const simplaArr = [
 {year: '1988', title: 'tile1', location: 'b'},

 {year: '1988', title: 'title2', location: 'a'},

 {year: '1989', title: 'title3', location: 'c'}
];
 => 
const simpleArr2 = [
{
    year: '1988', 
    places: [
        {title: 'title1', location: 'b'},
        {title: 'title2', location: 'a'},
    ]
},
{
    year: '1989',
    places: [{title: 'title3', location: 'c'}]
}]

const simplaArr = [
 {year: '1988', title: 'tile1', location: 'b'},

 {year: '1988', title: 'title2', location: 'a'},

 {year: '1989', title: 'title3', location: 'c'}
];
 => 
const simpleArr2 = [
{
    year: '1988', 
    places: [
        {title: 'title1', location: 'b'},
        {title: 'title2', location: 'a'},
    ]
},
{
    year: '1989',
    places: [{title: 'title3', location: 'c'}]
}]

const simpleArr = [
  { year: "1988", title: "tile1", location: "b" },
  { year: "1988", title: "title2", location: "a" },
  { year: "1989", title: "title3", location: "c" }
];
const newObj = {};
simpleArr.forEach((el) => {
  const year = el.year;
  const t = el.title;
  const loc = el.location;
  if (!Array.isArray(newObj[year])) newObj[year] = [];
  newObj[year].push({ title: t, location: loc });
});
const result =[]
for(let year in newObj){
  result.push({ year: year, places: newObj[year] })
}
console.log(result)

const simplaArr = [
 {year: '1988', title: 'tile1', location: 'b'},

 {year: '1988', title: 'title2', location: 'a'},

 {year: '1989', title: 'title3', location: 'c'}
];
 => 
const simpleArr2 = [
{
    year: '1988', 
    places: [
        {title: 'title1', location: 'b'},
        {title: 'title2', location: 'a'},
    ]
},
{
    year: '1989',
    places: [{title: 'title3', location: 'c'}]
}]

Object.values(simplaArr.reduce((map, { year, ...place }) =>
  Object.assign(map, {
        [year]: {
            year, places: (
                map[year] &&
                map[year].places || []
            ).concat(place)
        }
    }), {}
))

Object.values(simplaArr.reduce((map, { year, ...place }) =>
  Object.assign(map, {
        [year]: {
            year, places: (
                map[year] &&
                map[year].places || []
            ).concat(place)
        }
    }), {}
))

Object.values(simplaArr.reduce((map, { year, ...place }) =>
  Object.assign(map, {
        [year]: {
            year, places: (
                map[year] &&
                map[year].places || []
            ).concat(place)
        }
    }), {}
))