For polling task status I am using this part of the API

// part of a bigger object
map: {
	pending: status.getPendingMapCount(),
	pendingBytes: status.getPendingMapSize(),
	total: status.getTotalMapCount()
},
reduce: {
	pending: status.getPendingReduceCount(),
	pendingBytes: status.getPendingReduceSize(),
	total: status.getTotalReduceCount()
},
summarize: {
	pending: status.getPendingOutputCount(),
	pendingBytes: status.getPendingOutputSize(),
	total: status.getTotalOutputCount()
}