Although it probably isn't allowed to have trailing commas in arrays and obect initializers according to ECMA-262, many modern Javascript engines allow them.
I prefer coding styles with trailing commas usage, could the trailing commas removal feature make it into yuicompressor ?

Desired behavior:

[1,2,3,] => [1,2,3]
{ 'a' : 'b', } => { 'a' : 'b' }

Attaching a simple patch.