Turf.js merge looks for features in GeoJSON - node.js

Question

I'm building a mapping app with Node.js. We have about 40,000 polygons that display on the map so I'm trying to improve performance by merging them where possible. Turf.js has a merge function that seems like the ticket. I haven't been able to get it to work though.
Here is the code where I'm trying to use turf in my controller.
var mongoose = require('mongoose');
var bodyParser = require('body-parser');
var turf = require('turf');
var fs = require('fs');
exports.show = function(req, res) {
res.render('map', {
title: 'Map'
});
};
exports.postSearch = function(req, res) {
// Bunch of query stuff goes into array below, (omitted for post)
mongoose.model('Claim').find({
$and:array
}, function(err, polygons){
// fs.writeFile('poly.txt', polygons);
var test = turf.merge(polygons);
res.json(test);
});
};
I put that fs.writeFile in there to get a sample of the geojson returned from mongodb. This is what I get:
{
properties: {
TTLTPCD: 'CL',
RNHCTRS: 389,
TNRTPCD: 'C'
},
geometry: {
coordinates: [
[Object]
],
type: 'Polygon'
},
type: 'Feature',
_id: '56d2a2061c601e547e1099ee'
}, {
properties: {
TTLTPCD: 'CS',
RNHCTRS: 261,
TNRTPCD: 'C'
},
geometry: {
coordinates: [
[Object]
],
type: 'Polygon'
},
type: 'Feature',
_id: '56d2a2071c601e547e109d37'
},
// this repeats a couple hundred times on my test query.
I get a stack trace but it doesn't make sense to me:
PROJECT_FOLDER/node_modules/turf/node_modules/turf-merge/index.js:55
var merged = clone(polygons.features[0]),
^
TypeError: Cannot read property '0' of undefined
at Object.module.exports [as merge] (/media/ng/DATA/LighthouseLabs/ClaimMaps/nodeMaps/MapEmGems/node_modules/turf/node_modules/turf-merge/index.js:55:39)
at Query.<anonymous> (/media/ng/DATA/LighthouseLabs/ClaimMaps/nodeMaps/MapEmGems/controllers/map.js:75:14)
at /media/ng/DATA/LighthouseLabs/ClaimMaps/nodeMaps/MapEmGems/node_modules/kareem/index.js:177:19
at /media/ng/DATA/LighthouseLabs/ClaimMaps/nodeMaps/MapEmGems/node_modules/kareem/index.js:109:16
at doNTCallback0 (node.js:430:9)
at process._tickCallback (node.js:359:13)
Turf seems to be looking for a features key in the geojson, but there is none. Does anyone have a solution for this?
###################### Edit after ghybs's answer
OK, ghybs sort of solved the geojson formatting issue. I changed the last bit of the controller to this:
mongoose.model('Claim').find({
$and:array
}, function(err, polygons){
polygons = {
type: "FeatureCollection",
features: [polygons] };
fs.writeFile('poly.txt', polygons);
var test = turf.merge(polygons);
fs.writeFile('test.txt', test);
res.json(test);
});
};
I put a second fs.writeFile to see what turf.merge was returning. The original geojson was not an array so I added the []'s. No more turf error. My map can't understand the output though.
Now poly.txt gives me this:
[object Object]
and test.txt contains this:
{ properties: null,
geometry: undefined,
type: 'Feature',
_id: '56d2a2061c601e547e1099ee',
__v: undefined },{ properties: null,
geometry: undefined,
type: 'Feature',
_id: '56d2a2071c601e547e109d37',
__v: undefined },
// This repeats 336 times
So I'm one step closer, I think. Any ideas? The map worked fine with the original data that was in poly.txt. I'm trying to get the same thing but merged.

Answer 1

The polygons object from your fs.writeFile() debug is not GeoJSON compliant. Turf expects a GeoJSON compliant FeatureCollection, which must have a features member.
See also the doc on Turf merge method, it gives you a code sample of what a GeoJSON compliant FeatureCollection should look like.
So it looks like you should simply wrap your polygons object in a FeatureCollection to make it compliant:
polygons = {
type: "FeatureCollection",
features: polygons // assuming polygons is an array
};
EDIT following the question edit
If your initial polygons really outputs "[Object]" in the geometry coordinates, rather than an array of coordinates, turf will not be able to understand your polygons geometries.
But if you say it was working before trying to merge, the issue would probably be something else.
How do you know your polygons are not an array? Are you sure doing [polygons] is the right solution?
For the next issues, please open a different question.