bm25.js

BM25 = {};

// see attached file
BM25.stopWords = [];

// TODO: https://tartarus.org/martin/PorterStemmer/js.txt
BM25.stemmer = function(term) {
    return term;
}

BM25.tokenize = function(text) {
    text = text
        .toLowerCase()
        .replace(/\W/g, ' ')
        .replace(/\s+/g, ' ')
        .trim()
        .split(' ')
        .map(function(a) { return BM25.stemmer(a); });

    // Filter out stopWords
    let out = [];
    for (let i = 0, len = text.length; i < len; i++) {
        if (BM25.stopWords.indexOf(text[i]) === -1) {
            out.push(text[i]);
        }
    }

    return out;
};

BM25.addDocument = function(doc) {
    if (typeof doc.id === 'undefined') { throw new Error(1000, 'ID is a required property of documents.'); };
    if (typeof doc.body === 'undefined') { throw new Error(1001, 'Body is a required property of documents.'); };

    // Raw tokenized list of words
    let tokens = BM25.tokenize(doc.body);

    // Will hold unique terms and their counts and frequencies
    let _terms = {};

    // docObj will eventually be added to the documents database
    let docObj = {id: doc.id, tokens: tokens, body: doc.body};

    // Count number of terms
    docObj.termCount = tokens.length;

    // Increment totalDocuments
    this.totalDocuments++;

    // Readjust averageDocumentLength
    this.totalDocumentTermLength += docObj.termCount;
    this.averageDocumentLength = this.totalDocumentTermLength / this.totalDocuments;

    // Calculate term frequency
    // First get terms count
    for (let i = 0, len = tokens.length; i < len; i++) {
        let term = tokens[i];
        if (!_terms[term]) { 
            _terms[term] = {
                count: 0,
                freq: 0
            }; 
        };
        _terms[term].count++;
    }

    // Then re-loop to calculate term frequency.
    // We'll also update inverse document frequencies here.
    let keys = Object.keys(_terms);
    for (let i = 0, len = keys.length; i < len; i++) {
        let term = keys[i];
        // Term Frequency for this document.
        _terms[term].freq = _terms[term].count / docObj.termCount;

        // Inverse Document Frequency initialization
        if (!this.terms) {
            this.terms = {};
        }
        if (!this.terms[term]) {
            this.terms[term] = {
                n: 0, // Number of docs this term appears in, uniquely
                idf: 0
            };
        }

        this.terms[term].n++;
    };

    // Calculate inverse document frequencies
    // This is SLOWish so if you want to index a big batch of documents,
    // comment this out and run it once at the end of your addDocuments run
    // If you're only indexing a document or two at a time you can leave this in.
    // this.updateIdf();

    // Add docObj to docs db
    docObj.terms = _terms;
    if (!this.documents) this.documents = {};
    this.documents[docObj.id] = docObj;
};

BM25.updateIdf = function() {
    let keys = Object.keys(this.terms);
    for (let i = 0, len = keys.length; i < len; i++) {
        let term = keys[i];
        let num = (this.totalDocuments - this.terms[term].n + 0.5);
        let denom = (this.terms[term].n + 0.5);
        this.terms[term].idf = Math.max(Math.log10(num / denom), 0.01);
    }
};

BM25.search = function(query) {

    let queryTerms = BM25.tokenize(query);
    let results = [];

    // Look at each document in turn. There are better ways to do this with inverted indices.
    let keys = Object.keys(this.documents);
    for (let j = 0, nDocs = keys.length; j < nDocs; j++) {
        let id = keys[j];
        // The relevance score for a document is the sum of a tf-idf-like
        // calculation for each query term.
        this.documents[id]._score = 0;

        // Calculate the score for each query term
        for (let i = 0, len = queryTerms.length; i < len; i++) {
            let queryTerm = queryTerms[i];

            // We've never seen this term before so IDF will be 0.
            // Means we can skip the whole term, it adds nothing to the score
            // and isn't in any document.
            if (typeof this.terms[queryTerm] === 'undefined') {
                continue;
            }

            // This term isn't in the document, so the TF portion is 0 and this
            // term contributes nothing to the search score.
            if (typeof this.documents[id].terms[queryTerm] === 'undefined') {
                continue;
            }

            // The term is in the document, let's go.
            // The whole term is :
            // IDF * (TF * (k1 + 1)) / (TF + k1 * (1 - b + b * docLength / avgDocLength))

