find.js

import {
  compose,
  findIndex,
  cond,
} from 'lodash/fp';

const MY_LIST = [/* items here */];

const findMyItemIndex = findIndex(item => item === 'MY_ITEM');
const changeItemToSomething = () => // returns NEW object (immutable)

const doMagic = (item, list) => compose(
  cond([
    [(index) => index === -1, () => list],
    [(index) => index > -1, (index) => changeItemToSomething(item, list)],
  ]),
  findMyItemIndex(item),
)(list);

doMagic({a: 1}, MY_LIST);