feat(linter) eslint plugin unicorn: prefer array flat (#1650)

crates/oxc_linter/src/rules.rs

@@ -187,6 +187,7 @@ mod unicorn {
     pub mod number_literal_case;
     pub mod numeric_separators_style;
     pub mod prefer_add_event_listener;
+    pub mod prefer_array_flat;
     pub mod prefer_array_flat_map;
     pub mod prefer_array_some;
     pub mod prefer_blob_reading_methods;
@@ -384,6 +385,7 @@ oxc_macros::declare_all_lint_rules! {
     unicorn::numeric_separators_style,
     unicorn::prefer_add_event_listener,
     unicorn::prefer_array_flat_map,
+    unicorn::prefer_array_flat,
     unicorn::prefer_array_some,
     unicorn::prefer_blob_reading_methods,
     unicorn::prefer_code_point,

crates/oxc_linter/src/rules/unicorn/no_array_reduce.rs

@@ -64,7 +64,7 @@ impl Rule for NoArrayReduce {
             return;
         };
 
-        let Some(member_expr) = (call_expr).callee.get_member_expr() else {
+        let Some(member_expr) = call_expr.callee.get_member_expr() else {
             return;
         };
 

crates/oxc_linter/src/rules/unicorn/prefer_array_flat.rs

@@ -0,0 +1,416 @@
+use oxc_ast::{
+    ast::{
+        Argument, ArrayExpressionElement, BindingPatternKind, CallExpression, Expression, Statement,
+    },
+    AstKind,
+};
+use oxc_diagnostics::{
+    miette::{self, Diagnostic},
+    thiserror::Error,
+};
+use oxc_macros::declare_oxc_lint;
+use oxc_span::Span;
+
+use crate::{
+    ast_util::is_method_call,
+    context::LintContext,
+    rule::Rule,
+    utils::get_first_parameter_name,
+    utils::{get_return_identifier_name, is_empty_array_expression, is_prototype_property},
+    AstNode,
+};
+
+#[derive(Debug, Error, Diagnostic)]
+#[error("eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.")]
+#[diagnostic(severity(warning), help(r"Call `.flat()` on the array instead."))]
+struct PreferArrayFlatDiagnostic(#[label] pub Span);
+
+#[derive(Debug, Default, Clone)]
+pub struct PreferArrayFlat;
+
+declare_oxc_lint!(
+    /// ### What it does
+    ///
+    /// Prefers `Array#flat()` over legacy techniques to flatten arrays.    ///
+    ///
+    /// ### Why is this bad?
+    ///
+    /// ES2019 introduced a new method [`Array#flat()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat) that flatten arrays.
+    ///
+    /// This rule aims to standardize the use of `Array#flat()` over legacy techniques to flatten arrays.
+    ///
+    /// ### Example
+    /// ```javascript
+    /// // Bad
+    /// const foo = array.flatMap(x => x);
+    /// const foo = array.reduce((a, b) => a.concat(b), []);
+    /// const foo = array.reduce((a, b) => [...a, ...b], []);
+    /// const foo = [].concat(maybeArray);
+    /// const foo = [].concat(...array);
+    /// const foo = [].concat.apply([], array);
+    /// const foo = Array.prototype.concat.apply([], array);
+    /// const foo = Array.prototype.concat.call([], maybeArray);
+    /// const foo = Array.prototype.concat.call([], ...array);
+    ///
+    /// // Good
+    /// const foo = array.flat();
+    /// const foo = [maybeArray].flat();
+    /// ```
+    PreferArrayFlat,
+    correctness
+);
+
+impl Rule for PreferArrayFlat {
+    fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
+        let AstKind::CallExpression(call_expr) = node.kind() else {
+            return;
+        };
+
+        check_array_flat_map_case(call_expr, ctx);
+        check_array_reduce_case(call_expr, ctx);
+        check_array_concat_case(call_expr, ctx);
+        check_array_prototype_concat_case(call_expr, ctx);
+    }
+}
+
+// `array.flatMap(x => x)`
+fn check_array_flat_map_case<'a>(call_expr: &CallExpression<'a>, ctx: &LintContext<'a>) {
+    if !is_method_call(call_expr, None, Some(&["flatMap"]), Some(1), Some(1)) {
+        return;
+    }
+
+    let Argument::Expression(first_argument) = call_expr.arguments.get(0).unwrap() else {
+        return;
+    };
+
+    let Expression::ArrowExpression(first_argument) = first_argument else {
+        return;
+    };
+
+    if first_argument.r#async || first_argument.params.parameters_count() != 1 {
+        return;
+    }
+
+    let Some(first_param_name) = get_first_parameter_name(&first_argument.params) else {
+        return;
+    };
+
+    let Some(return_param_name) = get_return_identifier_name(&first_argument.body) else {
+        return;
+    };
+
+    if first_param_name != return_param_name {
+        return;
