Understanding Vue's Diff Algorithm: Rules, Implementation, and Optimizations

In front‑end development, especially when working with Vue, developers often need to know exactly which array elements have changed after a mutation and where those changes occur. This article explains the underlying diff algorithm used by Vue to compare old and new virtual nodes (vnode) and to generate minimal DOM updates.

The algorithm first distinguishes between depth‑first traversal of virtual nodes and same‑level comparison. Depth‑first traversal explores child nodes recursively, while same‑level comparison checks sibling nodes under the same parent.

Two essential vnode properties are highlighted:

key : the developer‑defined ":key" attribute;

sel : a combination of the element’s tag name, id, and class.

The article walks through the construction of a vnode, the creation of an index map for the old vnode array, and the three main phases of the diff process:

Head‑tail comparison : Using pointers oldStartIdx , oldEndIdx , newStartIdx , and newEndIdx , the algorithm performs head‑to‑head, tail‑to‑tail, and head‑to‑tail checks to quickly identify unchanged nodes.

Index comparison : When simple head‑tail checks fail (e.g., new nodes are inserted or old nodes are removed), an index map of the old array is built to locate moved nodes efficiently. This step also handles worst‑case O(n) complexity.

Post‑traversal handling : After the main loop, the algorithm deals with remaining nodes—adding any new nodes that were not processed or removing leftover old nodes.

The core ideas are: simple equality checks, building an old‑array index, head‑tail and index‑based vnode movement, and batch processing of remaining additions or deletions.

Vue 3 introduces further optimizations by filtering out static vnodes. If a vnode is identified as static, the diff step is skipped and only the reference is updated. The article also mentions other vnode types such as comment and fragment , and shows the corresponding source snippets.

Overall, the diff algorithm combines forward/backward comparisons with an index map to achieve efficient O(n) updates, minimizing DOM mutations while keeping the UI in sync with data changes.