Core: Use Array#flat where supported

src/core.js

@@ -6,7 +6,7 @@ define( [
 	"./var/arr",
 	"./var/getProto",
 	"./var/slice",
-	"./var/concat",
+	"./var/flat",
 	"./var/push",
 	"./var/indexOf",
 	"./var/class2type",
@@ -19,7 +19,7 @@ define( [
 	"./var/isWindow",
 	"./core/DOMEval",
 	"./core/toType"
-], function( arr, getProto, slice, concat, push, indexOf,
+], function( arr, getProto, slice, flat, push, indexOf,
 	class2type, toString, hasOwn, fnToString, ObjectFunctionString,
 	trim, support, isWindow, DOMEval, toType ) {
 
@@ -395,7 +395,7 @@ jQuery.extend( {
 		}
 
 		// Flatten any nested arrays
-		return concat.apply( [], ret );
+		return flat.call( ret );
 	},
 
 	// A global GUID counter for objects

src/var/flat.js

@@ -0,0 +1,14 @@
+define( [
+	"./arr",
+	"./concat"
+], function( arr, concat ) {
+	"use strict";
+
+	return arr.flat ? function() {
+
+		// Flat with depth = 1 to give the same result of concat.apply.
+		return arr.flat.call( this, 1 );
+	} : function() {
+		return concat.apply( [], this );
+	};
+} );

test/unit/core.js

@@ -691,7 +691,7 @@ QUnit.test( "map()", function( assert ) {
 } );
 
 QUnit.test( "jQuery.map", function( assert ) {
-	assert.expect( 25 );
+	assert.expect( 28 );
 
 	var i, label, result, callback;
 
@@ -791,6 +791,17 @@ QUnit.test( "jQuery.map", function( assert ) {
 		return k % 2 ? k : [ k, k, k ];
 	} );
 	assert.equal( result.join( "" ), "00012223", "Array results flattened (#2616)" );
+
+	result = jQuery.map( [ [ [ 1, 2 ], 3 ], 4 ], function( v, k ) {
+		return v;
+	} );
+	assert.equal( result.length, 3, "Array flatten only one level down" );
+	assert.ok( Array.isArray( result[ 0 ] ), "Array flatten only one level down" );
+
+	result = jQuery.map( Array( 300000 ), function( v, k ) {
+		return k;
+	} );
+	assert.equal( result.length, 300000, "Able to map 300000 records without any problems (#4320)" );
 } );
 
 QUnit.test( "jQuery.merge()", function( assert ) {