            // IDF is pre-calculated for the whole docset.
            let idf = this.terms[queryTerm].idf;
            // Numerator of the TF portion.
            let num = this.documents[id].terms[queryTerm].count * (this.k1 + 1);
            // Denomenator of the TF portion.
            let denom = this.documents[id].terms[queryTerm].count + (this.k1 * (1 - this.b + (this.b * this.documents[id].termCount / this.averageDocumentLength)));

            // Add this query term to the score
            this.documents[id]._score += idf * num / denom;
        }

        if (!isNaN(this.documents[id]._score) && this.documents[id]._score > 0) {
            results.push(this.documents[id]);
        }
    }

    results.sort(function(a, b) { return b._score - a._score; });
    return results.slice(0, 10);
};

// TODO: add example

BM25.stopwords.js

BM25.stopwods = ['a', 'about', 'above', 'across', 'after', 'afterwards', 'again', 'against', 'all', 'almost', 'alone', 'along', 'already', 'also', 'although', 'always', 'am', 'among', 'amongst', 'amoungst', 'amount', 'an', 'and', 'another', 'any', 'anyhow', 'anyone', 'anything', 'anyway', 'anywhere', 'are', 'around', 'as', 'at', 'back', 'be', 'became', 'because', 'become', 'becomes', 'becoming', 'been', 'before', 'beforehand', 'behind', 'being', 'below', 'beside', 'besides', 'between', 'beyond', 'bill', 'both', 'bottom', 'but', 'by', 'call', 'can', 'cannot', 'cant', 'co', 'computer', 'con', 'could', 'couldnt', 'cry', 'de', 'describe', 'detail', 'do', 'done', 'down', 'due', 'during', 'each', 'eg', 'eight', 'either', 'eleven', 'else', 'elsewhere', 'empty', 'enough', 'etc', 'even', 'ever', 'every', 'everyone', 'everything', 'everywhere', 'except', 'few', 'fifteen', 'fify', 'fill', 'find', 'fire', 'first', 'five', 'for', 'former', 'formerly', 'forty', 'found', 'four', 'from', 'front', 'full', 'further', 'get', 'give', 'go', 'had', 'has', 'hasnt', 'have', 'he', 'hence', 'her', 'here', 'hereafter', 'hereby', 'herein', 'hereupon', 'hers', 'herse', '', 'him', 'himse', '', 'his', 'how', 'however', 'hundred', 'i', 'ie', 'if', 'in', 'inc', 'indeed', 'interest', 'into', 'is', 'it', 'its', 'itse', '', 'keep', 'last', 'latter', 'latterly', 'least', 'less', 'ltd', 'made', 'many', 'may', 'me', 'meanwhile', 'might', 'mill', 'mine', 'more', 'moreover', 'most', 'mostly', 'move', 'much', 'must', 'my', 'myse', '', 'name', 'namely', 'neither', 'never', 'nevertheless', 'next', 'nine', 'no', 'nobody', 'none', 'noone', 'nor', 'not', 'nothing', 'now', 'nowhere', 'of', 'off', 'often', 'on', 'once', 'one', 'only', 'onto', 'or', 'other', 'others', 'otherwise', 'our', 'ours', 'ourselves', 'out', 'over', 'own', 'part', 'per', 'perhaps', 'please', 'put', 'rather', 're', 'same', 'see', 'seem', 'seemed', 'seeming', 'seems', 'serious', 'several', 'she', 'should', 'show', 'side', 'since', 'sincere', 'six', 'sixty', 'so', 'some', 'somehow', 'someone', 'something', 'sometime', 'sometimes', 'somewhere', 'still', 'such', 'system', 'take', 'ten', 'than', 'that', 'the', 'their', 'them', 'themselves', 'then', 'thence', 'there', 'thereafter', 'thereby', 'therefore', 'therein', 'thereupon', 'these', 'they', 'thick', 'thin', 'third', 'this', 'those', 'though', 'three', 'through', 'throughout', 'thru', 'thus', 'to', 'together', 'too', 'top', 'toward', 'towards', 'twelve', 'twenty', 'two', 'un', 'under', 'until', 'up', 'upon', 'us', 'very', 'via', 'was', 'we', 'well', 'were', 'what', 'whatever', 'when', 'whence', 'whenever', 'where', 'whereafter', 'whereas', 'whereby', 'wherein', 'whereupon', 'wherever', 'whether', 'which', 'while', 'whither', 'who', 'whoever', 'whole', 'whom', 'whose', 'why', 'will', 'with', 'within', 'without', 'would', 'yet', 'you', 'your', 'yours', 'yourself', 'yourselves'];

Hi,
⚠️ at line 134

var num = this.documents[id].terms[queryTerm].count * (this.k1 + 1);

If I'm correct it's not the .count but the .freq to get.