Invariant Violation: A state mutation was detected between dispatches, in the path

I would like to concatenate one item in the array with another one and remove the second item from the array. When I tried the old way, I get the state mutation detected error. When I tried the Object.assign, I am unable to get the values concatenated.

EDIT:
What is the equivalent of interests[makeIndex].value = ``${interests[makeIndex].value}, ${interests[secondPreferredMakeIndex].value}`` using Object.assign?

For e.g., for the below code segment the output I expect is,

Preferred Make - before Chevrolet
Preferred Make - after Chevrolet, Porsche

// Interests array from state
let interests = [
  {
    type: 'Preferred Make',
    value: 'Chevrolet',
  },
  {
    type: 'Second Preferred Make',
    value: 'Porsche',
  },
  {
    type: 'Preferred Model',
    value: 'Corvette',
  },
  {
    type: 'Second Preferred Model',
    value: 'Macan',
  }
];

console.log("Preferred Make - before", interests[0].value);

const secondPreferredMakeIndex = interests
  .map(x => x.type)
  .indexOf('Second Preferred Make');

if (secondPreferredMakeIndex > -1) {
  let makeIndex = interests.map(x => x.type).indexOf('Preferred Make');

  if (makeIndex > -1) {
    // For the below, I get mutation error. But it works
    // interests[makeIndex].value = `${interests[makeIndex].value}, ${interests[secondPreferredMakeIndex].value}`;

    // Concatenate and use Object.assign to avoid mutation
    interests = Object.assign([], interests, { makeIndex: `${interests[makeIndex].value}, ${interests[secondPreferredMakeIndex].value}` });
    /*
    // Tried the below as well in vain
    interests = Object.assign([], interests, {
      makeIndex: {
        type: 'Preferred Make',
        value: `${interests[makeIndex].value}, ${interests[secondPreferredMakeIndex].value}`
      },
    });
    */
  }

  // Delete the second Preferred Make
  interests.splice(secondPreferredMakeIndex, 1);
}

console.log("Preferred Make - after", interests[0].value);

Appreciate the helps