Using Lodash Map with Objects

Lodash’s _.map method is designed to be used with arrays, but playing around with it I found a couple of uses with objects that I hope you will find useful.

Require Lodash in your Node server app or use import in your React front-end client app

const _ = require('lodash') // Node
import _ from 'lodash' // React

Given the object below return an array of object keys where the value is true.

let quote_state = { btc: true, usdt: false, eth: false }

If we were to use _.map as we do with an array we would NOT get the desired result.

const _quote_filter = _.map(quote_state, (val, key) => {

   if (val) {
      return key
   }

})

console.log(_quote_filter) //=> [ 'btc', undefined, undefined ]

However, if we use _.map as an iteration block — similar to a loop — then we can use the val and key arguments to set conditions and push items onto an array.

const _quote_filter = []

_.map(quote_state, (val, key) => {

   if (val) {
      _quote_filter.push(key)
   }

})

console.log(_quote_filter) //=> [ 'btc' ]

You can also achieve the above with _.each, however, you cannot use _.each in the example below where we want to map object keys onto their respective sub_object and return an array.

let markets = {
   'BTC/USD': {
      buys: 0,
      sells: 3
   },
   'DASH/BTC': {
      buys: 3,
      sells: 1
   },
   'ETH/BTC': {
      buys: 3,
      sells: 2
   }
}
const _markets_arr = _.map(markets, (obj, key) => {
   obj.symbol = key
   return obj
})
console.log(_markets_arr) 
//=> [ 
       { buys: 0, sells: 3, symbol: 'BTC/USD' },
       { buys: 3, sells: 1, symbol: 'DASH/BTC' },
       { buys: 3, sells: 2, symbol: 'ETH/BTC' } 
   ]

Although, if you need to set conditions and filter out objects you will need to use _.map as mentioned before.

const _markets_arr = []

_.map(markets, (val, key) => {
   val.symbol = key
   if (val.buys > 0) {
      _markets_arr.push(val)
   }
})

console.log(_markets_arr)
//=> [ 
       { buys: 3, sells: 1, symbol: 'DASH/BTC' },
       { buys: 3, sells: 2, symbol: 'ETH/BTC' } 
   ]

You could use _.filter to do the same as above, HOWEVER, I have found both _.filter and vanilla JavaScript’s Array.prototype.filter do not work on certain JSON responses.

This is especially true when working with Elasticsearch queries called from a Node server and then fed to the front end via Socket.io.

As a bonus trick using _.each with an object — You can map an object key onto it’s respective sub-object whilst retaining the original object’s structure.

_.each(markets, (obj, key) => {
   obj.symbol = key
})

console.log(markets)
//=> {
       'BTC/USD': { buys: 0, sells: 3, symbol: 'BTC/USD' },
       'DASH/BTC': { buys: 3, sells: 1, symbol: 'DASH/BTC' },
       'ETH/BTC': { buys: 3, sells: 2, symbol: 'ETH/BTC' }
    }