[6510]

I guess...

  x.concat([y*2])

would return the array (but makes a duplicate)

Anyway, I find this to be a whole lot more sensible:

  x=[];
  for(y of [1,2,3,4,5]){
    if(y%2===1)x.push(y*2)
  }

Or even!

  y=[1,2,3,4,5];
  x=[];
  // map reduce/flatmap/map/filter etc omg wtf
  for( i=0; i < y.length; i++ ){
    if( y[i]%2 === 1 ){ x.push( y[i] * 2 ); }
  }

I cant even tell what language this is but there is nothing here that needs fixing.