Hello @kartik,
I would use the path npm module and then do something like this:
var path = require('path');
app.use('/scripts', express.static(path.join(__dirname, 'node_modules/bootstrap/dist')));
NOTE: we use path.join to make paths joining using system agnostic way, i.e. on windows and unix we have different path separators (/ and )
Hope this is helpful!!
Thank you!!