Add vue/no-negated-v-if-condition rule

.changeset/early-worlds-reply.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-vue': minor
+---
+
+Add new `vue/no-negated-v-if-condition` rule

docs/rules/index.md

@@ -237,6 +237,7 @@ For example:
 | [vue/no-empty-component-block] | disallow the `<template>` `<script>` `<style>` block to be empty | :wrench: | :hammer: |
 | [vue/no-import-compiler-macros] | disallow importing Vue compiler macros | :wrench: | :warning: |
 | [vue/no-multiple-objects-in-class] | disallow passing multiple objects in an array to class |  | :hammer: |
+| [vue/no-negated-v-if-condition] | disallow negated conditions in v-if/v-else | :bulb: | :hammer: |
 | [vue/no-potential-component-option-typo] | disallow a potential typo in your component property | :bulb: | :hammer: |
 | [vue/no-ref-object-reactivity-loss] | disallow usages of ref objects that can lead to loss of reactivity |  | :warning: |
 | [vue/no-restricted-block] | disallow specific block |  | :hammer: |
@@ -482,6 +483,7 @@ The following rules extend the rules provided by ESLint itself and apply them to
 [vue/no-multiple-slot-args]: ./no-multiple-slot-args.md
 [vue/no-multiple-template-root]: ./no-multiple-template-root.md
 [vue/no-mutating-props]: ./no-mutating-props.md
+[vue/no-negated-v-if-condition]: ./no-negated-v-if-condition.md
 [vue/no-parsing-error]: ./no-parsing-error.md
 [vue/no-potential-component-option-typo]: ./no-potential-component-option-typo.md
 [vue/no-ref-as-operand]: ./no-ref-as-operand.md

docs/rules/no-negated-v-if-condition.md

@@ -0,0 +1,63 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/no-negated-v-if-condition
+description: disallow negated conditions in v-if/v-else
+---
+
+# vue/no-negated-v-if-condition
+
+> disallow negated conditions in v-if/v-else
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> _**This rule has not been released yet.**_ </badge>
+- :bulb: Some problems reported by this rule are manually fixable by editor [suggestions](https://eslint.org/docs/developer-guide/working-with-rules#providing-suggestions).
+
+## :book: Rule Details
+
+This rule disallows negated conditions in `v-if` and `v-else-if` directives which have an `v-else` branch.
+
+Negated conditions make the code less readable. When there's an `else` clause, it's better to use a positive condition and switch the branches.
+
+<eslint-code-block :rules="{'vue/no-negated-v-if-condition': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <div v-if="foo">First</div>
+  <div v-else>Second</div>
+
+  <div v-if="!foo">First</div>
+  <div v-else-if="bar">Second</div>
+
+  <div v-if="!foo">Content</div>
+
+  <div v-if="a !== b">Not equal</div>
+
+  <!-- ✗ BAD -->
+  <div v-if="!foo">First</div>
+  <div v-else>Second</div>
+
+  <div v-if="a !== b">First</div>
+  <div v-else>Second</div>
+
+  <div v-if="foo">First</div>
+  <div v-else-if="!bar">Second</div>
+  <div v-else>Third</div>
+</template>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :couple: Related Rules
+
+- [no-negated-condition](https://eslint.org/docs/latest/rules/no-negated-condition)
+- [unicorn/no-negated-condition](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-negated-condition.md)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/no-negated-v-if-condition.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/no-negated-v-if-condition.js)

lib/index.js

@@ -141,6 +141,7 @@ const plugin = {
     'no-multiple-slot-args': require('./rules/no-multiple-slot-args'),
     'no-multiple-template-root': require('./rules/no-multiple-template-root'),
     'no-mutating-props': require('./rules/no-mutating-props'),
+    'no-negated-v-if-condition': require('./rules/no-negated-v-if-condition'),
     'no-parsing-error': require('./rules/no-parsing-error'),
     'no-potential-component-option-typo': require('./rules/no-potential-component-option-typo'),
     'no-ref-as-operand': require('./rules/no-ref-as-operand'),

