Mapping properties of javascript objects from under_score to camelCase and back again

propertyMapping.js

String.prototype.toCamel = function () {
    return this.replace(/(_[a-zA-Z])/g, function ($1) {
        return $1.toUpperCase().replace('_', '');
    });
};

String.prototype.toUnderscore = function () {
    return this.replace(/([A-Z])/g, function ($1) {
        return "_" + $1.toLowerCase();
    });
};

function withCamelCaseKeys(obj) {
    const newObjectWithCamelCaseKeys = {}
    for (const key of Object.keys(obj)) {
        if (obj[key] !== null && typeof obj[key] === 'object') {
            newObjectWithCamelCaseKeys[key.toCamel()] = withCamelCaseKeys(obj[key])
        } else {
            newObjectWithCamelCaseKeys[key.toCamel()] = obj[key]
        }
    }
    return newObjectWithCamelCaseKeys
}

function withUnderscoreKeys(obj) {
    const newObjectWithUnderscoreKeys = {}
    for (const key of Object.keys(obj)) {
        if (obj[key] !== null && typeof obj[key] === 'object') {
            newObjectWithUnderscoreKeys[key.toUnderscore()] = withUnderscoreKeys(obj[key])
        } else {
            newObjectWithUnderscoreKeys[key.toUnderscore()] = obj[key]
        }
    }
    return newObjectWithUnderscoreKeys
}

let obj = {camel_case_test: 123, object_test: {child_object_test_key: "test", child_object_test_key2: "test2"}}

const newThing = withCamelCaseKeys(obj)
console.debug(newThing)

const newThing2 = withUnderscoreKeys(newThing)
console.debug(newThing2)