Performance: improve .trim performance for large strings contains lots of whitespaces

vlsi · vlsi · commit 185d4b5c1529 · 2022-04-25T15:56:49.000+03:00
The old implementation took O(N^2) time to trim the string when multiple adjacent spaces
were present.

For instance, consider the string "A          B" (10 spaces between A and B).
Then old regexp /[\s]+$/ would take 10 steps until it realizes
the regexp does not match at position 1.
Then it would try to match the regexp at position 2, and it would take 9 steps.
Then 8 steps for position 3, ... so it would take 10*10/2 steps until
it figures out the regexp does not match.

The new approach is to require "non-whitespace" char before the whitespace run,
so it spends just one step for each space in the string.
diff --git a/src/jquery/core.js b/src/jquery/core.js
@@ -17,7 +17,12 @@ var findProp,
 
 	// Support: Android <=4.0 only
 	// Make sure we trim BOM and NBSP
-	rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
+	ltrim = /^[\s\uFEFF\xA0]+/,
+	// Require that the "whitespace run" starts from a non-whitespace
+	// to avoid O(N^2) behavior when the engine would try matching "\s$" at each space position.
+	// It is important that non-whitespace char is mandatory for regexp performance reasons,
+	// however, it does not break correctness since whitespace-only string will be trimmed by ltrim above.
+	rtrim = /([^\s\uFEFF\xA0])[\s\uFEFF\xA0]+$/;
 
 migratePatchFunc( jQuery.fn, "init", function( arg1 ) {
 	var args = Array.prototype.slice.call( arguments );
@@ -106,12 +111,26 @@ migrateWarnProp( jQuery.expr, ":", jQuery.expr.pseudos, "expr-pre-pseudos",
 
 // Prior to jQuery 3.1.1 there were internal refs so we don't warn there
 if ( jQueryVersionSince( "3.1.1" ) ) {
-	migratePatchAndWarnFunc( jQuery, "trim", function( text ) {
-		return text == null ?
-			"" :
-			( text + "" ).replace( rtrim, "" );
-	}, "trim",
-	"jQuery.trim is deprecated; use String.prototype.trim" );
+	var useDefaultTrim = false;
+	if (String.prototype.trim) {
+		useDefaultTrim = "_" === " \uFEFF\xA0_\uFEFF\xA0 ".trim();
+	}
+
+	if (useDefaultTrim) {
+		migratePatchAndWarnFunc( jQuery, "trim", function( text ) {
+			return text == null ?
+				"" :
+				( text + "" ).trim();
+		}, "trim",
+		"jQuery.trim is deprecated; use String.prototype.trim" );
+	} else {
+		migratePatchAndWarnFunc( jQuery, "trim", function( text ) {
+			return text == null ?
+				"" :
+				( text + "" ).replace( ltrim, "" ).replace( rtrim, "$1" );
+		}, "trim",
+		"jQuery.trim is deprecated; use String.prototype.trim" );
+	}
 }
 
 // Prior to jQuery 3.2 there were internal refs so we don't warn there
