Openlayers & NPM: can't load local .json file - json

Question

I try to load a .json file in Openlayers from a local URI. As recommended, I'm using npm. However, the file doesn't seem to load. The file example.json is stored in the directory data, which is a subdirectory of where the main.js is located. The json file I'm using is valid according to jsonLINT.
var vectorLayer = new VectorLayer({
source: new VectorSource({
url: 'data/example.json',
format: new GeoJSON()
}),
style: function(feature) {
style.getText().setText(feature.get('name'));
return style;
}
});
var map = new Map({
target: 'map',
layers: [ vectorLayer],
// ...
});
Messages in the console:
XHR GEThttp://localhost:1234/data/example.json [HTTP/1.1 200 OK 4ms]
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
XHR GEThttp://localhost:1234/data/example.json [HTTP/1.1 304 Not Modified 14ms]
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
I guess, the file doesn't load at all. If I change the adress to some file that isn't in the directory, I'll get the same messages. On reload, I'd get the 304 error code twice. If I type in 'localhost:1234/data/example.json' as URL, it'll retrieve my main page (as with localhost:1234), not the json file I'd expect to see.
I'm a beginner with npm so I assume the problem is how files are handled with npm.