+    }
+
+    ctx.diagnostic(PreferArrayFlatDiagnostic(call_expr.span));
+}
+
+// `array.reduce((a, b) => a.concat(b), [])`
+// `array.reduce((a, b) => [...a, ...b], [])`
+fn check_array_reduce_case<'a>(call_expr: &CallExpression<'a>, ctx: &LintContext<'a>) {
+    if !is_method_call(call_expr, None, Some(&["reduce"]), Some(2), Some(2)) {
+        return;
+    }
+    let Argument::Expression(Expression::ArrowExpression(first_argument)) =
+        call_expr.arguments.get(0).unwrap()
+    else {
+        return;
+    };
+    let Argument::Expression(second_argument) = call_expr.arguments.get(1).unwrap() else {
+        return;
+    };
+
+    if first_argument.r#async
+        || first_argument.params.parameters_count() != 2
+        || !is_empty_array_expression(second_argument)
+    {
+        return;
+    }
+
+    let Some((first_parameter, second_parameter)) = ({
+        match (
+            &first_argument.params.items[0].pattern.kind,
+            &first_argument.params.items[1].pattern.kind,
+        ) {
+            (
+                BindingPatternKind::BindingIdentifier(ref first_param),
+                BindingPatternKind::BindingIdentifier(second_param),
+            ) => Some((&first_param.name, &second_param.name)),
+
+            _ => None,
+        }
+    }) else {
+        return;
+    };
+
+    let Some(Statement::ExpressionStatement(expr_stmt)) = first_argument.body.statements.get(0)
+    else {
+        return;
+    };
+
+    // `array.reduce((a, b) => a.concat(b), [])`
+    if let Expression::CallExpression(concat_call_expr) = &expr_stmt.expression {
+        if is_method_call(concat_call_expr, None, Some(&["concat"]), Some(1), Some(1)) {
+            if let Argument::Expression(Expression::Identifier(first_argument_ident)) =
+                &concat_call_expr.arguments[0]
+            {
+                if first_argument_ident.name != second_parameter {
+                    return;
+                }
+
+                let Expression::Identifier(second_argument_ident) =
+                    concat_call_expr.callee.get_member_expr().unwrap().object()
+                else {
+                    return;
+                };
+
+                if second_argument_ident.name != first_parameter {
+                    return;
+                }
+
+                ctx.diagnostic(PreferArrayFlatDiagnostic(call_expr.span));
+            }
+        }
+    }
+
+    if let Expression::ArrayExpression(array_expr) = &expr_stmt.expression {
+        if array_expr.elements.len() != 2 {
+            return;
+        }
+
+        let Some((first_element, second_element)) = ({
+            match (&array_expr.elements[0], &array_expr.elements[1]) {
+                (
+                    ArrayExpressionElement::SpreadElement(first_element),
+                    ArrayExpressionElement::SpreadElement(second_element),
+                ) => match (&first_element.argument, &second_element.argument) {
+                    (
+                        Expression::Identifier(first_element),
+                        Expression::Identifier(second_element),
+                    ) => Some((first_element, second_element)),
+                    _ => None,
+                },
+                _ => None,
+            }
+        }) else {
+            return;
+        };
+
+        if first_element.name != first_parameter || second_element.name != second_parameter {
+            return;
+        }
+
+        ctx.diagnostic(PreferArrayFlatDiagnostic(call_expr.span));
+    };
+}
+
+// `[].concat(maybeArray)`
+// `[].concat(...array)`
+fn check_array_concat_case<'a>(call_expr: &CallExpression<'a>, ctx: &LintContext<'a>) {
+    if is_method_call(call_expr, None, Some(&["concat"]), Some(1), Some(1)) {
+        // `array.concat(maybeArray)`
+        if let Expression::ArrayExpression(array_expr) =
+            call_expr.callee.get_member_expr().unwrap().object()
+        {
+            if !array_expr.elements.is_empty() {
+                return;
+            }
+            ctx.diagnostic(PreferArrayFlatDiagnostic(call_expr.span));
+        }
+    }
+}
+
+// - `[].concat.apply([], array)` and `Array.prototype.concat.apply([], array)`
+// - `[].concat.call([], maybeArray)` and `Array.prototype.concat.call([], maybeArray)`
+// - `[].concat.call([], ...array)` and `Array.prototype.concat.call([], ...array)`
