var foo = {};

mutate1( foo );

// should be included
[ 'a', 'b', 'c' ].forEach( function ( letter, i ) {
	foo[ letter ] = i;
});

[ 'd', 'e', 'f' ].forEach( ( letter, i ) => {
	foo[ letter ] = i;
});

function mutate1 ( obj ) {
	obj.mutated = 1;
}

console.log( foo );