leaflet.js map is not showing up

Re-read the Leaflet quick-start tutorial, in particular this bit:

It’s worth noting that Leaflet is provider-agnostic, meaning that it doesn’t enforce a particular choice of providers for tiles, and it doesn’t even contain a single provider-specific line of code.

Leaflet doesn't add any default map data. It's up to you to tell Leaflet which data (vector features, tile layers) you want to show.

Here is your corrected code: http://plnkr.co/edit/E7dw2AuNbLneYpz51Qdi?p=preview

There is no tile provider in your code, so nothing is showing in your map.

Check out the source of http://leafletjs.com/examples/quick-start-example.html

var mymap = L.map('mapid').setView([51.505, -0.09], 13);

L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibWFwYm94IiwiYSI6ImNpandmbXliNDBjZWd2M2x6bDk3c2ZtOTkifQ._QA7i5Mpkd_m30IGElHziw', {
    maxZoom: 18,
    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>',
    id: 'mapbox.streets'
}).addTo(mymap);

If you don't want tiles from mapbox, you can use openstreet map

L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
}).addTo(mymap);