Update Nested Object on DynamoDB using Javascript/Typescript node.js

getDynamoExpression.js

/**
 * Recursive function to get UpdateExpression,ExpressionAttributeValues & ExpressionAttributeNames to update a nested object on dynamoDB
 * All levels of the nested object must exist previously on dynamoDB, this only update the value, does not create the branch.
 * Only works with objects of objects, not tested with Arrays.
 * @param obj , the object to update.
 * @param k , the seed is any value, takes sense on the last iteration.
 */
function getDynamoExpression(obj, k) {

    const key = Object.keys(obj);

    let UpdateExpression = 'SET ';
    let ExpressionAttributeValues = {};
    let ExpressionAttributeNames = {};
    let response = {
        UpdateExpression: ' ',
        ExpressionAttributeNames: {},
        ExpressionAttributeValues: {}
    };

    //https://stackoverflow.com/a/16608074/1210463

    /**
     * true when input is object, this means on all levels except the last one.
     */
    if (((!!obj) && (obj.constructor === Object))) {

        response = getDynamoExpression(obj[key[0]], key);
        UpdateExpression = 'SET #' + key + '.' + response['UpdateExpression'].substring(4); //substring deletes 'SET ' for the mid level values.
        ExpressionAttributeNames = {['#' + key]: key.toString(), ...response['ExpressionAttributeNames']};
        ExpressionAttributeValues = response['ExpressionAttributeValues'];

    } else {
        UpdateExpression = 'SET   = :' + k;
        ExpressionAttributeValues = {
            [':' + k]: obj
        }
    }

    //removes trailing dot on the last level
    if (UpdateExpression.indexOf(". ")) {
        UpdateExpression = UpdateExpression.replace(". ", "");
    }

    return {UpdateExpression, ExpressionAttributeValues, ExpressionAttributeNames};
}

//you can try many levels.
const obj = {
    level1: {
        level2: {
            level3: {
                level4: 'value'
            }
        }
    }
}