lib/rules/no-negated-v-if-condition.js

@@ -0,0 +1,203 @@
+/**
+ * @author Wayne Zhang
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+const utils = require('../utils')
+
+/**
+ * @typedef { VDirective & { value: (VExpressionContainer & { expression: Expression | null } ) | null  } } VIfDirective
+ */
+
+/**
+ * @param {Expression} expression
+ * @returns {boolean}
+ */
+function isNegatedExpression(expression) {
+  return (
+    (expression.type === 'UnaryExpression' && expression.operator === '!') ||
+    (expression.type === 'BinaryExpression' &&
+      (expression.operator === '!=' || expression.operator === '!=='))
+  )
+}
+
+/**
+ * @param {VElement} node
+ * @returns {VElement|null}
+ */
+function getNextSibling(node) {
+  if (!node.parent?.children) {
+    return null
+  }
+
+  const siblings = node.parent.children
+  const currentIndex = siblings.indexOf(node)
+
+  for (let i = currentIndex + 1; i < siblings.length; i++) {
+    const sibling = siblings[i]
+    if (sibling.type === 'VElement') {
+      return sibling
+    }
+  }
+
+  return null
+}
+
+/**
+ * @param {VElement} element
+ * @returns {boolean}
+ */
+function isDirectlyFollowedByElse(element) {
+  const nextElement = getNextSibling(element)
+  return nextElement ? utils.hasDirective(nextElement, 'else') : false
+}
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'disallow negated conditions in v-if/v-else',
+      categories: undefined,
+      url: 'https://eslint.vuejs.org/rules/no-negated-v-if-condition.html'
+    },
+    fixable: null,
+    hasSuggestions: true,
+    schema: [],
+    messages: {
+      negatedCondition: 'Unexpected negated condition in v-if with v-else.',
+      fixNegatedCondition:
+        'Convert to positive condition and swap if/else blocks.'
+    }
+  },
+  /** @param {RuleContext} context */
+  create(context) {
+    const sourceCode = context.getSourceCode()
+    const templateTokens =
+      sourceCode.parserServices.getTemplateBodyTokenStore &&
+      sourceCode.parserServices.getTemplateBodyTokenStore()
+
+    /**
+     * @param {VIfDirective} node
+     */
+    function checkNegatedCondition(node) {
+      if (!node.value?.expression) {
+        return
+      }
+
+      const expression = node.value.expression
+      const element = node.parent.parent
+
+      if (
+        !isNegatedExpression(expression) ||
+        !isDirectlyFollowedByElse(element)
+      ) {
+        return
+      }
+
+      const elseElement = getNextSibling(element)
+      if (!elseElement) {
+        return
+      }
+
+      context.report({
+        node: expression,
+        messageId: 'negatedCondition',
+        suggest: [
+          {
+            messageId: 'fixNegatedCondition',
+            *fix(fixer) {
+              yield* convertNegatedCondition(fixer, expression)
+              yield* swapElementContents(fixer, element, elseElement)
+            }
+          }
+        ]
+      })
+    }
+
+    /**
+     * @param {RuleFixer} fixer
+     * @param {Expression} expression
+     */
+    function* convertNegatedCondition(fixer, expression) {
+      if (
+        expression.type === 'UnaryExpression' &&
+        expression.operator === '!'
+      ) {
+        const token = templateTokens.getFirstToken(expression)
+        if (token?.type === 'Punctuator' && token.value === '!') {
+          yield fixer.remove(token)
+        }
+        return
+      }
+
+      if (expression.type === 'BinaryExpression') {
+        const operatorToken = templateTokens.getTokenAfter(
+          expression.left,
+          (token) =>
+            token?.type === 'Punctuator' && token.value === expression.operator
+        )
+
+        if (!operatorToken) return
+
+        if (expression.operator === '!=') {
+          yield fixer.replaceText(operatorToken, '==')
+        } else if (expression.operator === '!==') {
+          yield fixer.replaceText(operatorToken, '===')
+        }
+      }
+    }
+
+    /**
+     * @param {VElement} element
+     * @returns {string}
+     */
+    function getElementContent(element) {
+      if (element.children.length === 0) return ''
+      if (!element.endTag) return ''
+
+      const contentStart = element.startTag.range[1]
+      const contentEnd = element.endTag.range[0]
+
+      return sourceCode.text.slice(contentStart, contentEnd)
+    }
+
+    /**
+     * @param {RuleFixer} fixer
+     * @param {VElement} ifElement
+     * @param {VElement} elseElement
+     */
+    function* swapElementContents(fixer, ifElement, elseElement) {
+      if (!ifElement.endTag || !elseElement.endTag) {
+        return
+      }
+
+      const ifContent = getElementContent(ifElement)
+      const elseContent = getElementContent(elseElement)
+
+      if (ifContent === elseContent) {
+        return
+      }
+
+      yield fixer.replaceTextRange(
+        [ifElement.startTag.range[1], ifElement.endTag.range[0]],
+        elseContent
+      )
+      yield fixer.replaceTextRange(
+        [elseElement.startTag.range[1], elseElement.endTag.range[0]],
+        ifContent
+      )
+    }
+
+    return utils.defineTemplateBodyVisitor(context, {
+      /** @param {VIfDirective} node */
+      "VAttribute[directive=true][key.name.name='if']"(node) {
+        checkNegatedCondition(node)
+      },
+      /** @param {VIfDirective} node */
+      "VAttribute[directive=true][key.name.name='else-if']"(node) {
+        checkNegatedCondition(node)
+      }
+    })
+  }
+}