I am looking for the best way to replace or add to elements of an array without deleting the original reference. Here is the set up:

var a = [], b = [], c, i, obj;
for ( i = 0; i < 100000; i++ ) { a[ i ] = i; b[ i ] = 10000 - i; }
obj.data_list = a;

Now we want to concatenate b INTO a without changing the reference to a, since it is used in obj.data_list. Here is one method:

for ( i = 0; i < b.length; i++ ) { a.push( b[ i ] ); }

This seems to be a somewhat terser and 8x (on V8) faster method:

a.splice.apply( a, [ a.length, 0 ].concat( b ) );

I have found this useful when iterating over an "in-place" array and don't want to touch the elements as I go (a good practice). I start a new array (let's call it keep_list) with the initial arguments and then add the elements I wish to retain. Finally I use this apply method to quickly replace the truncated array:

var keep_list = [ 0, 0 ];
for ( i = 0; i < a.length; i++ ){ 
  if ( some_condition ){ keep_list.push( a[ i ] );
}

// truncate array
a.length = 0;

// And replace contents
a.splice.apply( a, keep_list );

There are a few problems with this solution:

• there is a max call stack size limit of around 50k on V8
• I have not tested on other JS engines yet.
• This solution is a bit cryptic

Has anyone found a better way?