Import libraries from command line
npm install underscore --save npm install @types/underscore --save
Add the following array to the tsconfig.app.json
"types": [ "underscore" ]
In order to use underscore, import the following all modules
import * as _ from 'underscore';
And finally, use underscore in your class
var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);