let a = {a:1}, b = {b:2};
let target = {};
// skips non-enumerable properties
Object.defineProperty(b, 'c', {
value: 3,
enumerable: false
});
Object.assign(target, a, b);
console.log( b );
// {b: 2, c: 3}
console.log( target );
// {a: 1, b: 2}