index.ts

...
 proof(indexOfLeaf: number) {
    let pathElements: string[] = [];
    let pathIndices: number[] = [];
    const leaf = this.storage.get(MerkleTree.indexToKey(0, indexOfLeaf));
    if (!leaf) throw new Error("leaf not found");
    // store sibling into pathElements and target's indices into pathIndices
    const handleIndex = (level: number, currentIndex: number, siblingIndex: number) => {
      const siblingValue = this.storage.get(MerkleTree.indexToKey(level, siblingIndex)) || this.zeros[level];
      pathElements.push(siblingValue);
      pathIndices.push(currentIndex % 2);
    };
    this.traverse(indexOfLeaf, handleIndex);
    return {
      root: this.root(),
      pathElements,
      pathIndices,
      leaf: leaf,
    };
  }
  
  ...
  
  // traverse from leaf to root with handler for target node and sibling node
  private traverse(indexOfLeaf: number, handler: (level: number, currentIndex: number, siblingIndex: number) => void) {
    let currentIndex = indexOfLeaf;
    for (let i = 0; i < this.levels; i++) {
      let siblingIndex;
      if (currentIndex % 2 === 0) {
        siblingIndex = currentIndex + 1;
      } else {
        siblingIndex = currentIndex - 1;
      }
      handler(i, currentIndex, siblingIndex);
      currentIndex = Math.floor(currentIndex / 2);
    }
  }
}