Avoid reading outside of collection bounds

Consider the following collection:

    const array = ['a', 'b', 'c'];

Retrieving `array[0]` can be done relatively quickly. However, when the property doesn’t exist on the receiver, JavaScript engines must continue to look up the prototype chain until either the property is found or the chain ends. This is inherently slower than *not* doing any prototype chain lookups. Retrieving an out-of-bounds index, e.g. `array[3]`, triggers this scenario, resulting in decreased performance.

This patch changes the way some loops are written to avoid running into the slow case unnecessarily.

Author: mathiasbynens

dist/sizzle.js

@@ -6,7 +6,7 @@
  * Released under the MIT license
  * https://js.foundation/
  *
- * Date: 2016-12-02
+ * Date: 2017-08-29
  */
 (function( window ) {
 
@@ -658,7 +658,7 @@ setDocument = Sizzle.setDocument = function( node ) {
 		// getElementById is not reliable as a find shortcut
 		Expr.find["ID"] = function( id, context ) {
 			if ( typeof context.getElementById !== "undefined" && documentIsHTML ) {
-				var node, i, elems,
+				var node, i, elems, length,
 					elem = context.getElementById( id );
 
 				if ( elem ) {
@@ -671,8 +671,10 @@ setDocument = Sizzle.setDocument = function( node ) {
 
 					// Fall back on getElementsByName
 					elems = context.getElementsByName( id );
+					length = elems.length;
 					i = 0;
-					while ( (elem = elems[i++]) ) {
+					for ( ; i < length; i++) {
+						elem = elems[ i ];
 						node = elem.getAttributeNode("id");
 						if ( node && node.value === id ) {
 							return [ elem ];
@@ -702,11 +704,13 @@ setDocument = Sizzle.setDocument = function( node ) {
 				tmp = [],
 				i = 0,
 				// By happy coincidence, a (broken) gEBTN appears on DocumentFragment nodes too
-				results = context.getElementsByTagName( tag );
+				results = context.getElementsByTagName( tag ),
+				length = results.length;
 
 			// Filter out possible comments
 			if ( tag === "*" ) {
-				while ( (elem = results[i++]) ) {
+				for ( ; i < length; i++) {
+					elem = results[ i ];
 					if ( elem.nodeType === 1 ) {
 						tmp.push( elem );
 					}
@@ -1054,17 +1058,19 @@ Sizzle.uniqueSort = function( results ) {
 	var elem,
 		duplicates = [],
 		j = 0,
-		i = 0;
+		i = 0,
+		length = results.length;
 
 	// Unless we *know* we can detect duplicates, assume their presence
 	hasDuplicate = !support.detectDuplicates;
 	sortInput = !support.sortStable && results.slice( 0 );
 	results.sort( sortOrder );
 
 	if ( hasDuplicate ) {
-		while ( (elem = results[i++]) ) {
-			if ( elem === results[ i ] ) {
-				j = duplicates.push( i );
+		for ( ; i < length; i++ ) {
+			elem = results[ i ];
+			if ( i + 1 < length && elem === results[ i + 1 ] ) {
+				j = duplicates.push( i + 1 );
 			}
 		}
 		while ( j-- ) {