feat: add new prefer-nothing rule (#140)

add new prefer-nothing rule

README.md

@@ -70,6 +70,7 @@ If you want more fine-grained configuration, you can instead add a snippet like
 - [lit/no-template-map](docs/rules/no-template-map.md)
 - [lit/no-useless-template-literals](docs/rules/no-useless-template-literals.md)
 - [lit/no-value-attribute](docs/rules/no-value-attribute.md)
+- [lit/prefer-nothing](docs/rules/prefer-nothing.md)
 - [lit/quoted-expressions](docs/rules/quoted-expressions.md)
 
 

docs/rules/prefer-nothing.md

@@ -0,0 +1,43 @@
+# Enforces use of `nothing` constant over empty templates (prefer-nothing)
+
+`nothing` is a constant provided by lit which may be used to render
+nothing. This is far more efficient than creating an empty template.
+
+This means you can do something like:
+
+```ts
+_render() {
+  if (!condition) {
+    return nothing;
+  }
+
+  return html`Hello there`;
+}
+```
+
+## Rule Details
+
+This rule enforces the use of `nothing` rather than empty templates.
+
+The following patterns are considered warnings:
+
+```ts
+html``;
+const tpl = html``;
+
+function render() {
+  return html``;
+}
+```
+
+The following patterns are not warnings:
+
+```ts
+html`foo`;
+html` `; // whitespace
+```
+
+## When Not To Use It
+
+If you prefer using empty templates or don't yet have lit 2.x (which provides
+the `nothing` constant), you should not use this rule.

src/rules/prefer-nothing.ts

@@ -0,0 +1,52 @@
+/**
+ * @fileoverview Enforces use of `nothing` constant over empty templates
+ * @author James Garbutt <https://github.com/43081j>
+ */
+
+import {Rule} from 'eslint';
+import * as ESTree from 'estree';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+const rule: Rule.RuleModule = {
+  meta: {
+    docs: {
+      description: 'Enforces use of `nothing` constant over empty templates',
+      category: 'Best Practices',
+      url: 'https://github.com/43081j/eslint-plugin-lit/blob/master/docs/rules/prefer-nothing.md'
+    },
+    schema: [],
+    messages: {
+      preferNothing:
+        '`nothing` is preferred over empty templates when you want to render' +
+        ' nothing'
+    }
+  },
+
+  create(context): Rule.RuleListener {
+    return {
+      TaggedTemplateExpression: (node: ESTree.Node): void => {
+        if (
+          node.type === 'TaggedTemplateExpression' &&
+          node.tag.type === 'Identifier' &&
+          node.tag.name === 'html'
+        ) {
+          if (
+            node.quasi.expressions.length === 0 &&
+            node.quasi.quasis.length === 1 &&
+            node.quasi.quasis[0].value.raw === ''
+          ) {
+            context.report({
+              node,
+              messageId: 'preferNothing'
+            });
+          }
+        }
+      }
+    };
+  }
+};
+
+export = rule;

src/test/rules/prefer-nothing_test.ts

@@ -0,0 +1,59 @@
+/**
+ * @fileoverview Enforces use of `nothing` constant over empty templates
+ * @author James Garbutt <https://github.com/43081j>
+ */
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+import rule = require('../../rules/prefer-nothing');
+import {RuleTester} from 'eslint';
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    sourceType: 'module',
+    ecmaVersion: 2015
+  }
+});
+
+ruleTester.run('prefer-nothing', rule, {
+  valid: ['html`foo`', 'css``', 'nonsense``', 'html`${x}`', 'html` `'],
+
+  invalid: [
+    {
+      code: 'html``',
+      errors: [
+        {
+          messageId: 'preferNothing',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: 'const x = html``;',
+      errors: [
+        {
+          messageId: 'preferNothing',
+          line: 1,
+          column: 11
+        }
+      ]
+    },
+    {
+      code: 'function render() { return html``; }',
+      errors: [
+        {
+          messageId: 'preferNothing',
+          line: 1,
+          column: 28
+        }
+      ]
+    }
+  ]
+});