You can use Array.concat like bellow:-
var arr = [['object1', 'object2'],['object1'],['object1','object2','object3']];
var flattened = [].concat.apply([],arr);
flattened
will be your expected array.
ES 2020 gives the flat, also flatMap if you want to iterate over, to flat lists of lists:
[['object1'], ['object2']].flat() // ['object1', 'object2']