feat(linter): eslint-plugin-unicorn(prefer-array-flat-map) (#997)

part of #684 


[prefer-array-flat-map](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-flat-map.md)

crates/oxc_linter/src/rules.rs

@@ -131,6 +131,7 @@ mod unicorn {
     pub mod no_instanceof_array;
     pub mod no_thenable;
     pub mod no_unnecessary_await;
+    pub mod prefer_array_flat_map;
 }
 
 oxc_macros::declare_all_lint_rules! {
@@ -239,6 +240,7 @@ oxc_macros::declare_all_lint_rules! {
     unicorn::no_unnecessary_await,
     unicorn::no_thenable,
     unicorn::filename_case,
+    unicorn::prefer_array_flat_map,
     import::named,
     import::no_cycle,
     import::no_self_import,

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

@@ -0,0 +1,135 @@
+use oxc_ast::{
+    ast::{Argument, Expression},
+    AstKind,
+};
+use oxc_diagnostics::{
+    miette::{self, Diagnostic},
+    thiserror::Error,
+};
+use oxc_macros::declare_oxc_lint;
+use oxc_span::Span;
+
+use crate::{context::LintContext, rule::Rule, AstNode};
+
+#[derive(Debug, Error, Diagnostic)]
+#[error("eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.")]
+#[diagnostic(severity(warning), help("Prefer `.flatMap(…)` over `.map(…).flat()`."))]
+struct PreferArrayFlatMapDiagnostic(#[label] pub Span);
+
+#[derive(Debug, Default, Clone)]
+pub struct PreferArrayFlatMap;
+
+declare_oxc_lint!(
+    /// ### What it does
+    ///
+    /// Prefers the use of `.flatMap()` when `map().flat()` are used together.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// It is slightly more efficient to use `.flatMap(…)` instead of `.map(…).flat()`.
+    ///
+    /// ### Example
+    /// ```javascript
+    /// const bar = [1,2,3].map(i => [i]).flat(); // ✗ fail
+    ///
+    /// const bar = [1,2,3].flatMap(i => [i]); // ✓ pass
+    /// ```
+    PreferArrayFlatMap,
+    correctness
+);
+
+impl Rule for PreferArrayFlatMap {
+    fn run<'a>(&self, node: &AstNode<'a>, ctx: &LintContext<'a>) {
+        let AstKind::CallExpression(flat_call_expr) = node.kind() else { return };
+
+        if flat_call_expr.arguments.len() > 1 {
+            return;
+        }
+
+        let callee = &flat_call_expr.callee.without_parenthesized();
+
+        if let Expression::MemberExpression(v) = callee {
+            if let Some(static_property_name) = v.static_property_name() {
+                if static_property_name != "flat" {
+                    return;
+                }
+            }
+        }
+
+        if let Some(first_arg) = flat_call_expr.arguments.first() {
+            if let Argument::Expression(Expression::NumberLiteral(number_lit)) = first_arg {
+                if number_lit.raw != "1" {
+                    return;
+                }
+            } else {
+                return;
+            }
+        }
+
+        let Expression::MemberExpression(member_expr) = callee else { return };
+
+        let Expression::CallExpression(map_call_expr) =
+            member_expr.object().without_parenthesized()
+        else {
+            return;
+        };
+
+        if let Expression::MemberExpression(map_member_expr) =
+            &map_call_expr.callee.without_parenthesized()
+        {
+            if let Some(property_name) = map_member_expr.static_property_name() {
+                if property_name != "map" {
+                    return;
+                }
+            }
+        };
+
+        ctx.diagnostic(PreferArrayFlatMapDiagnostic(flat_call_expr.span));
+    }
+}
+
+#[test]
+#[allow(clippy::too_many_lines)]
+fn test() {
+    use crate::tester::Tester;
+
+    let pass = vec![
+        ("const bar = [1,2,3].map()", None),
+        ("const bar = [1,2,3].map(i => i)", None),
+        ("const bar = [1,2,3].map((i) => i)", None),
+        ("const bar = [1,2,3].map((i) => { return i; })", None),
+        ("const bar = foo.map(i => i)", None),
+        ("const bar = [[1],[2],[3]].flat()", None),
+        ("const bar = [1,2,3].map(i => [i]).sort().flat()", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(2)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(1, null)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(Infinity)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(Number.POSITIVE_INFINITY)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(Number.MAX_VALUE)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(Number.MAX_SAFE_INTEGER)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(...[1])", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(0.4 +.6)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(+1)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(foo)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(foo.bar)", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(1.00)", None),
+    ];
+
+    let fail = vec![
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat()", None),
+        ("const bar = [[1],[2],[3]].map(i => [i]).flat(1,)", None),
+        ("const bar = [1,2,3].map(i => [i]).flat()", None),
+        ("const bar = [1,2,3].map((i) => [i]).flat()", None),
+        ("const bar = [1,2,3].map((i) => { return [i]; }).flat()", None),
+        ("const bar = [1,2,3].map(foo).flat()", None),
+        ("const bar = foo.map(i => [i]).flat()", None),
+        ("const bar = { map: () => {} }.map(i => [i]).flat()", None),
+        ("const bar = [1,2,3].map(i => i).map(i => [i]).flat()", None),
+        ("const bar = [1,2,3].sort().map(i => [i]).flat()", None),
+        ("const bar = (([1,2,3].map(i => [i]))).flat()", None),
+        ("let bar = [1,2,3] . map( x => y ) . flat () // 🤪", None),
+        ("const bar = [1,2,3].map(i => [i]).flat(1);", None),
+    ];
+
+    Tester::new(PreferArrayFlatMap::NAME, pass, fail).test_and_snapshot();
+}

crates/oxc_linter/src/snapshots/prefer_array_flat_map.snap

@@ -0,0 +1,96 @@
+---
+source: crates/oxc_linter/src/tester.rs
+expression: prefer_array_flat_map
+---
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [[1],[2],[3]].map(i => [i]).flat()
+   ·             ──────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [[1],[2],[3]].map(i => [i]).flat(1,)
+   ·             ────────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map(i => [i]).flat()
+   ·             ────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map((i) => [i]).flat()
+   ·             ──────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map((i) => { return [i]; }).flat()
+   ·             ──────────────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map(foo).flat()
+   ·             ───────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = foo.map(i => [i]).flat()
+   ·             ────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = { map: () => {} }.map(i => [i]).flat()
+   ·             ──────────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map(i => i).map(i => [i]).flat()
+   ·             ────────────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].sort().map(i => [i]).flat()
+   ·             ───────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = (([1,2,3].map(i => [i]))).flat()
+   ·             ────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ let bar = [1,2,3] . map( x => y ) . flat () // 🤪
+   ·           ─────────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+  ⚠ eslint-plugin-unicorn(prefer-array-flat-map): `Array.flatMap` performs `Array.map` and `Array.flat` in one step.
+   ╭─[prefer_array_flat_map.tsx:1:1]
+ 1 │ const bar = [1,2,3].map(i => [i]).flat(1);
+   ·             ─────────────────────────────
+   ╰────
+  help: Prefer `.flatMap(…)` over `.map(…).flat()`.
+
+