Union of multiple Features/polygons yet in version 6

[jfoclpf]

turf-union in all its docs mentions the union of only two (multi)polygons or geojson's

https://www.npmjs.com/package/@turf/union

It would be nice and useful if we could unite several (multi)polygons or geojson's

let union
union = turf.union(poly1, poly2)
union = turf.union(poly1, poly2, poly3)
union = turf.union(poly1, poly2, poly3, ..., polyn)

const polygons = [poly1, poly2, poly3, ..., polyn]
union = turf.union(...polygons)

[jfoclpf]

btw, I know I can do that recursively myself, but it would be useful to have that functionality out of the box under the hood

[jfoclpf]

apparently it is already possible, although docs in NPM are not that clear

[jfoclpf]

doesn't seem to work

[twelch]

Please provide a simple reproduceable example of your issue

[jfoclpf]

Basically it does not merge 3 or more polygons

var poly1 = turf.polygon([[
    [-82.574787, 35.594087],
    [-82.574787, 35.615581],
    [-82.545261, 35.615581],
    [-82.545261, 35.594087],
    [-82.574787, 35.594087]
]], {"fill": "#0f0"});
var poly2 = turf.polygon([[
    [-82.560024, 35.585153],
    [-82.560024, 35.602602],
    [-82.52964, 35.602602],
    [-82.52964, 35.585153],
    [-82.560024, 35.585153]
]], {"fill": "#00f"});
var poly3 = turf.polygon([[
    [-82.560024, 35.585153],
    [-83.56004, 35.602202],
    [-83.52964, 35.602602],
    [-82.52964, 35.585153],
    [-81.560024, 35.585151]
]], {"fill": "#f00"});

var union = turf.union(poly1, poly2, poly3)

[jfoclpf]

I realized that in version 7 you'll allow union of multiple polygons via featureCollection

var poly1 = turf.polygon([[
    [-82.574787, 35.594087],
    [-82.574787, 35.615581],
    [-82.545261, 35.615581],
    [-82.545261, 35.594087],
    [-82.574787, 35.594087]
]], {"fill": "#0f0"});
var poly2 = turf.polygon([[
    [-82.560024, 35.585153],
    [-82.560024, 35.602602],
    [-82.52964, 35.602602],
    [-82.52964, 35.585153],
    [-82.560024, 35.585153]
]], {"fill": "#00f"});
var poly3 = turf.polygon([[
    [-82.560024, 35.585153],
    [-83.56004, 35.602202],
    [-83.52964, 35.602602],
    [-82.52964, 35.585153],
    [-81.560024, 35.585151]
]], {"fill": "#f00"});

var union = turf.union(turf.featureCollection([poly1, poly2, poly3]));

//addToMap
var addToMap = [poly1, poly2, poly3, union];

I understand it's a breaking change, thus new major version, but it would be much easier, without breaking change, to allow yet in version 6

let union
union = turf.union(poly1, poly2)
union = turf.union(poly1, poly2, poly3)
union = turf.union(poly1, poly2, poly3, ..., polyn)

const polygons = [poly1, poly2, poly3, ..., polyn]
union = turf.union(...polygons)

[jfoclpf]

A workaround yet in version 6 is to use reduce

const polygons = [poly1, poly2, poly3, ..., polyn]
const union = polygons.reduce((a, b) => turf.union(a, b), polygons[0])

But is way too slow

[JamesLMilner]

I do not believe there will be any attempts to backport this to version 6, as it is an API breaking change.

[jfoclpf]

It is not a breaking change in the way I showed

[jfoclpf]

@JamesLMilner maybe I was not clear enough

In version 6 union accepts 3 parameters, the first 2 parameters being each a (Multi)Polygon Feature and the third parameter being an options JS Object.

It would be perfectly feasible to detect in a multitude of input parameters whether a certain parameter is the options Object or a (Multi)Polygon Feature.

Current function API would perfectly still work

turf.union(poly1, poly2, options)

But furthermore this would also work

turf.union(poly1, poly2, poly3, options)
turf.union(poly1, poly2, poly3, ..., polyN, options)

Thus, no breaking change, just a minor version change according to Semver.

I would do a PR but I am not versed in Typescript.