This NPM module is a simple repackaging of https://github.com/rads/sqlite-okapi-bm25 so that it can work out of the box with a simple npm install sqlite-okapi-bm25 instead of having to run the Makefile explicitly. The function is documented more explicitly in that repository.

This SQLite extension creates a SQL function called okapi_bm25 that returns the Okapi BM25 ranking for results of a full-text search. Okapi BM25 is a modern ranking function that calculates a score for each result based on its relevance to the search query. This extension only works with MATCH queries on FTS4 tables.

First, install the module via npm install sqlite3-okapi-bm25, then you can attach the extension to a sqlite connection.

var sqlite3 = require('sqlite3');

var db = new sqlite3.Database(':memory:')
require('sqlite3-fts4-rank')(db);

// Do stuff with the new rank UDF!
var sql = "SELECT docid, okapi_bm25(matchinfo(documents, 'pcnalx'), 0) AS rank " +
  "FROM documents " +
  "WHERE documents MATCH '\"serious mail\"' " +
  "ORDER BY rank DESC  " +
  " LIMIT 10 OFFSET 0;"
db.each(sql, function(err, row) {
  console.log(row.id + ": " + row.info);
});

• https://github.com/zhm/node-spatialite - Used as an example of building a sqlite3 extension
• http://n8.io/converting-a-c-library-to-gyp - A post by TooTallNate on building gyp extensions
• http://www.sqlite.org/cvstrac/wiki?p=LoadableExtensions
• https://github.com/rads/sqlite-okapi-bm25