Edit in JSFiddle

function foo(x) {
  x.push(4);
  document.writeln(x + '</br>');
  x.length = 0;
  x.push(5, 6, 7, 8);
  document.writeln(x + '</br>');
}
var a = [1, 2, 3];
foo(a);
document.writeln(a);