minify: removes duplicates from CSS selector lists

CHANGELOG.md

@@ -1,5 +1,22 @@
 # Changelog
 
+## Unreleased
+
+* Minification now removes duplicates from CSS selector lists
+
+    This release introduces the following CSS minification optimization:
+
+    ```css
+    /* Original input */
+    div, div { color: red }
+
+    /* Old output (with --minify) */
+    div,div{color:red}
+
+    /* New output (with --minify) */
+    div{color:red}
+    ```
+
 ## 0.17.13
 
 * Work around an issue with `NODE_PATH` and Go's WebAssembly internals ([#3001](https://github.com/evanw/esbuild/issues/3001))

internal/css_parser/css_parser_selector.go

@@ -15,6 +15,7 @@ func (p *parser) parseSelectorList(opts parseSelectorOpts) (list []css_ast.Compl
 	list = append(list, sel)
 
 	// Parse the remaining selectors
+skip:
 	for {
 		p.eat(css_lexer.TWhitespace)
 		if !p.eat(css_lexer.TComma) {
@@ -25,6 +26,16 @@ func (p *parser) parseSelectorList(opts parseSelectorOpts) (list []css_ast.Compl
 		if !good {
 			return
 		}
+
+		// Omit duplicate selectors
+		if p.options.MinifySyntax {
+			for _, existing := range list {
+				if sel.Equal(existing, nil) {
+					continue skip
+				}
+			}
+		}
+
 		list = append(list, sel)
 	}
 

internal/css_parser/css_parser_test.go

@@ -1774,6 +1774,17 @@ func TestMangleAlpha(t *testing.T) {
 	expectPrintedLowerMangle(t, "a { color: #000000FF }", "a {\n  color: #000;\n}\n")
 }
 
+func TestMangleDuplicateSelectors(t *testing.T) {
+	expectPrinted(t, "a, a { color: red }", "a,\na {\n  color: red;\n}\n")
+	expectPrintedMangle(t, "a, a { color: red }", "a {\n  color: red;\n}\n")
+	expectPrintedMangle(t, "a, b { color: red }", "a,\nb {\n  color: red;\n}\n")
+	expectPrintedMangle(t, "a, a.foo, a.foo, a.bar, a { color: red }", "a,\na.foo,\na.bar {\n  color: red;\n}\n")
+
+	expectPrintedMangle(t, "@media screen { a, a { color: red } }", "@media screen {\n  a {\n    color: red;\n  }\n}\n")
+	expectPrintedMangle(t, "@media screen { a, b { color: red } }", "@media screen {\n  a,\n  b {\n    color: red;\n  }\n}\n")
+	expectPrintedMangle(t, "@media screen { a, a.foo, a.foo, a.bar, a { color: red } }", "@media screen {\n  a,\n  a.foo,\n  a.bar {\n    color: red;\n  }\n}\n")
+}
+
 func TestMangleDuplicateSelectorRules(t *testing.T) {
 	expectPrinted(t, "a { color: red } b { color: red }", "a {\n  color: red;\n}\nb {\n  color: red;\n}\n")
 	expectPrintedMangle(t, "a { color: red } b { color: red }", "a,\nb {\n  color: red;\n}\n")