Q:

how to highlight a chosen line on a leaflet map?

I want to draw a map with few routes drawn on it.
I want to have a dropbox with numbers 1,..,n
when an item in the dropbox is chosen, the corresponding route is highlighted on the map.

I have started using "leaflet".
how do I highlight a line? I have used "weight" but it's more a border to a line. I would like to see the line is getting bolder.

here is my code:

document.onload = loadMap();

function loadMap() {
  var map = L.map('map').setView([37.8, -96], 4);


  L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
    attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors,<a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
    maxZoom: 18,
    id: 'mapbox.streets',
    accessToken: 'pk.eyJ1IjoiZW======V6ZTdlb2V5cyJ9.3HqHQ4BMRvSPaYe8ToA7YQ'
  }).addTo(map);


  var marker = L.marker([51.5, -0.09]).addTo(map);


  var myLines = [{
    "type": "LineString",
    "properties": {
      "id": "1"
    }
    "coordinates": [
      [-100, 40],
      [-105, 45],
      [-110, 55]
    ]
  }, {
    "type": "LineString",
    "properties": {
      "id": "2"
    }
    "coordinates": [
      [-105, 40],
      [-110, 45],
      [-115, 55]
    ]
  }];

  var myLayer = L.geoJson().addTo(map);
  myLayer.addData(myLines);


  geojson = L.geoJson(myLines, {
    onEachFeature: onEachFeature
  }).addTo(map);

}



function highlightFeature(e) {
  var layer = e.target;

  layer

  layer.setStyle({
    weight: 25,
    color: '#ff3300',
    dashArray: '',
    fillOpacity: 0.7
  });

  if (!L.Browser.ie && !L.Browser.opera) {
    layer.bringToFront();
  }
}

function resetHighlight(e) {
  geojson.resetStyle(e.target);


  layer.setStyle({
    weight: 5,
    color: '#0000ff',
    dashArray: '',
    fillOpacity: 0.7
  });
}


function onEachFeature(feature, layer) {
  layer.on({
    mouseover: highlightFeature,
    mouseout: resetHighlight,
    // click: zoomToFeature
  });
}

$('select[name="dropdown"]').change(function() {

  var item = $(this).val();
  alert("call the do something function on option " + item);
  //how to make the chosen line highlighted ??

});

A:

weight property is not changing line border, it changes stroke width in pixels. You get border effect because you are adding lines twice. Here:
myLayer.addData(myLines);

And here:
geojson = L.geoJson(myLines, {
    onEachFeature: onEachFeature
  }).addTo(map);

When a polyline is hovered, top layer's style is changed, but because you are adding polylines twice, there still remains a polyline from the lower layer. As it is described here, default stroke opacity is 0.5 (setting fillOpacity is redundant for the polyline by the way, for changing stroke-opacity opacity property is used). Polyline from the top layer becomes semi-transparent, and that makes the illusion of the border effect. 
So, you can just remove this line myLayer.addData(myLines); and get the expected result.
I've made a fiddle, where your example is corrected.

