How to create a reference implementation for a Tree in JavaScript

function Tree(data) {
  this.data = data;
  this.children = [];
}
Tree.prototype.addChild = function(data) {
  const child = new Tree(data);
  this.children.push(child);
};
Tree.prototype.contains = function(target) {
  if (this.data === target) {
    return true;
  }
  
  for (let i = 0; i < this.children.length; i++) {
    const child = this.children[i];
    if (child.contains(target)) {
      return true;
    }
  }
  return false;
};
function inOrderTraversal(tree, callback = identity, array = []) {
    if (tree) {
      inOrderTraversal(tree.left, callback, array);
      visit(array, tree.data);
      inOrderTraversal(tree.right, callback,array);
    }
    return array.map(callback);
}
function preOrderTraversal(tree, callback = identity, array = []) {
    if (tree) {
      visit(array, tree.data);
      preOrderTraversal(tree.left, callback, array);
      preOrderTraversal(tree.right, callback, array);
    }
    return array.map(callback);
}
function postOrderTraversal(tree, callback = identity, array = []) {
  if (tree) {
    postOrderTraversal(tree.left, callback, array);
    postOrderTraversal(tree.right, callback, array);
    visit(array, tree.data);
  }
  return array.map(callback);
}
function visit(array, data) {
  array.push(data);
}
function identity(element) {
  return element
}
function BinaryTree(data) {
  this.data = data;
  this.left = null;
  this.right = null;
}
BinaryTree.prototype.addChild = function(data) {
  const child = new BinaryTree(data);
  if (!this.left) {
    this.left = child;
  } else if (!this.right) {
    this.right = child;
  } else {
    // we could handle adding grandchildren from the root tree here.
  }
}
BinaryTree.prototype.contains = function(target, containsTarget = false) {
  if (this.data === target) {
    return true;  
  }
  
  if (this.left && this.left.contains(target, containsTarget)) {
    containsTarget = true;
  } else if (this.right && this.right.contains(target, containsTarget)) {
    containsTarget = true;
  }
  
  return containsTarget;
};