Mongodb 4.2 update pipeline creator which updates modified field if anything changed

makeUpdatePipeline.ts

import mongoose, { LeanDocument } from "mongoose";
import '@/lib/PromiseAll';

// Used because Object.keys doesn't give us types for field names
export function keys<T extends object>(obj: T) : (keyof T)[] {
    return Object.keys(obj) as (keyof T)[];
}

// Spelled incorrectly on purpose
const defaultIgnoreType = '__UNMODIFEID__';

export function makeUpdatePipeline<T extends mongoose.Document<any>>(updFields: Partial<LeanDocument<T>>, modFieldName = 'modified', ignoreType = defaultIgnoreType) {
    // Maps the input document values; anything that is new remains unchanged
    // and anything which is the same as the document we're matching against
    // gets changed to the value of _ignoreType_.
    // Note that using `k` and `v` for key names on this object is important
    // so that $arrayToObject works in stage 2
    const updateArray = keys(updFields).map(fieldName => ({
        k: fieldName,
        v: { $cond: {
            if: {$eq: [`\$${fieldName}`, updFields[fieldName]]},
            then: ignoreType,
            else: updFields[fieldName]
        } }
    }));
    const pipeline = [
        // Stage 1: Filter the update fields
        // we split the document putting the original document at `_root` and making a new
        // _update which is updateArray but we filter out anything with a value
        // of `ignoreType` (which we calculated when we built updateArray)
        // from what is on the object; the $cond in the `updateArray` does that
        { $project: {
            _id: 0,
            _update: {$filter: {
                input: updateArray,
                as: "curItem",
                cond: {$ne: <any>["$$curItem.v", ignoreType]}
            }},
            _root: "$$ROOT",
        }},

        // Stage 2: Update the document
        // We merge three objects to create the new output document:
        { $replaceRoot: {
            newRoot: { $mergeObjects: [
                "$_root",                      // Object 1: the original document
                {$arrayToObject: "$_update"},  // Object 2: our filtered updateArray converted to an object
                {$cond: {                      // Object 3: the modified field *if* updateArray isn't empty
                    if: {$gt: [ {$reduce: { input: "$_update", initialValue: 0, in: {$sum: 1}}}, 0]},
                    then: {[modFieldName]: "$$NOW"},
                    else: {}
                }},
            ]},
        } },
    ];
    return pipeline;
}