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Core: Tiny efficiency fix to jQuery.extend / jQuery.fn.extend (#4246)
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Read target[name] only when it's needed.

In addition to doing the property read-only when needed, this
avoids a slow path in V8 (see the issue for more details).

Fixes gh-4245
Closes gh-4246
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marjakh authored and mgol committed Dec 12, 2018
1 parent 13f3cd1 commit 4ffb1df
Showing 1 changed file with 8 additions and 6 deletions.
14 changes: 8 additions & 6 deletions src/core.js
Expand Up @@ -156,7 +156,6 @@ jQuery.extend = jQuery.fn.extend = function() {

// Extend the base object
for ( name in options ) {
src = target[ name ];
copy = options[ name ];

// Prevent never-ending loop
Expand All @@ -167,14 +166,17 @@ jQuery.extend = jQuery.fn.extend = function() {
// Recurse if we're merging plain objects or arrays
if ( deep && copy && ( jQuery.isPlainObject( copy ) ||
( copyIsArray = Array.isArray( copy ) ) ) ) {
src = target[ name ];

if ( copyIsArray ) {
copyIsArray = false;
clone = src && Array.isArray( src ) ? src : [];

// Ensure proper type for the source value
if ( copyIsArray && !Array.isArray( src ) ) {
clone = [];
} else if ( !copyIsArray && !jQuery.isPlainObject( src ) ) {
clone = {};
} else {
clone = src && jQuery.isPlainObject( src ) ? src : {};
clone = src;
}
copyIsArray = false;

// Never move original objects, clone them
target[ name ] = jQuery.extend( deep, clone, copy );
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