https://leetcode.com/problems/serialize-and-deserialize-bst/

serdeBST.js

function serialize ( root ) {
    const queue = root ? [ root ] : []
    const nodes = []

    while ( queue.length ) {
        nodes.push( node = queue.shift() )

        if ( node ) {
            queue.push( node.left )
            queue.push( node.right )    
        }
    }

    return nodes.map( node => node ? node.val : '' ).join( ',' )
}

function deserialize ( data ) {
    const nodes = data.split( ',' ).map(value => value ? new TreeNode( Number( value ) ) : null )    
    const root = nodes.shift()
    const queue = root ? [ root ] : []

    while ( queue.length ) {
        const node = queue.shift()

        if ( node ) {
            node.left = nodes.shift()
            node.right = nodes.shift()

            if ( nodes.length ) {
                queue.push( node.left )
                queue.push( node.right )
            }
        }
    }
    
    return root
}