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I've been breaking my head over a simple generalization of the combination of several Leaflet examples:

  • Make a polyline between markers,
  • But use Leaflet.MarkerCluster as we may have multiple in exactly identical locations,
  • Have multiple custom markers,
  • Number each marker along the path,
  • Have several of such numbered paths,
  • Read all the needed data from a file (dynamic, geoJson).
  • Center the map neatly around all the data.

Now for the life of me I can't get this to work: If I have a single path, it works swimmingly. If I have more than one path, the first is made 100% correctly (locations, icons, titles, popups, and polyline) and all the rest is missing; plus the 'fit bounds' fails so it's at the default zoom and location.

I've simplified my problem to the extent that I'm only using Leaflet and the Leaflet.MarkerCluster plugin --- it's not the different markers and numbers on them where it goes wrong. [The CSS of that plugin is also missing/unnecessary; zoom in and there's no clustering anyway.]

Here's a CodePen, https://codepen.io/Marvino-il-Marziano/pen/YvQbqR ...

I'm wondering if I'm mis-initializing a variable so it can take only one value where it should be an array? Because I don't seem to be overwriting something (as the first not the last data appear). [Possibly significant: If I start the first loop-variable (j= 0,1,2) at 1 instead of 0, I get a blank result --- or is that insignificant, just JS terminating arrays when it finds a null, so here it's the index-0?]

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OK, problem solved: It was indeed a failure to initialize.

Specifically, latlngs[j] wasn't initialized, only its container (by latlngs=[[]];). It seems that upon initializing the container, JS silently initializes the minimum useful amount (namely 1, the zero-indexed latlngs[0]=[]), which makes sense resource-wise and usability-wise. [If it hadn't initiated that first one, I'd have immediately spotted the problem, though!]

So the solution is simply to insert latlngs[j]=[]; at the beginning of the outer loop.

If it helps other beginners: The way I found my core problem was [prompted by https://stackoverflow.com/questions/603987/what-is-the-javascript-equivalent-of-var-dump-or-print-r-in-php ] through using my browser's Inspect/ Inspect Element function, which has a Console; and the Console gave

TypeError: latlngs[j] is undefined [Learn More]

so I didn't even have to do what the linked suggestion was (namely, make a variable to report a state and read it in the Console).

[I actually did other things to try to work it out, probably the most useful (but still superfluous) being to manually perform the outer loop (I mean: splitting the data in three separate variables; then making two copies of the loop, preceding each one consecutively by var j=0;, then j=1, and j=2; and adjusting the name of the data-variable correctly). It failed in the same way as the original problem and for the same reason.]

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