// We have some state like:
const state1 = {
  data: { some: 'stuff' },
  people: ['blah', 'blah', 'blah'],
  memberships: [
    { id: 1, roles: [], other: 'stuff' },
    { id: 2, roles: ['client'], more: 'junk' }
  ]
};


// We want to be able to add a role to a particular membership (without mutation) (but without ramda):
const addRole = (id, role) => (obj) => {
  const newMemberships = obj.memberships.map((membership) => {
    if (membership.id == id) {
      const newRoles = membership.roles.concat(role)
      return Object.assign({}, membership, {roles: newRoles})
    } else {
      return membership
    }
  })
  return Object.assign({}, obj, {memberships: newMemberships})
}


// For example:
addRole(2, 'admin')(state1)
// …returns a new object, in which `data`, `people`, and membership id 1 are all the exact same objects as before (not copies):
{
  data: { some: 'stuff' },
  people: ['blah', 'blah', 'blah'],
  memberships: [
    { id: 1, roles: [], other: 'stuff' },
    { id: 2, roles: ['client', 'admin'], more: 'junk' }
  ]
}