尝试循环list_cities并计算list_places中出现的次数：

const list_cities = 
    [
     {$id: "15FG", name: "Pittsburg"},
     {$id: "50HS", name: "Los Angeles"}
    ]
    
    const list_places = [
     {$id: "59HE", fk_city: "15FG", name: "Some Place", description: "I have text here", slug: "some-place"},
     {$id: "94KR", fk_city: "15FG", name: "Another Place", description: "This is the description", slug: "another-place",},
     {$id: "05HE", fk_city: "50HS", name: "The last Place" , description: "More text here", slug: "the-last-place"}
    ] 
    
const cityCount = [];
for (const c of list_cities) {
    cityCount.push({
    city: c.name,
    places: list_places.filter(p => p.fk_city === c.$id).length
  })
}
console.log(cityCount);

如果list_places很长，你可以通过先创建一个查找表fk_city，然后再创建一个查找表map来避免多次迭代。

const list_cities = [{ $id: '15FG', name: 'Pittsburg' }, { $id: '50HS', name: 'Los Angeles' },];
const list_places = [{ $id: '59HE', fk_city: '15FG', name: 'Some Place', description: 'I have text here', slug: 'some-place', }, { $id: '94KR', fk_city: '15FG', name: 'Another Place', description: 'This is the description', slug: 'another-place', }, { $id: '05HE', fk_city: '50HS', name: 'The last Place', description: 'More text here', slug: 'the-last-place', },];

const placeCount = {};
for (const { fk_city } of list_places) {
  placeCount[fk_city] = (placeCount[fk_city] ?? 0) + 1;
}

const result = list_cities.map((city) => ({
  ...city,
  placeCount: placeCount[city.$id],
}));

console.log(result);

这个代码对我有效，请检查它可以帮助你

list_cities.forEach((resp) => {
  list_places.forEach((res) => resp.$id === res.fk_city && (resp.places ? (resp.places = resp.places + 1): (resp.places = 1))) 
  return resp
});

const city =  list_cities.map((resp) => {
  return {city: resp.name, places: resp.places}
});
console.log(city); // use this variable