feat(lint): noUselessThisAlias (#23)

CHANGELOG.md

@@ -216,6 +216,10 @@ New entries must be placed in a section entitled `Unreleased`.
 
   This rule disallows declaration merging between an interface and a class.
 
+- Add [noUselessThisAlias](https://biomejs.dev/lint/rules/noUselessThisAlias/)
+
+  This rule disallows useless aliasing of `this` in arrow functions.
+
 - Add [useArrowFunction](https://biomejs.dev/lint/rules/usearrowfunction/)
 
   This rule proposes turning function expressions into arrow functions.

crates/rome_diagnostics_categories/src/categories.rs

@@ -98,6 +98,7 @@ define_categories! {
     "lint/nursery/noStaticOnlyClass": "https://biomejs.dev/lint/rules/noStaticOnlyClass",
     "lint/nursery/noUnsafeDeclarationMerging": "https://biomejs.dev/lint/rules/noUnsafeDeclarationMerging",
     "lint/nursery/noUselessEmptyExport": "https://biomejs.dev/lint/rules/noUselessEmptyExport",
+    "lint/nursery/noUselessThisAlias": "https://biomejs.dev/lint/rules/noUselessThisAlias",
     "lint/nursery/noVoid": "https://biomejs.dev/lint/rules/noVoid",
     "lint/nursery/useAriaPropTypes": "https://biomejs.dev/lint/rules/useAriaPropTypes",
     "lint/nursery/useArrowFunction": "https://biomejs.dev/lint/rules/useArrowFunction",

