20171117.js

let categories;
const example = {
  username: "user1",
  categories: [
    {
      name: "namae1",
      type: "type1"
    },
    {
      name: "namae2",
      type: "type2"
    }
  ]
}

//nameの値を全て変更したanswer2を新規作成
categories = example.categories.map((obj) => Object.assign({}, obj, { name: "name" }));
const answer2 = Object.assign({}, example, { categories });
console.log(answer2);

//"namae1"の値のみを変更したanswer3を新規作成
categories = example.categories.map((obj) => {
  if(obj.name !== "namae1") return obj;
  return Object.assign({}, obj, { name: "name" });
});
const answer3 = Object.assign({}, example, { categories });

//example自体の値を変える
example.categories.forEach((obj) => obj.name = "name");