Chunked reduce for PouchDB’s allDocs: instead of storing the entire output of allDocs in memory and then calling reduce (fold), repeatedly run the reduction on small chunks of allDocs. This should reduce memory requirements at a (slight) CPU cost.

allDocs-reduce.ts

/* For this example, npm install pouchdb pouchdb-adapter-node-websql sqlite3 */

import PouchDB = require('pouchdb');
PouchDB.plugin(require('pouchdb-adapter-node-websql'));

export type Db = PouchDB.Database<{}>;
export let db: Db = new PouchDB('mypouchdb.db', { adapter: 'websql' });

async function allDocsReduce(db: Db, opts, func, init, limit: number = 25, verbose: boolean = false) {
    opts = Object.assign({}, opts); // copy
    opts.limit = limit;
    while (1) {
        let res = await db.allDocs(opts);
        if (res.rows.length === 0) {
            return init;
        }
        init = res.rows.reduce(func, init);
        opts.startkey = res.rows[res.rows.length - 1].key;
        opts.skip = 1;
        if (verbose) {
            console.log(init);
        }
    }
}


async function init() {
    await db.bulkDocs(Array.from(Array(20), (_, n) => ({ _id: '' + n, n: Math.random() })));
    console.log((await db.allDocs({ include_docs: true })).rows.map(o => o.doc))
}
// init()

async function test() {
    // console.log((await db.allDocs({include_docs:true})).rows.map(o=>o.doc));
    console.log('max', await allDocsReduce(db, { include_docs: true }, (prev, next) => Math.max(next.doc.n, prev), 0))
    console.log('max', await allDocsReduce(db, { include_docs: true }, (prev, next) => Math.max(next.doc.n, prev), 0, 1))
    console.log('max', await allDocsReduce(db, { include_docs: true }, (prev, next) => Math.max(next.doc.n, prev), 0, 10))

    console.log('min', await allDocsReduce(db, { include_docs: true }, (prev, next) => Math.min(next.doc.n, prev), Infinity, 2))
    console.log('min', await allDocsReduce(db, { include_docs: true, descending: true }, (prev, next) => Math.min(next.doc.n, prev), Infinity, 2, true))
}
test()

/* Outputs:

max 0.9801551321128437
max 0.9801551321128437
max 0.9801551321128437
min 0.023215878756427166
0.09281023315135406
0.07004790542727712
0.07004790542727712
0.07004790542727712
0.07004790542727712
0.07004790542727712
0.07004790542727712
0.07004790542727712
0.023215878756427166
0.023215878756427166
min 0.023215878756427166

*/