使用地图获取数组中每个项目的数量

Question

我正在尝试获取每个字符串项在数组中出现的次数，并使用 Map 将其打印出来。我基本上是从 firebase 函数中读取数据，但现在，我正在使用一个数组来测试代码以获取每个项目的出现。我从here 尝试了以下代码，但它不起作用，我不知道为什么：

<script>
var appVersion= ["5012", "5021", "5012", "5011", "5012", "5021"];

var count = new Map();

appVersion.forEach(version=> count.set(count.get(version) + 1));

function show(versions) {
         document.write(versions);
         document.write("<br>");
      }
    count.forEach(show);
</script>

我们将不胜感激任何帮助或建议。

Answer 1

也许这有帮助

var appVersion= ["5012", "5021", "5012", "5011", "5012", "5021"];

let counter = {}
for (version of appVersion){
    counter[version] = 1 + (counter[version] || 0)
}

console.log(counter);

Answer 2

可能你想要这个

如 cmets 中所述，如果您希望最终对象采用这种格式：{ '5011': 1, '5012': 3, '5021': 2 } 你可以使用reduce

var appVersion= ["5012", "5021", "5012", "5011", "5012", "5021"];


map=new Map
appVersion.forEach(o=>{
   count=map.get(o)||0
    map.set(o,count+1)
})
r=[...map].reduce((acc,curr)=>{
  if (acc.length==0) {acc.push({[curr[0]]:curr[1]})}
  else  {acc[0][curr[0]]=curr[1]}
 return acc
},[])

console.log(r)

Answer 3

使用地图：

let contador = {};
appVersion.map(x => contador[x] = 1 + (contador[x] || 0));
console.log(contador);

Answer 4

见下文

var appVersion= ["5012", "5021", "5012", "5011", "5012", "5021"];

var count = new Map();

appVersion.map( (version) => count.get( version) != undefined ? count.set( version, count.get( version) + 1 ) : count.set( version, 1 ) );

console.log( [...count].sort() );