JS Object field mapper

object-field-mapper.js

import _ from 'lodash'

/**
 * Map fields of an object:
 * 
 * ```
 * ObjectUtils.mapFields(
 *   { a: 1, b: 2, c: 3, d: 4 },
 *   {
 *     a: num => num + 10,
 *     b: { newB: num => num + 20 },
 *     c: { C1: num => num + 20, C2: num => num + 30 }
 *   }
 * )
 * ```
 * -> `{ d: 4, a: 11, newB: 22, C1: 23, C2: 33 }`
 * 
 * @param {Object} obj Source object
 * @param {Object} mapper Map definition
 */
mapFields(obj, mapper) {
    const newObj = _.omit(obj, _.keys(mapper));

    for (const [oldKey, mapDef] of _.toPairs(mapper)) {
        const oldValue = obj[oldKey]
        let newFields;
        if (typeof mapDef === 'function') {
            newFields = { [oldKey]: mapDef(oldValue) }
        } else if (_.isPlainObject(mapDef)) {
            newFields = _.mapValues(mapDef, (mapDefSingleField, newKey) => {
                if (typeof mapDefSingleField === 'function') {
                    return mapDefSingleField(oldValue)
                }
                throw new Error(`invalid map definition type: ${typeof mapDefSingleField} for ${oldKey}->${newKey}`)
            })
        } else
            throw new Error(`invalid map definition type: ${typeof mapDef} for ${oldKey}`)

        _.assignWith(newObj, newFields, (oldVal, newVal, field) => {
            if (oldVal === undefined) return newVal;
            if (_.has(obj, field) && !_.has(mapper, field))
                throw new Error(`Re-mapping existing field '${field}'`)
            else
                throw new Error(`Duplicate mapping for '${field}': ${oldVal}, ${newVal}`)
        })
    }

    return newObj
}