feat(linter): Implement no-undef-init

crates/oxc_linter/src/ast_util.rs

@@ -3,10 +3,10 @@ use oxc_semantic::{AstNode, IsGlobalReference, NodeId, SymbolId};
 use oxc_span::{GetSpan, Span};
 use oxc_syntax::operator::{AssignmentOperator, BinaryOperator, LogicalOperator, UnaryOperator};
 
+use crate::context::LintContext;
 #[allow(clippy::wildcard_imports)]
 use oxc_ast::ast::*;
-
-use crate::context::LintContext;
+use oxc_syntax::scope::ScopeId;
 
 /// Test if an AST node is a boolean value that never changes. Specifically we
 /// test for:
@@ -400,6 +400,10 @@ pub fn is_function_node(node: &AstNode) -> bool {
     }
 }
 
+pub fn is_shadowed<'a>(scope_id: ScopeId, name: &'a str, ctx: &LintContext<'a>) -> bool {
+    ctx.scopes().find_binding(scope_id, name).is_some()
+}
+
 pub fn get_function_like_declaration<'b>(
     node: &AstNode<'b>,
     ctx: &LintContext<'b>,

crates/oxc_linter/src/rules.rs

@@ -109,6 +109,7 @@ mod eslint {
     pub mod no_ternary;
     pub mod no_this_before_super;
     pub mod no_undef;
+    pub mod no_undef_init;
     pub mod no_undefined;
     pub mod no_unexpected_multiline;
     pub mod no_unreachable;
@@ -578,6 +579,7 @@ oxc_macros::declare_all_lint_rules! {
     eslint::no_ternary,
     eslint::no_this_before_super,
     eslint::no_undef,
+    eslint::no_undef_init,
     eslint::no_undefined,
     eslint::no_unexpected_multiline,
     eslint::no_unreachable,

crates/oxc_linter/src/rules/eslint/no_alert.rs

@@ -4,6 +4,7 @@ use oxc_macros::declare_oxc_lint;
 use oxc_semantic::ScopeId;
 use oxc_span::{GetSpan, Span};
 
+use crate::ast_util::is_shadowed;
 use crate::{context::LintContext, rule::Rule, AstNode};
 
 fn no_alert_diagnostic(span: Span) -> OxcDiagnostic {
@@ -83,10 +84,6 @@ fn is_global_this_ref_or_global_window<'a>(
     false
 }
 
-fn is_shadowed<'a>(scope_id: ScopeId, name: &'a str, ctx: &LintContext<'a>) -> bool {
-    ctx.scopes().find_binding(scope_id, name).is_some()
-}
-
 impl Rule for NoAlert {
     fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
         let AstKind::CallExpression(call_expr) = node.kind() else {

