javascript - How to get the number of identical elements in an array

Question

There is an array, for example var numArr = ["A", "C", "B", "A", "C", "D", "A", "C"],How to filter out the same elements and the number of the same elements

Answer 1

A simple loop will appear

Answer 2

["A", "C", "B", "A", "C", "D", "A", "C","B"].reduce((r,v)=>{
  r[0].has(v)?r[1][v]?r[1][v]++:r[1][v]=2:r[0].add(v); 
  return r;
},[new Set,{}])[1]
// {A: 3, C: 3, B: 2}

Is this the effect? Select repeated elements (the number of occurrences is greater than or equal to 2) and count the number of occurrences of each repeated element?

Answer 3

function checkArray(para, arr) {
    let processArr = arr.filter(function(value) {
        return value == para;
    });

    return processArr.length;  // 这里返回数组长度或者相应处理
}

console.log(checkArray(1, numArr));

Answer 4

You can use Map

["1", "2", 2, 0, -0, NaN, NaN, [], [], {}, {}, undefined, , , null].reduce((m, k) => {
    return m.set(k, (m.get(k) || 0) + 1);
}, new Map());

Follow the following rules:

• NaN = NaN

• +0 = -0

• "1" != 1

• {} != {}

• [] != []

• undefined != null

• Empty elements are not counted

Answer 5

var obj={},arr=["A", "C", "B", "A", "C", "D", "A", "C","B"]
//for
for(var i=0,len=arr.length;i<len;i++){
    if(obj[arr[i]]){
        obj[arr[i]]++
    }
    else{
    obj[arr[i]]=1
    }
}
//forEach
arr.forEach(function(item,i){obj[arr[i]]?obj[arr[i]]++:obj[arr[i]]=1})