed in node is an
+ * empty string literal or not.
+ */
+function isEmptyString(node) {
+    return isStringLiteral(node) && node.value === "";
+}
+
 /**
  * Checks whether or not a node is a concatenating with an empty string.
  * @param {ASTNode} node - A BinaryExpression node to check.
  * @returns {boolean} Whether or not the node is a concatenating with an empty string.
  */
 function isConcatWithEmptyString(node) {
     return node.operator === "+" && (
-        (node.left.type === "Literal" && node.left.value === "") ||
-        (node.right.type === "Literal" && node.right.value === "")
+        (isEmptyString(node.left) && !isStringLiteral(node.right)) ||
+        (isEmptyString(node.right) && !isStringLiteral(node.left))
     );
 }
 
@@ -123,20 +145,16 @@ function isConcatWithEmptyString(node) {
  * @returns {boolean} Whether or not the node is appended with an empty string.
  */
 function isAppendEmptyString(node) {
-    return node.operator === "+=" && node.right.type === "Literal" && node.right.value === "";
+    return node.operator === "+=" && isEmptyString(node.right);
 }
 
 /**
- * Gets a node that is the left or right operand of a node, is not the specified literal.
- * @param {ASTNode} node - A BinaryExpression node to get.
- * @param {any} value - A literal value to check.
- * @returns {ASTNode} A node that is the left or right operand of the node, is not the specified literal.
+ * Returns the operand that is not an empty string from a flagged BinaryExpression.
+ * @param {ASTNode} node - The flagged BinaryExpression node to check.
+ * @returns {ASTNode} The operand that is not an empty string from a flagged BinaryExpression.
  */
-function getOtherOperand(node, value) {
-    if (node.left.type === "Literal" && node.left.value === value) {
-        return node.right;
-    }
-    return node.left;
+function getNonEmptyOperand(node) {
+    return isEmptyString(node.left) ? node.right : node.left;
 }
 
 //------------------------------------------------------------------------------
@@ -236,7 +254,7 @@ module.exports = {
                     context.report(
                         node,
                         "use `String({{code}})` instead.", {
-                            code: sourceCode.getText(getOtherOperand(node, ""))
+                            code: sourceCode.getText(getNonEmptyOperand(node))
                         });
                 }
             },
@@ -250,7 +268,7 @@ module.exports = {
                     context.report(
                         node,
                         "use `{{code}} = String({{code}})` instead.", {
-                            code: sourceCode.getText(getOtherOperand(node, ""))
+                            code: sourceCode.getText(getNonEmptyOperand(node))
                         });
                 }
             }

tests/lib/rules/no-implicit-coercion.js

@@ -75,7 +75,13 @@ ruleTester.run("no-implicit-coercion", rule, {
         {code: "var a = ~foo", options: [{boolean: true}]},
         {code: "var a = 1 * foo", options: [{boolean: true, allow: ["*"]}]},
         {code: "var a = +foo", options: [{boolean: true, allow: ["+"]}]},
-        {code: "var a = \"\" + foo", options: [{boolean: true, string: true, allow: ["+"]}]}
+        {code: "var a = \"\" + foo", options: [{boolean: true, string: true, allow: ["+"]}]},
+
+        // https://github.com/eslint/eslint/issues/7057
+        {code: "'' + 'foo'"},
+        {code: "'foo' + ''"},
+        {code: "foo += 'bar'"},
+        {code: "+42"}
     ],
     invalid: [
         {code: "!!foo", errors: [{message: "use `Boolean(foo)` instead.", type: "UnaryExpression"}]},