feat(linter) deepscan: bad char at comparison

crates/oxc_linter/src/rules.rs

@@ -18,6 +18,7 @@ mod import {
 mod deepscan {
     pub mod bad_array_method_on_arguments;
     pub mod bad_bitwise_operator;
+    pub mod bad_char_at_comparison;
     pub mod bad_comparison_sequence;
     pub mod bad_min_max_func;
     pub mod bad_replace_all_arg;
@@ -249,6 +250,7 @@ mod oxc {
 oxc_macros::declare_all_lint_rules! {
     deepscan::bad_array_method_on_arguments,
     deepscan::bad_bitwise_operator,
+    deepscan::bad_char_at_comparison,
     deepscan::bad_comparison_sequence,
     deepscan::bad_min_max_func,
     deepscan::bad_replace_all_arg,

crates/oxc_linter/src/rules/deepscan/bad_char_at_comparison.rs

@@ -0,0 +1,121 @@
+use oxc_ast::{ast::Expression, AstKind};
+use oxc_diagnostics::{
+    miette::{self, Diagnostic},
+    thiserror::Error,
+};
+use oxc_macros::declare_oxc_lint;
+use oxc_span::{GetSpan, Span};
+use oxc_syntax::operator::BinaryOperator;
+
+use crate::{ast_util::is_method_call, context::LintContext, rule::Rule, AstNode};
+
+#[derive(Debug, Error, Diagnostic)]
+#[error("deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method")]
+#[diagnostic(severity(warning), help("`String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false."))]
+struct BadCharAtComparisonDiagnostic(
+    #[label("`charAt` called here")] pub Span,
+    #[label("And compared with a string of length {2} here")] pub Span,
+    usize,
+);
+
+#[derive(Debug, Default, Clone)]
+pub struct BadCharAtComparison;
+
+declare_oxc_lint!(
+    /// ### What it does
+    ///
+    /// This rule warns when the return value of the `charAt` method is used to compare a string of length greater than 1.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// The `charAt` method returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+    ///
+    /// ### Example
+    /// ```javascript
+    /// // Bad: The return value of the `charAt` method is compared with a string of length greater than 1.
+    /// a.charAt(4) === 'a2';
+    /// a.charAt(4) === '/n';
+    ///
+    /// // Good: The return value of the `charAt` method is compared with a string of length 1.
+    /// a.charAt(4) === 'a'
+    /// a.charAt(4) === '\n';
+    /// ```
+    BadCharAtComparison,
+    correctness
+);
+
+impl Rule for BadCharAtComparison {
+    fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
+        let AstKind::CallExpression(call_expr) = node.kind() else { return };
+
+        if !is_method_call(call_expr, None, Some(&["charAt"]), Some(1), Some(1)) {
+            return;
+        }
+
+        let Some(parent) = ctx.nodes().parent_node(node.id()) else {
+            return;
+        };
+
+        let AstKind::BinaryExpression(binary_expr) = parent.kind() else { return };
+        if !matches!(
+            binary_expr.operator,
+            BinaryOperator::Equality
+                | BinaryOperator::Inequality
+                | BinaryOperator::StrictEquality
+                | BinaryOperator::StrictInequality
+        ) {
+            return;
+        };
+
+        let comparison_with = if binary_expr.left.span() == call_expr.span {
+            &binary_expr.right
+        } else {
+            &binary_expr.left
+        };
+
+        if let Expression::StringLiteral(string_lit) = comparison_with {
+            if !is_string_valid(string_lit.value.as_str()) {
+                ctx.diagnostic(BadCharAtComparisonDiagnostic(
+                    call_expr.span,
+                    string_lit.span,
+                    string_lit.value.len(),
+                ));
+            }
+        }
+    }
+}
+
+fn is_string_valid(str: &str) -> bool {
+    if str.len() < 2 || str.chars().count() == 1 {
+        return true;
+    }
+
+    false
+}
+
+#[test]
+fn test() {
+    use crate::tester::Tester;
+
+    let pass = vec![
+        r"a.charAt(4) === 'a'",
+        "a.charAt(4) === '\\n'",
+        "a.charAt(4) === '\t'",
+        r"a.charAt(4) === 'a'",
+        r"a.charAt(4) === '\ufeff'",
+        r"a.charAt(4) !== '\ufeff'",
+        "chatAt(4) === 'a2'",
+        "new chatAt(4) === 'a'",
+    ];
+
+    let fail = vec![
+        r"a.charAt(4) === 'aa'",
+        "a.charAt(4) === '/n'",
+        "a.charAt(3) === '/t'",
+        r"a.charAt(4) === 'ac'",
+        r"a.charAt(822) !== 'foo'",
+        r"a.charAt(4) === '\\ukeff'",
+    ];
+
+    Tester::new_without_config(BadCharAtComparison::NAME, pass, fail).test_and_snapshot();
+}

crates/oxc_linter/src/snapshots/bad_char_at_comparison.snap

@@ -0,0 +1,59 @@
+---
+source: crates/oxc_linter/src/tester.rs
+expression: bad_char_at_comparison
+---
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(4) === 'aa'
+   · ─────┬─────     ──┬─
+   ·      │            ╰── And compared with a string of length 2 here
+   ·      ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(4) === '/n'
+   · ─────┬─────     ──┬─
+   ·      │            ╰── And compared with a string of length 2 here
+   ·      ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(3) === '/t'
+   · ─────┬─────     ──┬─
+   ·      │            ╰── And compared with a string of length 2 here
+   ·      ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(4) === 'ac'
+   · ─────┬─────     ──┬─
+   ·      │            ╰── And compared with a string of length 2 here
+   ·      ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(822) !== 'foo'
+   · ──────┬──────     ──┬──
+   ·       │             ╰── And compared with a string of length 3 here
+   ·       ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+  ⚠ deepscan(bad-char-at-comparison): Invalid comparison with `charAt` method
+   ╭─[bad_char_at_comparison.tsx:1:1]
+ 1 │ a.charAt(4) === '\\ukeff'
+   · ─────┬─────     ────┬────
+   ·      │              ╰── And compared with a string of length 6 here
+   ·      ╰── `charAt` called here
+   ╰────
+  help: `String.prototype.charAt` returns a string of length 1. If the return value is compared with a string of length greater than 1, the comparison will always be false.
+
+