for...in loop through array of objects returns 'Cannot read property of undefined'

Question

I need to recursively navigate through a tree of objects until I find the matching element and push some data into it. Here's a simplified example of my structure:


post.comments = [
    {
        _id: a,
        content: 'Foo',
        replies: [
            {
                _id: aa,
                content: 'Foobar',
                replies: [
                    {
                        _id: aaa,
                        content: 'Foobarbaz',
                        replies: [...]
                    }
                ]
            },
            {
                _id: ab,
                content: 'Barfoo',
                replies: [...]
            }
        ]
    },
    {
        _id: b,
        content: 'Bar',
        replies: [...]
    },
    {
        _id: c,
        content: 'Bar',
        replies: [...]
    },
    ...
]

The replies can be nested infinitely, in theory.

And here's my recursive function:

function findNode(comments, id, data) {
    for (let key in comments) {
        currentNode = comments[key]
        if (currentNode._id.equals(id)) {
            currentNode.replies.push(data)
            break;
        } else {
            if (currentNode.replies.length) {
                findNode(currentNode.replies, id, data)
            }
            else {
                continue;
            }
        }
    }
}

findNode(post.comments, id, comment)

This function appears to only read through the child nodes of the first comment object, then exits with 'Cannot read property 'equals' of undefined' when it reaches a node without children (but I presumed that the continue; should pull it out of that branch of the tree?)

What is it I'm not getting about for loops and recursion?

EDIT: Sorry! string.equals(id) comes from Mongoose - it's equivalent to string == id for the puropses of this example array.

You need to declare currentNode with let or var or else it's global. — Pointy
– Pointy, Commented Jun 13, 2019 at 14:53
Where is String.equals defined? Did you mean if (currentNode._id == id) ? — James
– James, Commented Jun 13, 2019 at 15:07
where is the .equals method coming from? A vanilla comparison in JavaScript looks like comments.id === id. — Mulan
– Mulan, Commented Jun 13, 2019 at 15:27

horia · Accepted Answer · 2020-02-03 14:25:45Z

4

You can't use currentNode.replies.length if currentNode.replies is undefined. You should check if currentNode.replies is not null, would go somewhat like this:

if (currentNode.replies && currentNode.replies.length) { ...

EDIT: same goes for currentNode._id, check if it is not null before calling currentNode._id.equals.

edited Feb 3, 2020 at 14:25

answered Jun 13, 2019 at 14:57

horia

4195 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

lowercasename Over a year ago

Thank you! That solved the original problem - but now I'm getting RangeError: Maximum call stack size exceeded and I don't understand why! I think I'm covering all the cases??

Collectives™ on Stack Overflow

for...in loop through array of objects returns 'Cannot read property of undefined'

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related