As long as your data is exposed via a REST API then it doesn’t make a difference how the front-end fetches it. In fact that’s the point of separating the back-end concerns from the front-end. You’ll need to look into how to setup your MongoDB as a REST service by following something like this or this.

Once the data are exposed via REST you can use Azle’s call_api function to fetch the data:

az.call_api({
    "url" : "url_to_your_mongo_web_service",
    "parameters" : {},
    "done" : function(data) {
        // do something with data
        console.log(data)
    }
})

It’s just as easy to use the standard fetch API:

fetch(url).then(response => response.text()).then(data => {
    my_data = JSON.parse(data)
});

You’ll have to do some research and attempt building.