Basically, Object.assign is just reading and assigning properties and values one by one, that's why the following use case of Object.assign also works:

Object.assign(htmlElement.style, { fontSize: '12px', color: 'navy' });
// This is basically the same as
htmlElement.style.fontSize = '12px';
htmlElement.style.color = 'navy';

Object.assign(function() {}, { extraPropOnFunction });
// This is basically the same as
(function() {}).extraPropOnFunction = extraPropOnFunction;

Thus, Object.assign on a plain object will also face the same Monomorphism JIT issue of V8. If we predefine the shape of the object, V8 will have to de-optimize later. But if we don't define the shape, V8 will determine the shape itself.