Object Assign updating original source

Question

Today, I was working with redux.

When I was creating a copy with Object.assign with objects without inner arrays, it worked well.

But when I had to deal with an object like this:

{
      ID:'2',
      NAME:'GENERAL',
      FIELDS:[
        {
          ID:'1',
          NAME:'M2'
        }
      ]
    }

, Object assign would make a copy, but the changes that I made in the copy were happening in the original too.

I have to use lodash cloneDeep.

Anyone knows why this behaviour happens?