Open up the 'web console' in Chrome/Firefox with F12 or Ctrl + Shift + J

Be sure to select the tab 'Console'

let's do some typing:

    console.log('hello world')
    var ams = turf.point(52.372410, 4.896554);
    var jhb = turf.point(-26.193613, 28.027536);
    turf.distance(ams, jhb, 'kilometers');
    turf.distance(ams, jhb, 'miles');
    turf.distance(ams, jhb, 'radians');

    // add a polygon around south africa
    var polygon = turf.polygon([ [
        [-22.025510, 33.747421], [-22.025510, 16.872420], 
        [-34.814657, 16.872420], [-34.814657, 33.747421]
      ]
    ]);

    point_fc = turf.featurecollection([ams, jhb]);
    polygon_fc = turf.featurecollection([polygon]);
    turf.within(point_fc, polygon_fc);

    
this is made with turfjs
this is open source turf-example