javascript - LoDash - How to push one collections values into another via common key -

i have 2 collections.

var =  [  {unique_id: "001", state: "co"},  {unique_id: "001", state: "tx"},  {unique_id: "001", state: "cc"},  {unique_id: "002", state: "cc"},  {unique_id: "002", state: "ny"} ] 

and

var b =  [  {unique_id: "001", states:[]},  {unique_id: "002", states:[]} ] 

and want get:

var b =  [  {unique_id: "001", states:["co","tx","cc"]},  {unique_id: "002", states:["cc","ny"]} ] 

i should mention "b" array has stay in same order it's in , of unique_id's don't have value.

i've been trying use lodash https://lodash.com/ - if can solve lodash awesome!

the time complexity of solution suboptimal ( o(n^2) ), might think of ways match pushing values "b":

_.foreach(a, function(element1){   _.foreach(b, function(element2){     if (element2.unique_id === element1.unique_id) {      element2.states.push(element1.state);     }   }); }); 

perhaps better solution might index objects in b unique id using lodash's _.indexby method. example can index objects in b follows:

var c = _.indexby(b, 'unique_id') 

which result in:

{001:{unique_id: "001", states:[]}, 002: {unique_id: "002", states:[]}} 

since objects in our c array pointing same objects in memory in our b array, can directly mutate our objects in c , b reference updated objects in memory. so:

_.foreach(a, function(element1){   if (element1.unique_id in c) {     c[element1.unique_id].states.push(element1.state);   } }) 

now if take @ our b array, we'll see value is:

var b =  [  {unique_id: "001", states:["co","tx","cc"]},  {unique_id: "002", states:["cc","ny"]} ] 

the time complexity of solution should close o(n), better using nested _.foreach approach. code snippet:

var =   [   {unique_id: "001", state: "co"},   {unique_id: "001", state: "tx"},   {unique_id: "001", state: "cc"},   {unique_id: "002", state: "cc"},   {unique_id: "002", state: "ny"}  ];    var b =   [   {unique_id: "001", states:[]},   {unique_id: "002", states:[]}  ];    var c = _.indexby(b, 'unique_id');    _.foreach(a, function(element1){    if (element1.unique_id in c) {      c[element1.unique_id].states.push(element1.state);    }  });    document.writeln(json.stringify(b))

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