const obj = {
    M1:1,
    M2:2,
    M3:3,
    P1:1,
    P2:2,
    P3:2,
    P4:1
}

arr = [
     {name:"M",value:6},
     {name:"P",value:6}
 ]

转换规则没有描述清楚啊，arr 里面 value 6 哪里来的？
obj 里面也没有 6 啊

const arr = Object.entries(obj)
    .reduce((arr, [key, value]) => {
        const index = key.charCodeAt(0)
        if (arr[index]) arr[index] += value
        else arr[index] = value
        return arr
    }, [])
    .map((value, index) => ({
        name: String.fromCharCode(index),
        value
    }))
    .filter(i => i)

每个人心中的"优雅“是不一样的，并且往往”优雅“的代码会降低代码的可读性。

var obj = {
    M1:1,
    M2:2,
    M3:3,
    P1:1,
    P2:2,
    P3:2,
    P4:1
}

// Fist, group to {M: 6, P: 6}
var group = Object.keys(obj).reduce((group, key) => {
  const groupKey = key.replace(/^([a-z]+)\d+/i, '$1');
  group[groupKey] = obj[key] + (group[groupKey] || 0);
  return group;
}, {});

// Finally
var result = Object.keys(group).map(name => ({name, value: result[name]}))

console.log(result)

const arr = keys(obj).map((item)=>({name:item, value:obj[item]}))