SOURCE

function BinaryTree() {
    let Node = function (val) {
        this.val = val
        this.left = null
        this.right = null
    }

    this.root = null
}

var preorderTraversal = (root) => {
    let result = []

    var preorderTraverseNode = (node) => {
        if (node) {
            result.push(node.val)

            preorderTraverseNode(node.left)
            preorderTraverseNode(node.right)
        }
    }

    preorderTraverseNode(root)

    return result
}