You can use array reduce function. Inside the reduce callback check if the current object which is under iteration is an object or not. If it is an object then use JSON.stringify and concat with the accumulator.

const array = [
  'a',
  'b',
  {
    name: 'John doe',
    age: 25
  }
];

const val = array.reduce((acc, curr) => {

  if (typeof curr === 'object') {
    acc += JSON.stringify(curr);
  } else {
    acc += `${curr} `

  }


  return acc;
}, '');
console.log(val)

Using JSON.stringify on the entire array will have starting and ending [ and ] respectively, which is not what you are looking

const array = [
  'a',
  'b',
  {
    name: 'John doe',
    age: 25
  }
];

console.log(JSON.stringify(array))

Simple ! Try following :

var arr = [
  'a',
  'b',
  { name: 'John doe', age: 25 }
]

var newArr = arr.map(i => typeof i === "object" ? JSON.stringify(i) : i)

console.log(newArr)

output :

['a', 'b', '{"name":"John doe","age":25}']