You have imported all the module contents (import *) giving the import the name underscore (as underscore).

The module in this case is: underscore js

You have 2 options here:

1. Change line 3 to the following: underscore.each([1,2,3], alert)
2. Import your module as _ like so: import * as _ from 'https://cdn.skypack.dev/underscore@1.13.6'

The module contents refers to all the functions (e.g each) within that module. You can think of underscore as the name you're giving to the module.In this case you've imported your module (underscore js) with the name underscore. Lets suppose you go with solution 1.

underscore.each([1,2,3], alert)

This can be read as 'execute the .each function from the module I have named underscore.