+fn check_array_prototype_concat_case<'a>(call_expr: &CallExpression<'a>, ctx: &LintContext<'a>) {
+    let Some(member_expr) = call_expr.callee.get_member_expr() else {
+        return;
+    };
+
+    if let Expression::MemberExpression(member_expr_obj) = member_expr.object() {
+        let is_call_call = is_method_call(call_expr, None, Some(&["call"]), Some(2), Some(2));
+
+        if (is_call_call || is_method_call(call_expr, None, Some(&["apply"]), Some(2), Some(2)))
+            && is_prototype_property(member_expr_obj, "concat", Some("Array"))
+        {
+            if let Argument::Expression(first_argument) = &call_expr.arguments[0] {
+                if is_empty_array_expression(first_argument)
+                    && (is_call_call
+                        || !matches!(call_expr.arguments.get(1), Some(Argument::SpreadElement(_))))
+                {
+                    ctx.diagnostic(PreferArrayFlatDiagnostic(call_expr.span));
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn test() {
+    use crate::tester::Tester;
+
+    let pass = vec![
+        r"array.flatMap",
+        r"new array.flatMap(x => x)",
+        r"flatMap(x => x)",
+        r"array.notFlatMap(x => x)",
+        r"array[flatMap](x => x)",
+        r"array.flatMap(x => x, thisArgument)",
+        r"array.flatMap(...[x => x])",
+        r"array.flatMap(function (x) { return x; })",
+        r"array.flatMap(async x => x)",
+        r"array.flatMap(function * (x) { return x;})",
+        r"array.flatMap(() => x)",
+        r"array.flatMap((x, y) => x)",
+        r"array.flatMap(x => y)",
+        r"new array.reduce((a, b) => a.concat(b), [])",
+        r"array.reduce",
+        r"reduce((a, b) => a.concat(b), [])",
+        r"array[reduce]((a, b) => a.concat(b), [])",
+        r"array.notReduce((a, b) => a.concat(b), [])",
+        r"array.reduce((a, b) => a.concat(b), [], EXTRA_ARGUMENT)",
+        r"array.reduce((a, b) => a.concat(b), NOT_EMPTY_ARRAY)",
+        r"array.reduce((a, b, extraParameter) => a.concat(b), [])",
+        r"array.reduce((a,) => a.concat(b), [])",
+        r"array.reduce(() => a.concat(b), [])",
+        r"array.reduce((a, b) => {return a.concat(b); }, [])",
+        r"array.reduce(function (a, b) { return a.concat(b); }, [])",
+        r"array.reduce((a, b) => b.concat(b), [])",
+        r"array.reduce((a, b) => a.concat(a), [])",
+        r"array.reduce((a, b) => b.concat(a), [])",
+        r"array.reduce((a, b) => a.notConcat(b), [])",
+        r"array.reduce((a, b) => a.concat, [])",
+        r"new array.reduce((a, b) => [...a, ...b], [])",
+        r"array[reduce]((a, b) => [...a, ...b], [])",
+        r"reduce((a, b) => [...a, ...b], [])",
+        r"array.notReduce((a, b) => [...a, ...b], [])",
+        r"array.reduce((a, b) => [...a, ...b], [], EXTRA_ARGUMENT)",
+        r"array.reduce((a, b) => [...a, ...b], NOT_EMPTY_ARRAY)",
+        r"array.reduce((a, b, extraParameter) => [...a, ...b], [])",
+        r"array.reduce((a,) => [...a, ...b], [])",
+        r"array.reduce(() => [...a, ...b], [])",
+        r"array.reduce((a, b) => {return [...a, ...b]; }, [])",
+        r"array.reduce(function (a, b) { return [...a, ...b]; }, [])",
+        r"array.reduce((a, b) => [...b, ...b], [])",
+        r"array.reduce((a, b) => [...a, ...a], [])",
+        r"array.reduce((a, b) => [...b, ...a], [])",
+        r"array.reduce((a, b) => [a, ...b], [])",
+        r"array.reduce((a, b) => [...a, b], [])",
+        r"array.reduce((a, b) => [a, b], [])",
+        r"array.reduce((a, b) => [...a, ...b, c], [])",
+        r"array.reduce((a, b) => [...a, ...b,,], [])",
+        r"array.reduce((a, b) => [,...a, ...b], [])",
+        r"array.reduce((a, b) => [, ], [])",
+        r"array.reduce((a, b) => [, ,], [])",
+        r"[].concat",
+        r"new [].concat(array)",
+        r"[][concat](array)",
+        r"[].notConcat(array)",
+        r"[,].concat(array)",
+        r"({}).concat(array)",
+        r"[].concat()",
+        r"[].concat(array, EXTRA_ARGUMENT)",
+        r"new [].concat(...array)",
+        r"[][concat](...array)",
+        r"[].notConcat(...array)",
+        r"[,].concat(...array)",
+        r"({}).concat(...array)",
+        r"[].concat()",
+        r"[].concat(...array, EXTRA_ARGUMENT)",
+        r"new [].concat.apply([], array)",
+        r"[].concat.apply",
+        r"[].concat.apply([], ...array)",
