Update: Add templateString option in no-implicit-coercion (fixes #12866)

This adds the `templateString` option to `no-implicit-coercion`. This makes the
rule report the following code:

```js
`${foo}`
```

For backwards compatibility, this was added as a separate option instead of as
default behaviour.

docs/rules/no-implicit-coercion.md

@@ -33,10 +33,11 @@ This rule is aimed to flag shorter notations for the type conversion, then sugge
 
 This rule has three main options and one override option to allow some coercions as required.
 
-* `"boolean"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `boolean` type.
-* `"number"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `number` type.
-* `"string"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `string` type.
-* `"allow"` (`empty` by default) - Each entry in this array can be one of `~`, `!!`, `+` or `*` that are to be allowed.
+-   `"boolean"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `boolean` type.
+-   `"number"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `number` type.
+-   `"string"` (`true` by default) - When this is `true`, this rule warns shorter type conversions for `string` type.
+-   `"templateString"` (`false` by default) - When this is `true`, this rule warns shorter type conversions using template strings.
+-   `"allow"` (`empty` by default) - Each entry in this array can be one of `~`, `!!`, `+` or `*` that are to be allowed.
 
 Note that operator `+` in `allow` list would allow `+foo` (number coercion) as well as `"" + foo` (string coercion).
 
@@ -106,6 +107,24 @@ var s = String(foo);
 foo = String(foo);
 ```
 
+### templateString
+
+Examples of **incorrect** code for the default `{ "templateString": true }` option:
+
+```js
+/*eslint no-implicit-coercion: "error"*/
+
+`${foo}`;
+```
+
+Examples of **correct** code for the default `{ "templateString": false }` option:
+
+```js
+/*eslint no-implicit-coercion: "error"*/
+
+`${foo}`;
+```
+
 ### allow
 
 Using `allow` list, we can override and allow specific operators.

lib/rules/no-implicit-coercion.js

@@ -24,6 +24,7 @@ function parseOptions(options) {
         boolean: "boolean" in options ? options.boolean : true,
         number: "number" in options ? options.number : true,
         string: "string" in options ? options.string : true,
+        templateString: "templateString" in options ? options.templateString : false,
         allow: options.allow || []
     };
 }
@@ -180,6 +181,10 @@ module.exports = {
                     type: "boolean",
                     default: true
                 },
+                templateString: {
+                    type: "boolean",
+                    default: false
+                },
                 allow: {
                     type: "array",
                     items: {
@@ -299,6 +304,33 @@ module.exports = {
 
                     report(node, recommendation, true);
                 }
+            },
+
+            TemplateLiteral(node) {
+                if (!options.templateString) {
+                    return;
+                }
+
+                // `` or `${foo}${bar}`
+                if (node.expressions.length !== 1) {
+                    return;
+                }
+
+
+                //  `prefix${foo}`
+                if (node.quasis[0].value.raw !== "") {
+                    return;
+                }
+
+                //  `${foo}postfix`
+                if (node.quasis[1].value.raw !== "") {
+                    return;
+                }
+
+                const code = sourceCode.getText(node.expressions[0]);
+                const recommendation = `String(${code})`;
+
+                report(node, recommendation, true);
             }
         };
     }

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

@@ -248,6 +248,17 @@ ruleTester.run("no-implicit-coercion", rule, {
                 type: "BinaryExpression"
             }]
         },
+        {
+            code: "`${foo}`",
+            output: "String(foo)",
+            options: [{ templateString: true }],
+            parserOptions: { ecmaVersion: 6 },
+            errors: [{
+                messageId: "useRecommendation",
+                data: { recommendation: "String(foo)" },
+                type: "TemplateLiteral"
+            }]
+        },
         {
             code: "foo += \"\"",
             output: "foo = String(foo)",