Core: Use Array.prototype.flat where supported

Calling `Array.prototype.concat.apply( [], inputArray )` to flatten `inputArray`
crashes for large arrays; using `Array.prototype.flat` avoids these issues in
browsers that support it. In case it's necessary to support these large arrays
even in older browsers, a polyfill for `Array.prototype.flat` can be loaded.
This is already being done by many applications.

Fixes jquerygh-4320
Closes jquerygh-4459

src/core.js

@@ -6,7 +6,7 @@ define( [
 	"./var/arr",
 	"./var/getProto",
 	"./var/slice",
-	"./var/concat",
+	"./var/flat",
 	"./var/push",
 	"./var/indexOf",
 	"./var/class2type",
@@ -18,7 +18,7 @@ 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,
 	support, isWindow, DOMEval, toType ) {
 
@@ -397,7 +397,7 @@ jQuery.extend( {
 		}
 
 		// Flatten any nested arrays
-		return concat.apply( [], ret );
+		return flat( ret );
 	},
 
 	// A global GUID counter for objects

src/manipulation.js

@@ -1,7 +1,7 @@
 define( [
 	"./core",
 	"./core/isAttached",
-	"./var/concat",
+	"./var/flat",
 	"./var/isIE",
 	"./var/push",
 	"./core/access",
@@ -22,7 +22,7 @@ define( [
 	"./traversing",
 	"./selector",
 	"./event"
-], function( jQuery, isAttached, concat, isIE, push, access, rtagName,
+], function( jQuery, isAttached, flat, isIE, push, access, rtagName,
 	rscriptType, wrapMap, getAll, setGlobalEval, buildFragment,
 	dataPriv, dataUser, acceptData, DOMEval, nodeName ) {
 
@@ -103,7 +103,7 @@ function cloneCopyEvent( src, dest ) {
 function domManip( collection, args, callback, ignored ) {
 
 	// Flatten any nested arrays
-	args = concat.apply( [], args );
+	args = flat( args );
 
 	var fragment, first, scripts, hasScripts, node, doc,
 		i = 0,

src/var/flat.js

@@ -0,0 +1,15 @@
+define( [
+	"./arr"
+], function( arr ) {
+
+"use strict";
+
+// Support: IE 11+, Edge 18+
+// Provide fallback for browsers without Array#flat.
+return arr.flat ? function( array ) {
+	return arr.flat.call( array );
+} : function( array ) {
+	return arr.concat.apply( [], array );
+};
+
+} );

test/unit/core.js

@@ -681,7 +681,7 @@ QUnit.test( "map()", function( assert ) {
 } );
 
 QUnit.test( "jQuery.map", function( assert ) {
-	assert.expect( 25 );
+	assert.expect( 28 );
 
 	var i, label, result, callback;
 
@@ -781,6 +781,23 @@ 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" );
+
+	// Support: IE 11+, Edge 18+
+	// Skip the test in browsers without Array#flat.
+	if ( Array.prototype.flat ) {
+		result = jQuery.map( Array( 300000 ), function( v, k ) {
+			return k;
+		} );
+		assert.equal( result.length, 300000, "Able to map 300000 records without any problems (#4320)" );
+	} else {
+		assert.ok( "skip", "Array#flat doesn't supported on all browsers" );
+	}
 } );
 
 QUnit.test( "jQuery.merge()", function( assert ) {