how can I generate a new array by concatenate nested array objects by using lodash

Question

I have an array of objects, and each objects may contain another array of objects, I want to concatenate values of nested array and generate a new array.

For example

lessons = [
 {
   id: 1,
   description: string,
   material: [{obj1}, {obj2}]
 },
 {
   id: 2,
   description: string,
   material: [{obj3}]
 },
 {
   id: 3,
   description: string,
   material: [{obj4}, {obj5}, {obj6}]
 }
]

I want to generate a new array only contains material, expected output would like this

materials = [
   {obj1},
   {obj2},
   {obj3},
   {obj4},
   {obj5},
   {obj6}
 ]

how can I do that by using lodash?

Majed Badawi · Accepted Answer · 2021-02-28 13:26:00Z

2

You can use .flatMap to iterate over lessons, and return all the material lists in one array:

const lessons = [
 { id: 1, description: 'string', material: [{obj1:1}, {obj2:2}] },
 { id: 2, description: 'string', material: [{obj3:3}] },
 { id: 3, description: 'string', material: [{obj4:4}, {obj5:5}, {obj6:6}] }
];

const res = _.flatMap(lessons, ({ material = [] }) => material);

console.log(res);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js" integrity="sha512-WFN04846sdKMIP5LKNphMaWzU7YpMyCU245etK3g/2ARYbPK9Ub18eG+ljU96qKRCWh+quCY7yefSmlkQw1ANQ==" crossorigin="anonymous"></script>

edited Feb 28, 2021 at 13:26

answered Feb 28, 2021 at 13:19

Majed Badawi

28.5k4 gold badges30 silver badges55 bronze badges

I can't run the code snippet, it shows _ is not defined. I can see the usage is from lodash doc, can I just write it like : const res = flatMap(lessons, e => e.material || []) without underscore?
– RH-st
Commented Feb 28, 2021 at 13:32
1

@RH-st you stated that you need to use lodash so make sure to have it installed. If you want to use it normally, try this: lessons.flatMap(({ material = [] }) => material);
– Majed Badawi
Commented Feb 28, 2021 at 13:34
Amazing! That really help!! Thanks a lot, can I ask for deeper explanation? so lessons.flatMap() means lessons is already become the first argument which is the collection, therefore we only need to pass the second argument which is ({ material = [] }) => material be the iteratee function, right?
– RH-st
Commented Feb 28, 2021 at 13:48
1

@RH-st you're welcome. Basically, it's a function in Array.prototype and the argument is the callback function that gets executed in each iteration. This is the reference for the docs: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
– Majed Badawi
Commented Feb 28, 2021 at 13:52

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