BST Implementation is FP Style

binaryTree.js

const merge = (...args) => Object.assign({}, ...args)
const arrayFromObject = obj => Object.keys(obj).map(key => obj[key])
const getLastTreeIndex = tree => parseInt(Object.keys(tree).pop(), 10)
const getNextTreeIndex = tree => getLastTreeIndex(tree) + 1
const getNodeParentIndex = (tree, {value}) => {
    let currentNode = tree[0]
    let currentIndex = 0
    while (currentNode) {
        if (value >= currentNode.value) {
            currentNode = tree[currentNode.right]
            if (!currentNode) return currentIndex
            currentIndex = currentNode.index
        } else {
            currentNode = tree[currentNode.left]
            if (!currentNode) return currentIndex
            currentIndex = currentNode.index
        }
    }

    return null
}

const connectParentWithChild = (parent, childIndex, side) =>
    merge(parent, {[side]: childIndex})
const getRelationshipBetweenChildAndParent = (parent, child) =>
    child.value >= parent.value ? 'right' : 'left'

const appendToTreeIndex = (tree, node) => {
    return merge(
        tree,
        {[getNextTreeIndex(tree)]: merge(
            node,
            {index: getNextTreeIndex(tree)}
        )},
        {[getNodeParentIndex(tree, node)]: connectParentWithChild(
            tree[getNodeParentIndex(tree, node)],
            getNextTreeIndex(tree),
            getRelationshipBetweenChildAndParent(
                tree[getNodeParentIndex(tree, node)],
                node
            )
        )}
    )
}

const createNode = (value, index = 0) => ({
    left: null,
    right: null,
    index,
    value
})

const createInitialTree = initialValue => ({0: createNode(initialValue)})
const createBST = (initialValue, integers) =>
    integers.reduce((bst, integer) => appendToTreeIndex(bst, createNode(integer)), 
        createInitialTree(initialValue))

const wrapChildrenIntoParent = (parent, tree) => merge(
    parent,
    {left: tree[parent.left]},
    {right: tree[parent.right]}
)

const generateRandomArray = (count) => {
    const array = []
    for  (let i = 0; i < count; i++) {
        array.push(Math.floor(Math.random() * 100))
    }
    return array
}
const bst = createBST(42, generateRandomArray(5))
console.log(bst)