Drawing polyline in Leaflet?

Is your code pasted directly? If so,

var pointA = new L.LatLng(28.635308, 77.22496);
var pointB = new L.LatLng(28.984461, 77.70641);
var pointList = [pointA, pointB];

var firstpolyline = new L.polyline(pointList {
color: 'red',
weight: 3,
opacity: 0.5
smoothFactor: 1

});

has missing comma's on lines 5 & 8, and line 12 use firstpolyline.addTo(map). Make it

var pointA = new L.LatLng(28.635308, 77.22496);
var pointB = new L.LatLng(28.984461, 77.70641);
var pointList = [pointA, pointB];

var firstpolyline = new L.Polyline(pointList, {
    color: 'red',
    weight: 3,
    opacity: 0.5,
    smoothFactor: 1
});
firstpolyline.addTo(map);

addLayer doesn't work for me, had to do .addTo(map)

I used following code to draw polyline between various Locations:

var polylinePoints = [
    [37.781814, -122.404740],
    [37.781719, -122.404637],
    [37.781489, -122.404949],
    [37.780704, -122.403945],
    [37.780012, -122.404827]
  ];            
  
  var polyline = L.polyline(polylinePoints).addTo(map);

For more Details you can follow this Link.