getting the length of the array of objects inside an object

Question

I would like to map the object and get the number/length of replies that can be found in replies in this case 2. I have fetched this data from the API and is unable to get the number of replies that can be found here.

let data = [ 
 { 
 "feedbackId":32,
 "sender":"12345"
 "comment":"Test Comment",
 "replies":[ 
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":1,
 "comments":"So this is 1",
 "createdBy":"Jack",
 },
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":2,
 "comments":"2nd one",
 "createdBy":"Min",
 }
 ],
 }
]

This is what I've done. I am only able to get 1 as the length.

let repliesToRender = data.map((item) => {item.replies})
let numReplies = repliesToRender.length

Answer 1

You can calculate reply per feedback using Array.prototype.map

for calculting total number of replies you can use Array.prototype.reduce

let data = [{"feedbackId":32,"sender":"12345","comment":"Test Comment","replies":[{"feedbackLogId":32,"feedbackReplyLogId":1,"comments":"So this is 1","createdBy":"Jack"},{"feedbackLogId":32,"feedbackReplyLogId":2,"comments":"2nd one","createdBy":"Min"}]},{"feedbackId":32,"sender":"12345","comment":"Test Comment","replies":[{"feedbackLogId":32,"feedbackReplyLogId":1,"comments":"So this is 1","createdBy":"Jack"},{"feedbackLogId":32,"feedbackReplyLogId":2,"comments":"2nd one","createdBy":"Min"}]}]
let repliesCount = data.map(({replies}) => replies.length);
console.log('per replies',repliesCount);
let totalCount = data.reduce((a,c) => a + c.replies.length, 0);
console.log('total replies ',totalCount)

• Tushar Vaghela: This solutions will take 0.47900390625ms as it's using map and reduce to traverse array. Complexity will increase with more data.
• arizafar: @TusharVaghela it's map OR reduce depending upon the requirement, also what do you suggest otherwise? or its just an information? also how the complexity will increase with more data?
• Tushar Vaghela: I suggest using let {replies} = data[0];console.log(replies.length); This solution will take around 0.209716796875ms. With more number of replies, map or reduce will take more time.

Answer 2

You have 2 issue in your code,

let repliesToRender = data.map((item) => {item.replies})

should be

let repliesToRender = data.map((item) => item.replies)

And map return you array so your repliesToRender will be an array of array so you need

let numReplies = repliesToRender[0].length

Or just

let numReplies = data.replies.length

---

let data = [ 
 { 
 "feedbackId":32,
 "sender":"12345",
 "comment":"Test Comment",
 "replies":[ 
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":1,
 "comments":"So this is 1",
 "createdBy":"Jack",
 },
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":2,
 "comments":"2nd one",
 "createdBy":"Min",
 }
 ],
 }
]
let repliesToRender = data.map((item) => item.replies)
let numReplies = repliesToRender[0].length
console.log(numReplies)

Answer 3

If replies count is needed for each of the objects, then below will return array of replies count for respective objects in order,

let eachObjectsRepliesCount = data.map(({replies}) => replies.length);

If replies count is needed for all the objects combined, then below will return the total replies count,

let repliesToRender = data.map((item) => item.replies)
let totalRepliesCount = repliesToRender[0].length

Answer 4

Another approach. This will run faster in case of large data-sets, as it's not using loops.

let data = [ 
 { 
 "feedbackId":32,
 "sender":"12345",
 "comment":"Test Comment",
 "replies":[ 
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":1,
 "comments":"So this is 1",
 "createdBy":"Jack",
 },
 { 
 "feedbackLogId":32,
 "feedbackReplyLogId":2,
 "comments":"2nd one",
 "createdBy":"Min",
 }
 ],
 }
]
let {replies} = data[0];
console.log(replies.length);