Código Preciso

dani.js

var dataDb = [{
        id: 1,
        dataIni: new Date('2017-01-20'),
        dataFin: new Date('2017-01-24'),
        val: 50
    },
    {
        id: 2,
        dataIni: new Date('2017-01-25'),
        dataFin: new Date('2017-02-08'),
        val: 100
    }
];

function lastDay(month, year) {
    let d = new Date(year, month, 0);
    return d.getDate();
}

function cleanDatesVal(arr) {
    let objs = [];
    for (let i = 0; i < arr.length; i++) {
        let obj = {};
        let dataIniDays = arr[i].dataIni.getDate() + 1;
        let dataFinDays = arr[i].dataFin.getDate() + 1;
        let dataIniMonth = arr[i].dataIni.getMonth() + 1;
        let dataFinMonth = arr[i].dataFin.getMonth() + 1;
        let dataIniYear = arr[i].dataIni.getFullYear();
        let dataFinYear = arr[i].dataFin.getFullYear();

        if (dataIniMonth !== dataFinMonth) {
            let lastDayVal = lastDay(dataIniMonth, dataIniYear);
            let dayUntilFinal = (lastDayVal - dataIniDays) + 1;
            let diasToSave = dayUntilFinal + dataFinDays;
            let finalVal = (arr[i].val / diasToSave) * dayUntilFinal;

            obj['id'] = arr[i].id;
            obj['dias'] = dayUntilFinal;
            obj['val'] = finalVal;
            objs.push(obj);
            continue;
        }

        obj['id'] = arr[i].id;
        obj['dias'] = (dataFinDays - dataIniDays) + 1;
        obj['val'] = arr[i].val;
        objs.push(obj);
    }
    return objs;
}

function getFinalVal(arr) {
    let val = 0;
    for (let i = 0; i < arr.length; i++) {
        val += arr[i].val;
    }
    return val.toFixed(2);
}

var objs = cleanDatesVal(dataDb);
console.log(`---------------------------------`);
console.log(`Valor Final: ${getFinalVal(objs)}`);
console.log(`---------------------------------`);