[New] add jsx-no-useless-fragment rule

Co-Authored-By: Jordan Harband <ljharb@gmail.com>

README.md

@@ -170,6 +170,7 @@ Enable the rules that you would like to use.
 * [react/jsx-no-literals](docs/rules/jsx-no-literals.md): Prevent usage of unwrapped JSX strings
 * [react/jsx-no-target-blank](docs/rules/jsx-no-target-blank.md): Prevent usage of unsafe `target='_blank'`
 * [react/jsx-no-undef](docs/rules/jsx-no-undef.md): Disallow undeclared variables in JSX
+* [react/jsx-no-useless-fragment](docs/rules/jsx-no-useless-fragment.md): Disallow unnescessary fragments (fixable)
 * [react/jsx-one-expression-per-line](docs/rules/jsx-one-expression-per-line.md): Limit to one expression per line in JSX
 * [react/jsx-curly-brace-presence](docs/rules/jsx-curly-brace-presence.md): Enforce curly braces or disallow unnecessary curly braces in JSX
 * [react/jsx-fragments](docs/rules/jsx-fragments.md): Enforce shorthand or standard form for React fragments

docs/rules/jsx-no-useless-fragment.md

@@ -0,0 +1,54 @@
+# Disallow unnecessary fragments (react/jsx-no-useless-fragment)
+
+A fragment is redundant if it contains only one child, or if it is the child of a html element, and is not a [keyed fragment](https://reactjs.org/docs/fragments.html#keyed-fragments).
+
+**Fixable:** This rule is sometimes automatically fixable using the `--fix` flag on the command line.
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```jsx
+<>{foo}</>
+
+<><Foo /></>
+
+<p><>foo</></p>
+
+<></>
+
+<Fragment>foo</Fragment>
+
+<React.Fragment>foo</React.Fragment>
+
+<section>
+  <>
+    <div />
+    <div />
+  </>
+</section>
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+<>
+  <Foo />
+  <Bar />
+</>
+
+<>foo {bar}</>
+
+<> {foo}</>
+
+const cat = <>meow</>
+
+<SomeComponent>
+  <>
+    <div />
+    <div />
+  </>
+</SomeComponent>
+
+<Fragment key={item.id}>{item.value}</Fragment>
+```

index.js

@@ -35,6 +35,7 @@ const allRules = {
   'jsx-no-duplicate-props': require('./lib/rules/jsx-no-duplicate-props'),
   'jsx-no-literals': require('./lib/rules/jsx-no-literals'),
   'jsx-no-target-blank': require('./lib/rules/jsx-no-target-blank'),
+  'jsx-no-useless-fragment': require('./lib/rules/jsx-no-useless-fragment'),
   'jsx-one-expression-per-line': require('./lib/rules/jsx-one-expression-per-line'),
   'jsx-no-undef': require('./lib/rules/jsx-no-undef'),
   'jsx-curly-brace-presence': require('./lib/rules/jsx-curly-brace-presence'),

