// Complete the following underscore functions.

// Reference http://underscorejs.org/ for examples.

/* eslint-disable no-unused-vars */

const keys = obj => Object.keys(obj);

// Retrieve all the names of the object's properties.

// Return the keys as strings in an array.

// Based on http://underscorejs.org/#keys

const values = obj => Object.keys(obj).map(key => obj[key]);

// Return all of the values of the object's own properties.

// Ignore functions

// http://underscorejs.org/#values

// const mapObject = (obj, cb) => Object.keys(obj).map(key => { cb(obj[key]) });

const mapObject = (obj, cb) => {

// Like map for arrays, but for objects. Transform the value of each property in turn.

// http://underscorejs.org/#mapObject

const copy = {};

const items = keys(obj);

for (let i = 0, len = items.length; i < len; i++) {

copy[items[i]] = cb(obj[items[i]]);

}

return copy;

};

const pairs = obj => Object.keys(obj).map(key => [key, obj[key]]);

// Convert an object into a list of [key, value] pairs.

// http://underscorejs.org/#pairs

const invert = (obj) => {

// Returns a copy of the object where the keys have become the values and the values the keys.

// Assume that all of the object's values will be unique and string serializable.

// http://underscorejs.org/#invert

const copy = {};

const arr = pairs(obj);

for (let i = 0, len = arr.length; i < len; i++) {

copy[arr[i][1]] = arr[i][0];

}

return copy;

};

const defaults = (obj, defaultProps) => {

// Fill in undefined properties that match properties on the `defaultProps` parameter object.

// Return `obj`.

// http://underscorejs.org/#defaults

const objKeys = pairs(obj);

const defKeys = pairs(defaultProps);

for (let i = 0, len = defKeys.length; i < len; i++) {

const key = defKeys[i][0];

const val = defKeys[i][1];

if (!obj[key]) obj[key] = val;

}

return obj;

};

/* eslint-enable no-unused-vars */

module.exports = {

keys,

values,

mapObject,

pairs,

invert,

defaults,

};