Combine two retrieval rankings

WHAT YOU WILL BUILD

A deduplicated ranking with a visible, reproducible scoring rule.

A keyword engine and a semantic retriever can return different score scales. Reciprocal rank fusion uses positions instead: each list contributes 1 / (constant + rank), with rank starting at one. It rewards agreement without pretending the original scores are comparable.

FOLLOW ALONG

Work through the example.

1. 01

   Inspect the candidate IDs

   The first ranking favors sku-42; the second favors returns. Notice that returns and sku-42 occur in both lists. A document must keep the same ID across retrieval systems.

2. 02

   Run the fusion

   Run the downloaded file. returns comes first, followed by sku-42. Print scores to inspect the sum contributed by each source ranking.

3. 03

   Change one ranking

   Swap returns and sku-42 in the second list. Predict the new top document before running. Update the expected ranking assertion after checking the arithmetic.

4. 04

   Compare against a labeled question

   Decide which documents are relevant to a specific question, then compare each individual list and the fused list at the same K. A changed order is useful only if it retrieves better evidence.

THE COMPLETE LAB

Run it locally.

Run this command from the folder containing your downloaded file:

node rag-rank-fusion.mjs

// SaveMyToken local lab. Run with Node.js 22 or newer.
import assert from "node:assert/strict";

// Two supplied rankings: no embedding model is being run here.
const lexical = ["sku-42", "returns", "shipping"];
const semantic = ["returns", "refunds", "sku-42"];
const rankConstant = 60;
const scores = new Map();
for (const list of [lexical, semantic]) {
  [...new Set(list)].forEach((id, index) => {
    scores.set(id, (scores.get(id) ?? 0) + 1 / (rankConstant + index + 1));
  });
}
const ranked = [...scores].sort((a, b) => b[1] - a[1] || a[0].localeCompare(b[0]));
const result = ranked.map(([id]) => id);
console.log("Fused ranking:", result.join(", "));
assert.deepEqual(result, ["returns", "sku-42", "refunds", "shipping"]);
assert.equal(new Set(result).size, result.length);

Expected output for the unchanged example

Fused ranking: returns, sku-42, refunds, shipping

The assertions also check the baseline behavior. After changing an input, predict the result and update the relevant assertion.

EXPLAIN WHAT YOU LEARNED

Interview practice

Try answering aloud before opening the reference answer. These are original learning questions, not a record of any employer's interviews.