lib/rules/jsx-no-useless-fragment.js

@@ -0,0 +1,210 @@
+/**
+ * @fileoverview Disallow useless fragments
+ */
+
+'use strict';
+
+const pragmaUtil = require('../util/pragma');
+const jsxUtil = require('../util/jsx');
+const docsUrl = require('../util/docsUrl');
+
+function isJSXText(node) {
+  return !!node && (node.type === 'JSXText' || node.type === 'Literal');
+}
+
+/**
+ * @param {string} text
+ * @returns {boolean}
+ */
+function isOnlyWhitespace(text) {
+  return text.trim().length === 0;
+}
+
+/**
+ * @param {ASTNode} node
+ * @returns {boolean}
+ */
+function isNonspaceJSXTextOrJSXCurly(node) {
+  return (isJSXText(node) && !isOnlyWhitespace(node.raw)) || node.type === 'JSXExpressionContainer';
+}
+
+/**
+ * Somehow fragment like this is useful: <Foo content={<>ee eeee eeee ...</>} />
+ * @param {ASTNode} node
+ * @returns {boolean}
+ */
+function isFragmentWithOnlyTextAndIsNotChild(node) {
+  return node.children.length === 1 &&
+    isJSXText(node.children[0]) &&
+    !(node.parent.type === 'JSXElement' || node.parent.type === 'JSXFragment');
+}
+
+/**
+ * @param {string} text
+ * @returns {string}
+ */
+function trimLikeReact(text) {
+  const leadingSpaces = /^\s*/.exec(text)[0];
+  const trailingSpaces = /\s*$/.exec(text)[0];
+
+  const start = leadingSpaces.includes('\n') ? leadingSpaces.length : 0;
+  const end = trailingSpaces.includes('\n') ? text.length - trailingSpaces.length : text.length;
+
+  return text.slice(start, end);
+}
+
+/**
+ * Test if node is like `<Fragment key={_}>_</Fragment>`
+ * @param {JSXElement} node
+ * @returns {boolean}
+ */
+function isKeyedElement(node) {
+  return node.type === 'JSXElement' &&
+    node.openingElement.attributes &&
+    node.openingElement.attributes.some(jsxUtil.isJSXAttributeKey);
+}
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    fixable: 'code',
+    docs: {
+      description: 'Disallow unnecessary fragments',
+      category: 'Possible Errors',
+      recommended: false,
+      url: docsUrl('jsx-no-useless-fragment')
+    },
+    messages: {
+      NeedsMoreChidren: 'Fragments should contain more than one child - otherwise, there‘s no need for a Fragment at all.',
+      ChildOfHtmlElement: 'Passing a fragment to an HTML element is useless.'
+    }
+  },
+
+  create(context) {
+    const reactPragma = pragmaUtil.getFromContext(context);
+    const fragmentPragma = pragmaUtil.getFragmentFromContext(context);
+
+    /**
+     * Test whether a node is an padding spaces trimmed by react runtime.
+     * @param {ASTNode} node
+     * @returns {boolean}
+     */
+    function isPaddingSpaces(node) {
+      return isJSXText(node) &&
+        isOnlyWhitespace(node.raw) &&
+        node.raw.includes('\n');
+    }
+
+    /**
+     * Test whether a JSXElement has less than two children, excluding paddings spaces.
+     * @param {JSXElement|JSXFragment} node
+     * @returns {boolean}
+     */
+    function hasLessThanTwoChildren(node) {
+      if (!node || !node.children || node.children.length < 2) {
+        return true;
+      }
+
+      return (
+        node.children.length -
+        (+isPaddingSpaces(node.children[0])) -
+        (+isPaddingSpaces(node.children[node.children.length - 1]))
+      ) < 2;
+    }
+
+    /**
+     * @param {JSXElement|JSXFragment} node
+     * @returns {boolean}
+     */
+    function isChildOfHtmlElement(node) {
+      return node.parent.type === 'JSXElement' &&
+        node.parent.openingElement.name.type === 'JSXIdentifier' &&
+        /^[a-z]+$/.test(node.parent.openingElement.name.name);
+    }
+
+    /**
+     * @param {JSXElement|JSXFragment} node
+     * @return {boolean}
+     */
+    function isChildOfComponentElement(node) {
+      return node.parent.type === 'JSXElement' &&
+        !isChildOfHtmlElement(node) &&
+        !jsxUtil.isFragment(node.parent, reactPragma, fragmentPragma);
+    }
+
+    /**
+     * @param {ASTNode} node
+     * @returns {boolean}
+     */
+    function canFix(node) {
+      // Not safe to fix fragments without a jsx parent.
+      if (!(node.parent.type === 'JSXElement' || node.parent.type === 'JSXFragment')) {
+        // const a = <></>
+        if (node.children.length === 0) {
+          return false;
+        }
+
+        // const a = <>cat {meow}</>
+        if (node.children.some(isNonspaceJSXTextOrJSXCurly)) {
+          return false;
+        }
+      }
+
+      // Not safe to fix `<Eeee><>foo</></Eeee>` because `Eeee` might require its children be a ReactElement.
+      if (isChildOfComponentElement(node)) {
+        return false;
+      }
+
+      return true;
+    }
+
+    /**
+     * @param {ASTNode} node
+     * @returns {Function | undefined}
+     */
+    function getFix(node) {
+      if (!canFix(node)) {
+        return undefined;
+      }
+
+      return function fix(fixer) {
+        const opener = node.type === 'JSXFragment' ? node.openingFragment : node.openingElement;
+        const closer = node.type === 'JSXFragment' ? node.closingFragment : node.closingElement;
+        const childrenText = context.getSourceCode().getText().slice(opener.range[1], closer.range[0]);
+
+        return fixer.replaceText(node, trimLikeReact(childrenText));
+      };
+    }
+
+    function checkNode(node) {
+      if (isKeyedElement(node)) {
+        return;
+      }
+
+      if (hasLessThanTwoChildren(node) && !isFragmentWithOnlyTextAndIsNotChild(node)) {
+        context.report({
+          node,
+          messageId: 'NeedsMoreChidren',
+          fix: getFix(node)
+        });
+      }
+
+      if (isChildOfHtmlElement(node)) {
+        context.report({
+          node,
+          messageId: 'ChildOfHtmlElement',
+          fix: getFix(node)
+        });
+      }
+    }
+
+    return {
+      JSXElement(node) {
+        if (jsxUtil.isFragment(node, reactPragma, fragmentPragma)) {
+          checkNode(node);
+        }
+      },
+      JSXFragment: checkNode
+    };
+  }
+};

