Implementing BST in javascript
Interface

Why return the value added? That's unusual and unexpected.

Bug

It's unusual to allow adding duplicate values.
It would be better to ignore.

Avoid flag variables when possible

The flag variable inserted is unnecessary.

while (true) {
    if (item &gt; currNode.value) {
        if (currNode.rightNode) currNode = currNode.rightNode;
        else {
            currNode.rightNode = {
                value: item
            };
            return;
        }
    } else {
        if (currNode.leftNode) currNode = currNode.leftNode;
        else {
            currNode.leftNode = {
                value: item
            };
            return;
        }
    }
}


I replaced the insert = true statements with return.
If you really want to return item, then you can either use that instead of return, or use break.