+        r"[].concat.apply([], array, EXTRA_ARGUMENT)",
+        r"[].concat.apply([])",
+        r"[].concat.apply(NOT_EMPTY_ARRAY, array)",
+        r"[].concat.apply([,], array)",
+        r"[,].concat.apply([], array)",
+        r"[].concat[apply]([], array)",
+        r"[][concat].apply([], array)",
+        r"[].concat.notApply([], array)",
+        r"[].notConcat.apply([], array)",
+        r"new Array.prototype.concat.apply([], array)",
+        r"Array.prototype.concat.apply",
+        r"Array.prototype.concat.apply([], ...array)",
+        r"Array.prototype.concat.apply([], array, EXTRA_ARGUMENT)",
+        r"Array.prototype.concat.apply([])",
+        r"Array.prototype.concat.apply(NOT_EMPTY_ARRAY, array)",
+        r"Array.prototype.concat.apply([,], array)",
+        r"Array.prototype.concat[apply]([], array)",
+        r"Array.prototype[concat].apply([], array)",
+        r"Array[prototype].concat.apply([], array)",
+        r"Array.prototype.concat.notApply([], array)",
+        r"Array.prototype.notConcat.apply([], array)",
+        r"Array.notPrototype.concat.apply([], array)",
+        r"NotArray.prototype.concat.apply([], array)",
+        r"Array.prototype?.concat.apply([], array)",
+        r"object.Array.prototype.concat.apply([], array)",
+    ];
+
+    let fail = vec![
+        r"array.flatMap(x => x)",
+        r"function foo(){return[].flatMap(x => x)}",
+        r"foo.flatMap(x => x) instanceof Array",
+        r"array.reduce((a, b) => a.concat(b), [])",
+        r"function foo(){return[].reduce((a, b) => a.concat(b), [])}",
+        r"array.reduce((a, b) => [...a, ...b], [])",
+        r"array.reduce((a, b) => [...a, ...b,], [])",
+        r"function foo(){return[].reduce((a, b) => [...a, ...b,], [])}",
+        r"[].concat(maybeArray)",
+        r"[].concat( ((0, maybeArray)) )",
+        r"[].concat( ((maybeArray)) )",
+        r"[].concat( [foo] )",
+        r"[].concat( [[foo]] )",
+        r"function foo(){return[].concat(maybeArray)}",
+        r"[].concat(...array)",
+        r"[].concat(...(( array )))",
+        r"[].concat(...(( [foo] )))",
+        r"[].concat(...(( [[foo]] )))",
+        r"function foo(){return[].concat(...array)}",
+        r"class A extends[].concat(...array){}",
+        r"const A = class extends[].concat(...array){}",
+        r"[].concat.apply([], array)",
+        r"[].concat.apply([], ((0, array)))",
+        r"[].concat.apply([], ((array)))",
+        r"[].concat.apply([], [foo])",
+        r"[].concat.apply([], [[foo]])",
+        r"[].concat.call([], maybeArray)",
+        r"[].concat.call([], ((0, maybeArray)))",
+        r"[].concat.call([], ((maybeArray)))",
+        r"[].concat.call([], [foo])",
+        r"[].concat.call([], [[foo]])",
+        r"[].concat.call([], ...array)",
+        r"[].concat.call([], ...((0, array)))",
+        r"[].concat.call([], ...((array)))",
+        r"[].concat.call([], ...[foo])",
+        r"[].concat.call([], ...[[foo]])",
+        r"function foo(){return[].concat.call([], ...array)}",
+        r"Array.prototype.concat.apply([], array)",
+        r"Array.prototype.concat.apply([], ((0, array)))",
+        r"Array.prototype.concat.apply([], ((array)))",
+        r"Array.prototype.concat.apply([], [foo])",
+        r"Array.prototype.concat.apply([], [[foo]])",
+        r"Array.prototype.concat.call([], maybeArray)",
+        r"Array.prototype.concat.call([], ((0, maybeArray)))",
+        r"Array.prototype.concat.call([], ((maybeArray)))",
+        r"Array.prototype.concat.call([], [foo])",
+        r"Array.prototype.concat.call([], [[foo]])",
+        r"Array.prototype.concat.call([], ...array)",
+        r"Array.prototype.concat.call([], ...((0, array)))",
+        r"Array.prototype.concat.call([], ...((array)))",
+        r"Array.prototype.concat.call([], ...[foo])",
+        r"Array.prototype.concat.call([], ...[[foo]])",
+        r"[].concat.apply([], array)",
+        r"Array.prototype.concat.apply([], array)",
+        r"Array.prototype.concat.apply([], (0, array))",
+        r"Array.prototype.concat.call([], (0, array))",
+        r"async function a() { return [].concat(await getArray()); }",
+        r"[].concat(some./**/array)",
+        r"[/**/].concat(some./**/array)",
+        r"[/**/].concat(some.array)",
+    ];
+
+    Tester::new_without_config(PreferArrayFlat::NAME, pass, fail).test_and_snapshot();
+}

