A counter-example to Cody Lindley's "part 7". http://tech.pro/tutorial/1669/21-javascript-parts-i-struggle-to-remember-

args_and_params.js

var arg_ary = [ 1, 2, 3 ];
var arg_obj  = { foo: 'bar' };

function reset( param ) {
  param[0] = undefined;
  console.log( "in reset", param );
}

console.log( arg_ary ); // [ 1, 2, 3 ]
reset( arg_ary ); // [ undefined, 2, 3 ]
console.log( arg_ary ); // [ undefined, 2, 3 ]

console.log( arg_obj ); // { foo: 'bar' }
reset( arg_obj ); // { 0: undefined, foo: 'bar' }
console.log( arg_obj ); // { 0: undefined, foo: 'bar' }