lib/types.d.ts

@@ -9,6 +9,8 @@ declare global {
   type Token = eslint.AST.Token;
   type Fixer = eslint.Rule.RuleFixer;
   type JSXAttribute = ASTNode;
+  type JSXElement = ASTNode;
+  type JSXFragment = ASTNode;
   type JSXSpreadAttribute = ASTNode;
 
   interface Context extends eslint.SourceCode {

lib/util/jsx.js

@@ -26,6 +26,35 @@ function isDOMComponent(node) {
   return COMPAT_TAG_REGEX.test(name);
 }
 
+/**
+ * Test whether a JSXElement is a fragment
+ * @param {JSXElement} node
+ * @param {string} reactPragma
+ * @param {string} fragmentPragma
+ * @returns {boolean}
+ */
+function isFragment(node, reactPragma, fragmentPragma) {
+  const name = node.openingElement.name;
+
+  // <Fragment>
+  if (name.type === 'JSXIdentifier' && name.name === fragmentPragma) {
+    return true;
+  }
+
+  // <React.Fragment>
+  if (
+    name.type === 'JSXMemberExpression' &&
+    name.object.type === 'JSXIdentifier' &&
+    name.object.name === reactPragma &&
+    name.property.type === 'JSXIdentifier' &&
+    name.property.name === fragmentPragma
+  ) {
+    return true;
+  }
+
+  return false;
+}
+
 /**
  * Checks if a node represents a JSX element or fragment.
  * @param {object} node - node to check.
@@ -35,7 +64,21 @@ function isJSX(node) {
   return node && ['JSXElement', 'JSXFragment'].indexOf(node.type) >= 0;
 }
 
+/**
+ * Check if node is like `key={...}` as in `<Foo key={...} />`
+ * @param {ASTNode} node
+ * @returns {boolean}
+ */
+function isJSXAttributeKey(node) {
+  return node.type === 'JSXAttribute' &&
+    node.name &&
+    node.name.type === 'JSXIdentifier' &&
+    node.name.name === 'key';
+}
+
 module.exports = {
   isDOMComponent,
-  isJSX
+  isFragment,
+  isJSX,
+  isJSXAttributeKey
 };