Mapping objects in JavaScript (ES5)

mapping.js

const data = {
    a: 1,
    b: 2,
};

const newData = Object.keys(data)
    .map(function (key) {
        return {
            key: '_' + key,
            value: data[key] + 1,
        };
    })
    .reduce(function (obj, item) {
        obj[item.key] = item.value;
        return obj;
    }, {});