From a69d15f2990a2271b65b0e6c3ba7a4ee56fa4274 Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Wed, 8 Jan 2025 12:07:46 +0000
Subject: [PATCH] feat(minifier): compress `new Array(2)` -> `[,,]` (#8344)

For an integer value `n` smaller than 6, `Array(n)` can be compressed to `[,,]` (the number of `,` is `n`).
---
 .../peephole_substitute_alternate_syntax.rs   | 21 ++++++++++++++++++-
 tasks/minsize/minsize.snap                    |  4 ++--
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs b/crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs
index e35eed20c8a01..763405d231d70 100644
--- a/crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs
+++ b/crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs
@@ -762,7 +762,24 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
                         Some(ctx.ast.expression_array(span, ctx.ast.vec(), None))
                     }
                     // `new Array(8)` -> `Array(8)`
-                    else if let Expression::NumericLiteral(_) = arg {
+                    else if let Expression::NumericLiteral(n) = arg {
+                        // new Array(2) -> `[,,]`
+                        // this does not work with IE8 and below
+                        // learned from https://github.com/babel/minify/pull/45
+                        #[expect(clippy::cast_possible_truncation)]
+                        if n.value.fract() == 0.0 {
+                            let n_int = n.value as i64;
+                            if (1..=6).contains(&n_int) {
+                                return Some(ctx.ast.expression_array(
+                                    span,
+                                    ctx.ast.vec_from_iter((0..n_int).map(|_| {
+                                        ArrayExpressionElement::Elision(Elision { span: SPAN })
+                                    })),
+                                    None,
+                                ));
+                            }
+                        }
+
                         let callee = ctx.ast.expression_identifier_reference(SPAN, "Array");
                         let args = ctx.ast.move_vec(args);
                         Some(ctx.ast.expression_call(span, callee, NONE, args, false))
@@ -1143,6 +1160,8 @@ mod test {
         // One argument
         test("x = new Array(0)", "x = []");
         test("x = new Array(\"a\")", "x = [\"a\"]");
+        test("x = new Array(1)", "x = [,]");
+        test("x = new Array(6)", "x = [,,,,,,]");
         test("x = new Array(7)", "x = Array(7)");
         test("x = new Array(7n)", "x = [7n]");
         test("x = new Array(y)", "x = Array(y)");
diff --git a/tasks/minsize/minsize.snap b/tasks/minsize/minsize.snap
index de8d0ea670d32..1ab509dc6d1b3 100644
--- a/tasks/minsize/minsize.snap
+++ b/tasks/minsize/minsize.snap
@@ -11,7 +11,7 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 544.10 kB  | 71.75 kB   | 72.48 kB   | 26.16 kB   | 26.20 kB   | lodash.js 
 
-555.77 kB  | 272.91 kB  | 270.13 kB  | 90.92 kB   | 90.80 kB   | d3.js     
+555.77 kB  | 272.82 kB  | 270.13 kB  | 90.92 kB   | 90.80 kB   | d3.js     
 
 1.01 MB    | 460.22 kB  | 458.89 kB  | 126.83 kB  | 126.71 kB  | bundle.min.js
 
@@ -23,5 +23,5 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 6.69 MB    | 2.32 MB    | 2.31 MB    | 492.72 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.50 MB    | 3.49 MB    | 908.29 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.50 MB    | 3.49 MB    | 908.28 kB  | 915.50 kB  | typescript.js