BTreeNode with get immediate brother function

BTreeNode.js

export class BTreeNode {
  ...
  /**
   * Get the immediate with more values. If there no one with extra 
   * values, return one of the immediate brothers
   * @returns {BTreeNode}
   */
  getImmediateBrother() {
    // Get position of node in parent children list
    const index = this.parent.children.indexOf(this);
    if (index > 0 && this.parent.children[index-1].n > this.tree.order - 1) {
      // Previous child exists and has more than t-1 values
      return this.parent.children[index-1];
    }
    if (index < this.parent.n && this.parent.children[index+1].n > this.tree.order - 1) {
      // Next child exists and has more than t-1 values
      return this.parent.children[index+1];
    }
    // There is no brother with extra values, return any of them
    return index > 0 ? this.parent.children[index-1] : this.parent.children[index+1];
  }
}