crates/oxc_linter/src/snapshots/prefer_array_flat.snap

@@ -0,0 +1,425 @@
+---
+source: crates/oxc_linter/src/tester.rs
+expression: prefer_array_flat
+---
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ array.flatMap(x => x)
+   · ─────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].flatMap(x => x)}
+   ·                      ──────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ foo.flatMap(x => x) instanceof Array
+   · ───────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ array.reduce((a, b) => a.concat(b), [])
+   · ───────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].reduce((a, b) => a.concat(b), [])}
+   ·                      ────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ array.reduce((a, b) => [...a, ...b], [])
+   · ────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ array.reduce((a, b) => [...a, ...b,], [])
+   · ─────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].reduce((a, b) => [...a, ...b,], [])}
+   ·                      ──────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(maybeArray)
+   · ─────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat( ((0, maybeArray)) )
+   · ──────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat( ((maybeArray)) )
+   · ───────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat( [foo] )
+   · ──────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat( [[foo]] )
+   · ────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].concat(maybeArray)}
+   ·                      ─────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(...array)
+   · ───────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(...(( array )))
+   · ─────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(...(( [foo] )))
+   · ─────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(...(( [[foo]] )))
+   · ───────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].concat(...array)}
+   ·                      ───────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ class A extends[].concat(...array){}
+   ·                ───────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ const A = class extends[].concat(...array){}
+   ·                        ───────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], array)
+   · ──────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], ((0, array)))
+   · ─────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], ((array)))
+   · ──────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], [foo])
+   · ──────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], [[foo]])
+   · ────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], maybeArray)
+   · ──────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ((0, maybeArray)))
+   · ─────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ((maybeArray)))
+   · ──────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], [foo])
+   · ─────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], [[foo]])
+   · ───────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ...array)
+   · ────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ...((0, array)))
+   · ───────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ...((array)))
+   · ────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ...[foo])
+   · ────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.call([], ...[[foo]])
+   · ──────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ function foo(){return[].concat.call([], ...array)}
+   ·                      ────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], array)
+   · ───────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], ((0, array)))
+   · ──────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], ((array)))
+   · ───────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], [foo])
+   · ───────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], [[foo]])
+   · ─────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], maybeArray)
+   · ───────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ((0, maybeArray)))
+   · ──────────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ((maybeArray)))
+   · ───────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], [foo])
+   · ──────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], [[foo]])
+   · ────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ...array)
+   · ─────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ...((0, array)))
+   · ────────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ...((array)))
+   · ─────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ...[foo])
+   · ─────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], ...[[foo]])
+   · ───────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat.apply([], array)
+   · ──────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], array)
+   · ───────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.apply([], (0, array))
+   · ────────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ Array.prototype.concat.call([], (0, array))
+   · ───────────────────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ async function a() { return [].concat(await getArray()); }
+   ·                             ───────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [].concat(some./**/array)
+   · ─────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [/**/].concat(some./**/array)
+   · ─────────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat): Prefer Array#flat() over legacy techniques to flatten arrays.
+   ╭─[prefer_array_flat.tsx:1:1]
+ 1 │ [/**/].concat(some.array)
+   · ─────────────────────────
+   ╰────
+  help: Call `.flat()` on the array instead.
+
+

