Map/Reduce multi level JSON object
Turning a comment into a review (I provide an alternative implementation, but the basic ideas can be applied to the OP implementation).
First, some general thoughts.
As I mention in the comments regarding the parseInt function, a linter can help spot any problems. A popular one is ESLint. But perhaps more to the point, the data seems to already be a number, so there shouldn't be any need to parse it.
It can help to clean up the code quite a bit by extracting the filters, reducers, mappers, sorters, etc. outside as a variable and give them a descriptive name, for example:
// filter
const arrivedOrRescued = ({rescueStatus}) =&gt; rescueStatus &gt;= 3

// sorter
const byArrivalOrRescueTime = (a, b) =&gt;
  Date.parse(a.ambulanceArrivalTime || a.rescueTime)
    - Date.parse(b.ambulanceArrivalTime || b.rescueTime)

// reducer
const byRescuers = (acc, {rescuerGroups}) =&gt; {
  rescuerGroups.forEach(({
    rescues
    , rescuer1Abbreviation: r1
    , rescuer2Abbreviation: r2
  }) =&gt; {
    let group = `${r1}-${r2}`
    acc.has(group) ? acc.get(group).push(...rescues) : acc.set(group, rescues)
  })

  return acc
}

so getting a Map (just an example) by rescuers out of the swimlanes could then become
let rescuers = cases.filter(arrivedOrRescued).reduce(byRescuer, new Map())

and sorting their rescues could become
rescuers.forEach((rescues, abbrevs) =&gt; rescues.sort(byArrivalOrRescueTime))

after which the latest rescue is the last element of the array now that they have all been collected into a single array and sorted.
One thing I would like to promote here when there can be somewhat complex nested objects in the data, is to use JSDoc comments to document the data shape.
For example:
/*
 * @typedef {Object} Swimlane
 * @property {Array.&lt;RescuerGroup&gt;} rescuerGroups
 * @property {Number} rescueStatus
 * @property {String} rescueStatusName
 *
 * @typedef {Object} RescuerGroup
 * @property {Array.&lt;Rescue&gt;} rescues
 * @property {?Number} rescuer1
 * @property {?Number} rescuer2
 * @property {?String} rescuer1Abbreviation
 * @property {?String} rescuer2Abbreviation
 *
 * @typedef {Object} Rescue
 * @property {?String} ambulanceArrivalTime
 * @property {?String} callDateTime
 * @property {?String} callerName
 * @property {String} callerNumber
 * @property {?Number} callOutcomeId
 * @property {Number} emergencyCaseId
 * @property {Number} emergencyCodeId
 * @property {Number} emergencyNumber
 * @property {Number} isLargeAnimal
 * @property {Number} latitude
 * @property {Number} longitude
 * @property {{lat: Number, lng: Number}} latLngLiteral
 * @property {String} location
 * @property {Array.&lt;String&gt;} patients
 * @property {Number} rescuer1
 * @property {Number} rescuer2
 * @property {?String} rescueTime
 * @property {Number} RescueStatus
 */

This makes it easier for others especially in data transformations.
Just a note that I haven't tested any of this, so take the above code with a grain of salt. Also, many will probably tell me to use more braces and semicolons.