crates/oxc_linter/src/rules/eslint/no_undef_init.rs

@@ -0,0 +1,144 @@
+use crate::ast_util::is_shadowed;
+use crate::{context::LintContext, rule::Rule, AstNode};
+use oxc_ast::ast::{VariableDeclarationKind, VariableDeclarator};
+use oxc_ast::AstKind;
+use oxc_diagnostics::OxcDiagnostic;
+use oxc_macros::declare_oxc_lint;
+use oxc_span::{GetSpan, Span};
+
+fn no_undef_init(name: &str, span: Span) -> OxcDiagnostic {
+    OxcDiagnostic::warn(format!("It's not necessary to initialize {name} to undefined."))
+        .with_label(span.label("A variable that is declared and not initialized to any value automatically gets the value of undefined."))
+}
+
+#[derive(Debug, Default, Clone)]
+pub struct NoUndefInit;
+
+declare_oxc_lint!(
+    /// ### What it does
+    ///
+    /// Disallow initializing variables to `undefined`.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// A variable that is declared and not initialized to any value automatically gets the value of `undefined`.
+    /// It’s considered a best practice to avoid initializing variables to `undefined`.
+    ///
+    /// ### Examples
+    ///
+    /// Examples of **incorrect** code for this rule:
+    /// ```js
+    /// var a = undefined;
+    /// ```
+    ///
+    /// Examples of **correct** code for this rule:
+    /// ```js
+    /// var a;
+    /// ```
+    NoUndefInit,
+    style,
+    fix
+);
+
+impl Rule for NoUndefInit {
+    fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
+        if let AstKind::VariableDeclaration(var_decl) = node.kind() {
+            let faulty_declarations: Vec<&VariableDeclarator> = var_decl
+                .declarations
+                .iter()
+                .filter(|decl| {
+                    decl.init.as_ref().is_some_and(oxc_ast::ast::Expression::is_undefined)
+                })
+                .collect::<Vec<&VariableDeclarator>>();
+
+            if var_decl.kind != VariableDeclarationKind::Const
+                && !faulty_declarations.is_empty()
+                && !is_shadowed(node.scope_id(), "undefined", ctx)
+            {
+                for decl in faulty_declarations {
+                    if decl.kind == VariableDeclarationKind::Var
+                        || decl.id.kind.is_destructuring_pattern()
+                        || ctx
+                            .semantic()
+                            .trivias()
+                            .has_comments_between(Span::new(decl.id.span().start, decl.span().end))
+                    {
+                        let diagnostic_span = Span::new(decl.id.span().end, decl.span().end);
+                        let variable_name = &ctx.source_text()[decl.id.span()];
+                        ctx.diagnostic(no_undef_init(variable_name, diagnostic_span));
+                        continue;
+                    }
+                    let identifier = decl.id.get_binding_identifier().unwrap();
+                    if let Some(init) = &decl.init {
+                        let diagnostic_span = Span::new(identifier.span.end, init.span().end);
+                        ctx.diagnostic_with_fix(
+                            no_undef_init(identifier.name.as_str(), diagnostic_span),
+                            |fixer| fixer.delete_range(diagnostic_span),
+                        );
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn test() {
+    use crate::tester::Tester;
+
+    let pass = vec![
+        "var a;",
+        "const foo = undefined", // { "ecmaVersion": 6 },
+        "var undefined = 5; var foo = undefined;",
+        "class C { field = undefined; }", // { "ecmaVersion": 2022 }
+    ];
+
+    let fail = vec![
+        "var a = undefined;",
+        "var a = undefined, b = 1;",
+        "var a = 1, b = undefined, c = 5;",
+        "var [a] = undefined;", // { "ecmaVersion": 6 },
+        "var {a} = undefined;", // { "ecmaVersion": 6 },
+        "for(var i in [1,2,3]){var a = undefined; for(var j in [1,2,3]){}}",
+        "let a = undefined;",               // { "ecmaVersion": 6 },
+        "let a = undefined, b = 1;",        // { "ecmaVersion": 6 },
+        "let a = 1, b = undefined, c = 5;", // { "ecmaVersion": 6 },
+        "let [a] = undefined;",             // { "ecmaVersion": 6 },
+        "let {a} = undefined;",             // { "ecmaVersion": 6 },
+        "for(var i in [1,2,3]){let a = undefined; for(var j in [1,2,3]){}}", // { "ecmaVersion": 6 },
+        "let /* comment */a = undefined;", // { "ecmaVersion": 6 },
+        "let a/**/ = undefined;",          // { "ecmaVersion": 6 },
+        "let a /**/ = undefined;",         // { "ecmaVersion": 6 },
+        "let a//
+			= undefined;",         // { "ecmaVersion": 6 },
+        "let a = /**/undefined;",          // { "ecmaVersion": 6 },
+        "let a = //
+			undefined;",        // { "ecmaVersion": 6 },
+        "let a = undefined/* comment */;", // { "ecmaVersion": 6 },
+        "let a = undefined/* comment */, b;", // { "ecmaVersion": 6 },
+        "let a = undefined//comment
+			, b;", // { "ecmaVersion": 6 }
+    ];
+
+    let fix = vec![
+        ("let a = undefined;", "let a;", None),
+        ("let a = undefined, b = 1;", "let a, b = 1;", None),
+        ("let a = 1, b = undefined, c = 5;", "let a = 1, b, c = 5;", None),
+        (
+            "for(var i in [1,2,3]){let a = undefined; for(var j in [1,2,3]){}}",
+            "for(var i in [1,2,3]){let a; for(var j in [1,2,3]){}}",
+            None,
+        ),
+        ("let /* comment */a = undefined;", "let /* comment */a;", None),
+        ("let a = undefined/* comment */;", "let a/* comment */;", None),
+        ("let a = undefined/* comment */, b;", "let a/* comment */, b;", None),
+        (
+            "let a = undefined//comment
+			, b;",
+            "let a//comment
+			, b;",
+            None,
+        ),
+    ];
+    Tester::new(NoUndefInit::NAME, pass, fail).expect_fix(fix).test_and_snapshot();
+}

crates/oxc_linter/src/snapshots/no_undef_init.snap

@@ -0,0 +1,158 @@
+---
+source: crates/oxc_linter/src/tester.rs
+---
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ var a = undefined;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ var a = undefined, b = 1;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize b to undefined.
+   ╭─[no_undef_init.tsx:1:13]
+ 1 │ var a = 1, b = undefined, c = 5;
+   ·             ──────┬─────
+   ·                   ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize [a] to undefined.
+   ╭─[no_undef_init.tsx:1:8]
+ 1 │ var [a] = undefined;
+   ·        ──────┬─────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize {a} to undefined.
+   ╭─[no_undef_init.tsx:1:8]
+ 1 │ var {a} = undefined;
+   ·        ──────┬─────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:28]
+ 1 │ for(var i in [1,2,3]){var a = undefined; for(var j in [1,2,3]){}}
+   ·                            ──────┬─────
+   ·                                  ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = undefined;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = undefined, b = 1;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize b to undefined.
+   ╭─[no_undef_init.tsx:1:13]
+ 1 │ let a = 1, b = undefined, c = 5;
+   ·             ──────┬─────
+   ·                   ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize [a] to undefined.
+   ╭─[no_undef_init.tsx:1:8]
+ 1 │ let [a] = undefined;
+   ·        ──────┬─────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize {a} to undefined.
+   ╭─[no_undef_init.tsx:1:8]
+ 1 │ let {a} = undefined;
+   ·        ──────┬─────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:28]
+ 1 │ for(var i in [1,2,3]){let a = undefined; for(var j in [1,2,3]){}}
+   ·                            ──────┬─────
+   ·                                  ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:19]
+ 1 │ let /* comment */a = undefined;
+   ·                   ──────┬─────
+   ·                         ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a/**/ = undefined;
+   ·      ────────┬───────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a /**/ = undefined;
+   ·      ────────┬────────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ ╭─▶ let a//
+ 2 │ ├─▶             = undefined;
+   · ╰──── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = /**/undefined;
+   ·      ────────┬───────
+   ·              ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ ╭─▶ let a = //
+ 2 │ ├─▶             undefined;
+   · ╰──── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = undefined/* comment */;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = undefined/* comment */, b;
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+   ╰────
+  help: Delete this code.
+
+  ⚠ eslint(no-undef-init): It's not necessary to initialize a to undefined.
+   ╭─[no_undef_init.tsx:1:6]
+ 1 │ let a = undefined//comment
+   ·      ──────┬─────
+   ·            ╰── A variable that is declared and not initialized to any value automatically gets the value of undefined.
+ 2 │             , b;
+   ╰────
+  help: Delete this code.