How to extend an array with new array without declaration

Question

I am working on project where I need to maintain an array from json data returned from API, json can have tree, I have following code which is working fine but I wan to remove if conditions before assigning values to array elements

// data contains json 
let newArray = []
for(let d in data){
    for(let x in data[d]){
        if(typeof(newArray[d]) === 'undefined'){
            newArray[d] = []
        }
        if(typeof(newArray[d][data[d][x]['id']]) === 'undefined'){
            newArray[d][data[d][x]['id']] = []
        }
        newArray[d][data[d][x]['id']]['price'] = data[d][x]['price']
        newArray[d][data[d][x]['id']]['discount'] = data[d][x]['discount']
    }
}

In above code I have to check the array first and declare it as array if its not otherwise it returns undefined error, is there any way to get rid of there conditions and extend array as per requirement ?

newArray[d] looks suspicious. I hope newArray will not become a sparse array? (if so, use an object instead) You can make the code more DRY by storing the repetitive expressions in variables — CertainPerformance, Commented Aug 23, 2019 at 8:50
You seem to be using arrays as normal objects. What is data[d][x]['id'] ? Is it a string or is it a number ? — Titus, Commented Aug 23, 2019 at 9:06
@Titus it can be anything from string or number, I did check with let newArray = {} but it has same issue — Vikram, Commented Aug 23, 2019 at 9:15
It should be an object in the first place, not an array. JavaScript allows you to do sth like: let arr = []; arr['prop1'] = 'val1'; without complaining. But the problem is with serialization. If you try to JSON.stringify() such a thing, only number-like indexes will be kept in the result. For storing key-value pairs there are object literals ({ key1: 'value1' }) and this is what you should use here. Regarding your case, if you expect that let o = {}; o.prop1.prop2 = 'val1' will create { prop1: { prop2: 'val1' } } then the answer is no, you can't do that without additional effort. — Sebastian Kaczmarek, Commented Aug 23, 2019 at 9:32
@SebastianKaczmarek Thanks for explanation, it seems that using conditions is only solution in my case, however I can try with object at other places though — Vikram, Commented Aug 23, 2019 at 11:58

Aviso · Accepted Answer · 2019-08-23 09:52:06Z

2

You can you new ES6 spread operator like this

newAraay[d] = [...newArray,...Array(data[d][x]['id']),[...Array('price',data[d][x]['price'])]]

Like here in this snippet I am directly doing abc[1][3][4] = "new value" without explicitly initialising them

let abc = [];
abc[1]=  [...abc,...Array(3),[...Array(4),'new inserted value']]
console.log(abc);

edited Aug 23, 2019 at 9:52

answered Aug 23, 2019 at 9:30

Aviso

6954 silver badges12 bronze badges

1

It should be rather: abc[1]= [...abc, ...Array(3),[...Array(4),'new inserted value']]. Otherwise, when you try to do something like this: abc[1]= [...Array(2),[...Array(4),'new inserted value']] after first assing then it will be overridden
– Sebastian Kaczmarek
Commented Aug 23, 2019 at 9:43
@SebastianKaczmarek do you mean abc will be overridden ?
– Aviso
Commented Aug 23, 2019 at 9:46
1

Not the whole array, but the 1 index will be, if you try to do abc[1][3][4] = 'newvalue' and then abc[1][2][3] = 'anothervalue'
– Sebastian Kaczmarek
Commented Aug 23, 2019 at 9:48
yes , you are right ! I will edit the anwer , Thanx :)
– Aviso
Commented Aug 23, 2019 at 9:51

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CodeBanBan · Accepted Answer · 2019-08-23 09:10:21Z

1

newArray[d] = newArray[d] || []

You can understand this operation in this post

Or use Lodash Library

https://lodash.com/docs/4.17.11#set

edited Aug 23, 2019 at 9:10

answered Aug 23, 2019 at 8:52

CodeBanBan

485 bronze badges

its a declaration again, I want to be able to assign value, also it might work on first level element but not for inner elements like newArray[d][e][f] if I want to
– Vikram
Commented Aug 23, 2019 at 9:03

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