feat: new rule no-this-assign-in-render

README.md

@@ -69,12 +69,12 @@ If you want more fine-grained configuration, you can instead add a snippet like
 - [lit/no-template-arrow](docs/rules/no-template-arrow.md)
 - [lit/no-template-bind](docs/rules/no-template-bind.md)
 - [lit/no-template-map](docs/rules/no-template-map.md)
+- [lit/no-this-assign-in-render](docs/rules/no-this-assign-in-render.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)
 
-
 ## Shareable configurations
 
 ### Recommended

docs/rules/no-this-assign-in-render.md

@@ -0,0 +1,36 @@
+# Disallows assignments to members of `this` in render methods (no-this-assign-in-render)
+
+Assignments to `this` in the render method of a `LitElement` are generally a
+mistake since it should ideally act like a pure function.
+
+Property updates should usually happen in the other lifecycle methods
+(e.g. `updated`) or in event handlers.
+
+## Rule Details
+
+This rule disallows assigning to `this` in the render method.
+
+The following patterns are considered warnings:
+
+```ts
+render() {
+  this.prop = 5;
+}
+```
+
+The following patterns are not warnings:
+
+```ts
+render() {
+  const prop = 5;
+}
+
+updated() {
+  this.prop = 5;
+}
+```
+
+## When Not To Use It
+
+If you have non-observed properties you wish to update per render, you may
+want to disable this rule.

src/rules/no-this-assign-in-render.ts

@@ -0,0 +1,125 @@
+/**
+ * @fileoverview Disallows assignments to members of `this` in render methods
+ * @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:
+        'Disallows assignments to members of `this` in render methods',
+      category: 'Best Practices',
+      recommended: true,
+      url: 'https://github.com/43081j/eslint-plugin-lit/blob/master/docs/rules/no-this-assign-in-render.md'
+    },
+    schema: [],
+    messages: {
+      noThis:
+        'Members of `this` should not be assigned to in the render ' +
+        'method. It is likely you should do this elsewhere instead ' +
+        '(e.g. in `updated`)'
+    }
+  },
+
+  create(context): Rule.RuleListener {
+    let inRender = false;
+    let inComponent = false;
+
+    /**
+     * Class entered
+     *
+     * @param {ESTree.Class} node Node entered
+     * @return {void}
+     */
+    function classEnter(node: ESTree.Class): void {
+      if (
+        !node.superClass ||
+        node.superClass.type !== 'Identifier' ||
+        node.superClass.name !== 'LitElement'
+      ) {
+        return;
+      }
+
+      inComponent = true;
+    }
+
+    /**
+     * Class exited
+     *
+     * @return {void}
+     */
+    function classExit(): void {
+      inComponent = false;
+    }
+
+    /**
+     * Method entered
+     *
+     * @param {ESTree.MethodDefinition} node Node entered
+     * @return {void}
+     */
+    function methodEnter(node: ESTree.MethodDefinition): void {
+      if (
+        !inComponent ||
+        node.kind !== 'method' ||
+        node.static === true ||
+        node.key.type !== 'Identifier' ||
+        node.key.name !== 'render'
+      ) {
+        return;
+      }
+
+      inRender = true;
+    }
+
+    /**
+     * Method exited
+     *
+     * @return {void}
+     */
+    function methodExit(): void {
+      inRender = false;
+    }
+
+    /**
+     * Assignment expression entered
+     *
+     * @param {ESTree.AssignmentExpression} node Node entered
+     * @return {void}
+     */
+    function assignmentFound(node: ESTree.AssignmentExpression): void {
+      if (!inRender) {
+        return;
+      }
+
+      context.report({
+        node,
+        messageId: 'noThis'
+      });
+    }
+
+    return {
+      ClassExpression: (node: ESTree.Node): void =>
+        classEnter(node as ESTree.Class),
+      ClassDeclaration: (node: ESTree.Node): void =>
+        classEnter(node as ESTree.Class),
+      'ClassExpression:exit': classExit,
+      'ClassDeclaration:exit': classExit,
+      MethodDefinition: (node: ESTree.Node): void =>
+        methodEnter(node as ESTree.MethodDefinition),
+      'MethodDefinition:exit': methodExit,
+      'AssignmentExpression:has([left] ThisExpression)': (
+        node: ESTree.Node
+      ): void => assignmentFound(node as ESTree.AssignmentExpression)
+    };
+  }
+};
+
+export = rule;

src/test/rules/no-this-assign-in-render_test.ts

@@ -0,0 +1,113 @@
+/**
+ * @fileoverview Disallows assignments to members of `this` in render methods
+ * @author James Garbutt <https://github.com/43081j>
+ */
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+import rule = require('../../rules/no-this-assign-in-render');
+import {RuleTester} from 'eslint';
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    sourceType: 'module',
+    ecmaVersion: 2015
+  }
+});
+
+ruleTester.run('no-this-assign-in-render', rule, {
+  valid: [
+    'const x = 808;',
+    'class Foo { }',
+    `class Foo {
+        render() {
+          this.prop = 5;
+        }
+      }`,
+    `class Foo {
+        render() {
+          this.deep.prop = 5;
+        }
+      }`,
+    `class Foo extends LitElement {
+        render() {
+          const x = this.prop;
+        }
+      }`,
+    `class Foo extends LitElement {
+        render() {
+          const x = 5;
+        }
+      }`,
+    `class Foo extends LitElement {
+        static render() {
+          this.foo = 5;
+        }
+      }`
+  ],
+
+  invalid: [
+    {
+      code: `class Foo extends LitElement {
+        render() {
+          this.prop = 'foo';
+        }
+      }`,
+      errors: [
+        {
+          messageId: 'noThis',
+          line: 3,
+          column: 11
+        }
+      ]
+    },
+    {
+      code: `class Foo extends LitElement {
+        render() {
+          this.deep.prop = 'foo';
+        }
+      }`,
+      errors: [
+        {
+          messageId: 'noThis',
+          line: 3,
+          column: 11
+        }
+      ]
+    },
+    {
+      code: `const foo = class extends LitElement {
+        render() {
+          this.prop = 'foo';
+        }
+      }`,
+      errors: [
+        {
+          messageId: 'noThis',
+          line: 3,
+          column: 11
+        }
+      ]
+    },
+    {
+      code: `class Foo extends LitElement {
+        render() {
+          this['prop'] = 'foo';
+        }
+      }`,
+      errors: [
+        {
+          messageId: 'noThis',
+          line: 3,
+          column: 11
+        }
+      ]
+    }
+  ]
+});