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Find nearest common ancestor node

Q: Find nearest common ancestor node

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Hard > **How to read this doc:** all 4 real scenarios below — two leaves in different branches, a leaf and its own ancestor, a node and itself, and a leaf against another node's own ancestor — were verified directly against a real jsdom tree, confirming the exact correct real element every time. ## 1. The problem, restated Given two DOM nodes, find the single, deepest node that is an ancestor of (or IS) both of them — the point where their two separate paths up to the document root genuinely converge. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | One node is an ancestor of the other? | The answer should genuinely be that ancestor itself — an ancestor is trivially a common ancestor of itself and any of its own descendants. | | Same node passed twice? | The answer should genuinely be that node itself. | | Nodes guaranteed to be in

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The problem

Problem, as an interviewer would state it: "Given two DOM nodes, find their NEAREST COMMON ANCESTOR — the deepest single node that is an ancestor of (or equal to) BOTH given nodes."

Examples:

Code

findNearestCommonAncestor(nodeC, nodeE); // returns their shared ancestor, e.g. #root

Clarifying questions expected:

If one node is itself an ancestor of the other, is the answer that ancestor node itself?
If the same node is passed for both arguments, is the answer that node itself?
Can this assume the two nodes are genuinely in the SAME document tree, or must it handle the case where they are not?

Code / implementation expected: Yes — real, direct proof against a real jsdom tree, covering both a straightforward two-leaves case and the ancestor/self edge cases.

js-coding
utility
algorithm

Try it yourself

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Hard

How to read this doc: all 4 real scenarios below — two leaves in different branches, a leaf and its own ancestor, a node and itself, and a leaf against another node's own ancestor — were verified directly against a real jsdom tree, confirming the exact correct real element every time.

1. The problem, restated

Given two DOM nodes, find the single, deepest node that is an ancestor of (or IS) both of them — the point where their two separate paths up to the document root genuinely converge.

2. Clarifying questions a strong candidate asks

Question	Why it matters
One node is an ancestor of the other?	The answer should genuinely be that ancestor itself — an ancestor is trivially a common ancestor of itself and any of its own descendants.
Same

The solution is hidden so you can try first

6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: the nearest-common-ancestor finder produces the correct real DOM element across 4 scenarios in a real jsdom tree

Practice

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