crates/oxc_linter/src/utils/unicorn.rs

@@ -1,7 +1,10 @@
 mod boolean;
 pub use self::boolean::*;
 use oxc_ast::{
-    ast::{Expression, LogicalExpression, MemberExpression, Statement},
+    ast::{
+        BindingPatternKind, Expression, FormalParameters, FunctionBody, LogicalExpression,
+        MemberExpression, Statement,
+    },
     AstKind,
 };
 use oxc_semantic::AstNode;
@@ -61,13 +64,7 @@ pub fn is_prototype_property(
 
     match object {
         // `[].method`
-        Some("Array") => {
-            if let Expression::ArrayExpression(array_expr) = member_expr.object() {
-                array_expr.elements.len() == 0
-            } else {
-                false
-            }
-        }
+        Some("Array") => is_empty_array_expression(member_expr.object()),
 
         // `{}.method`
         Some("Object") => {
@@ -81,6 +78,14 @@ pub fn is_prototype_property(
     }
 }
 
+pub fn is_empty_array_expression(expr: &Expression) -> bool {
+    if let Expression::ArrayExpression(array_expr) = expr {
+        array_expr.elements.len() == 0
+    } else {
+        false
+    }
+}
+
 pub fn is_logical_expression(node: &AstNode) -> bool {
     matches!(
         node.kind(),
@@ -90,3 +95,44 @@ pub fn is_logical_expression(node: &AstNode) -> bool {
         })
     )
 }
+
+// gets the name of the first parameter of a function
+pub fn get_first_parameter_name<'a>(arg: &'a FormalParameters) -> Option<&'a str> {
+    let first_func_param = arg.items.get(0)?;
+    let BindingPatternKind::BindingIdentifier(first_func_param) = &first_func_param.pattern.kind
+    else {
+        return None;
+    };
+    Some(first_func_param.name.as_str())
+}
+
+pub fn get_return_identifier_name<'a>(body: &'a FunctionBody<'_>) -> Option<&'a str> {
+    match body.statements.get(0)? {
+        Statement::BlockStatement(block_stmt) => {
+            let Statement::ReturnStatement(return_stmt) = block_stmt.body.get(0)? else {
+                return None;
+            };
+
+            let Some(Expression::Identifier(ident)) = return_stmt.argument.as_ref() else {
+                return None;
+            };
+
+            Some(ident.name.as_str())
+        }
+        Statement::ReturnStatement(return_stmt) => {
+            let return_expr = return_stmt.argument.as_ref()?;
+            match return_expr {
+                Expression::Identifier(ident) => Some(ident.name.as_str()),
+                _ => None,
+            }
+        }
+        Statement::ExpressionStatement(expr_stmt) => {
+            let Expression::Identifier(ident) = &expr_stmt.expression else {
+                return None;
+            };
+
+            Some(ident.name.as_str())
+        }
+        _ => None,
+    }
+}