crates/rome_js_analyze/src/semantic_analyzers/nursery/no_useless_this_alias.rs

@@ -0,0 +1,178 @@
+use crate::{control_flow::AnyJsControlFlowRoot, semantic_services::Semantic, JsRuleAction};
+use rome_analyze::{context::RuleContext, declare_rule, ActionCategory, Rule, RuleDiagnostic};
+use rome_console::markup;
+use rome_diagnostics::Applicability;
+use rome_js_factory::make;
+use rome_js_semantic::ReferencesExtensions;
+use rome_js_syntax::{
+    AnyJsBinding, AnyJsBindingPattern, AnyJsExpression, JsArrowFunctionExpression,
+    JsAssignmentExpression, JsExpressionStatement, JsIdentifierBinding, JsIdentifierExpression,
+    JsThisExpression, JsVariableDeclaration, JsVariableDeclarator, T,
+};
+use rome_rowan::{AstNode, AstSeparatedList, BatchMutationExt};
+
+declare_rule! {
+    /// Disallow useless `this` aliasing.
+    ///
+    /// Arrow functions inherits `this` from their enclosing scope;
+    /// this makes `this` aliasing useless in this situation.
+    ///
+    /// Credits: https://typescript-eslint.io/rules/no-this-alias/
+    ///
+    /// ## Examples
+    ///
+    /// ### Invalid
+    ///
+    /// ```js,expect_diagnostic
+    /// class A {
+    ///     method() {
+    ///         const self = this;
+    ///         return () => {
+    ///             return self;
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ## Valid
+    ///
+    /// ```js
+    /// class A {
+    ///     method() {
+    ///         const self = this;
+    ///         return function() {
+    ///             this.g();
+    ///             return self;
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    pub(crate) NoUselessThisAlias {
+        version: "1.0.0",
+        name: "noUselessThisAlias",
+        recommended: true,
+    }
+}
+
+impl Rule for NoUselessThisAlias {
+    type Query = Semantic<JsVariableDeclarator>;
+    type State = JsIdentifierBinding;
+    type Signals = Option<Self::State>;
+    type Options = ();
+
+    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
+        let declarator = ctx.query();
+        let model = ctx.model();
+        let mut is_this_alias = if let Some(initializer) = declarator.initializer() {
+            let initializer = initializer.expression().ok()?.omit_parentheses();
+            if !JsThisExpression::can_cast(initializer.syntax().kind()) {
+                return None;
+            }
+            true
+        } else {
+            false
+        };
+        let Ok(AnyJsBindingPattern::AnyJsBinding(AnyJsBinding::JsIdentifierBinding(id))) = declarator.id() else {
+            // Ignore destructuring
+            return None;
+        };
+        let this_scope = declarator
+            .syntax()
+            .ancestors()
+            .find_map(AnyJsControlFlowRoot::cast)?;
+        for write in id.all_writes(model) {
+            let assign = JsAssignmentExpression::cast(write.syntax().parent()?)?;
+            let assign_right = assign.right().ok()?.omit_parentheses();
+            if !JsThisExpression::can_cast(assign_right.syntax().kind()) {
+                return None;
+            }
+            is_this_alias = true;
+        }
+        // This cehck is useful when the loop is not executed (no write).
+        if !is_this_alias {
+            return None;
+        }
+        for reference in id.all_references(model) {
+            let current_this_scope = reference
+                .syntax()
+                .ancestors()
+                .filter(|x| !JsArrowFunctionExpression::can_cast(x.kind()))
+                .find_map(AnyJsControlFlowRoot::cast)?;
+            if this_scope != current_this_scope {
+                // The aliasing is required because they have not the same `this` scope.
+                return None;
+            }
+        }
+        Some(id)
+    }
+
+    fn diagnostic(ctx: &RuleContext<Self>, _: &Self::State) -> Option<RuleDiagnostic> {
+        let declarator = ctx.query();
+        Some(
+            RuleDiagnostic::new(
+                rule_category!(),
+                declarator.range(),
+                markup! {
+                    "This aliasing of "<Emphasis>"this"</Emphasis>" is unnecessary."
+                },
+            )
+            .note(markup! {
+                "Arrow functions inherits `this` from their enclosing scope."
+            }),
+        )
+    }
+
+    fn action(ctx: &RuleContext<Self>, id: &Self::State) -> Option<JsRuleAction> {
+        let declarator = ctx.query();
+        let model = ctx.model();
+        let Some(var_decl) = declarator.syntax().ancestors().find_map(JsVariableDeclaration::cast) else {
+            return None;
+        };
+        let mut mutation = ctx.root().begin();
+        let this_expr = AnyJsExpression::from(make::js_this_expression(make::token(T![this])));
+        for read in id.all_reads(model) {
+            let syntax = read.syntax();
+            let syntax = syntax.parent()?;
+            let Some(expr) = JsIdentifierExpression::cast(syntax) else {
+                return None;
+            };
+            mutation.replace_node(expr.into(), this_expr.clone());
+        }
+        for write in id.all_writes(model) {
+            let syntax = write.syntax();
+            let syntax = syntax.parent()?;
+            let Some(statement) = JsExpressionStatement::cast(syntax.parent()?) else {
+                return None;
+            };
+            mutation.remove_node(statement);
+        }
+        let var_declarator_list = var_decl.declarators();
+        if var_declarator_list.len() == 1 {
+            mutation.remove_node(var_decl);
+        } else {
+            let mut deleted_comma = None;
+            for (current_declarator, current_comma) in var_declarator_list
+                .iter()
+                .zip(var_declarator_list.separators())
+            {
+                deleted_comma = current_comma.ok();
+                let current_declarator = current_declarator.ok()?;
+                if &current_declarator == declarator {
+                    break;
+                }
+            }
+            mutation.remove_node(declarator.clone());
+            mutation.remove_token(deleted_comma?);
+        }
+        Some(JsRuleAction {
+            category: ActionCategory::QuickFix,
+            applicability: Applicability::Always,
+            message: markup! {
+                "Use "<Emphasis>"this"</Emphasis>" instead of an alias."
+            }
+            .to_owned(),
+            mutation,
+        })
+    }
+}

crates/rome_js_analyze/tests/specs/nursery/noUselessThisAlias/invalid.js

@@ -0,0 +1,25 @@
+const self = this, v = 0, /*u*/ u = 2, self2 = this;
+
+function f() {
+    // assignment comment
+    const self = this;
+    return () => {
+        /*a*/self/*b*/.g();
+    }
+}
+
+function f() {
+    let self = this;
+    return () => {
+        self.g();
+    }
+}
+
+function f() {
+    var self;
+    self = this;
+    self = this;
+    return () => {
+        self.g();
+    }
+}