In this example i will try to explain immutability

immutability.js

let mutateObj = {
  first_name:'lorem',
  last_name:'ipsum'
};

let immutateObj = {
  first_name:'irum',
  last_name:'egestas'
};

const mutatingState = (obj) => {
  obj.first_name = 'ullamcorper';
  return obj;
}

const immutatingState = (obj) => {
  const copiedObj = Object.assign({},obj);
  copiedObj.first_name = 'facilisis';
  return copiedObj;
}

mutatingState(mutateObj);
console.log(mutateObj);
/*
above output {
  first_name:'ullamcorper',
  last_name:'ipsum'
};

in the above function we have mutated the orignal state
*/
const newObj = immutatingState(immutateObj);
console.log(immutateObj);
/*
newObj will be 
{
  first_name:'facilisis',
  last_name:'egestas'
};
and  immutateObj will not be changed and it value is
{
  first_name:'irum',
  last_name:'egestas'
};
here the orignal state is not change
*/