GeoJSON does not appear on my Leaflet map

I'm a beginner in Leaflet and I need help to deal with GeoJSON files.

I succeeded in creating a Leaflet basemap. I can add markers without troubles but I cannot manage to show a GeoJSON layer.

Here is the code I picked up from Leaflet Tutorials website https://leafletjs.com/examples/geojson/ :

var geojsonFeature = {
    "type": "Feature",
    "properties": {
        "name": "Coors Field",
        "amenity": "Baseball Stadium",
        "popupContent": "This is where the Rockies play!"
    },
    "geometry": {
        "type": "Point",

        "coordinates": [-104.99404, 39.75621]
    }
};
L.geoJSON(geojsonFeature).addTo(map);

Unfortunatly, the GeoJSON does not appear in my map.

Can you tell me where I missed something ?

---

Alright, you're fine Francisco, this part was OK. I changed the whole script and it works !

However, would you know how to load a GeoJSON from an external source ?

I tried this :

var geojsonLayer = new L.GeoJSON.AJAX("foo.geojson");       
geojsonLayer.addTo(map);

from Loading external GeoJSON file into Leaflet map?

but this definitely does not work.

Any ideas ?

---

I am not skilled enough to understand your advice.

Here is the whole code I am using to create my map :

I know it's a bit messy but this is actually my very first HTML script... I guess I did not include in a proper way the AJAX library.

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Actually, I just want a solution to replace the lines where I create the GeoJSON by something that tells javascript to load the GeoJSON stored locally in my folder. This :

var geojsonFeature = {
    "type": "Feature",
    "properties": {
        "name": "Coors Field",
        "amenity": "Baseball Stadium",
        "popupContent": "This is where the Rockies play!"
    },
    "geometry": {

        "type": "Point",
        "coordinates": [-104.99404, 39.75621]
    }
};

should be transformed into something like (I don't know if it exists) :

var geojsonFeature = [foo.geosjon]

where "foo.geosjson" is a file like :

{ "type": "FeatureCollection", "name": "abies", "crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } }, "features": [ { "type": "Feature"