Inserting Elements Into a Sorted Array
Good things:


There is a very descriptive docblock above the function that documents the arguments.
const is used for things that don't change, whereas let is used for values that can be re-assigned.


Suggestions


When inserting at the end, just use Array.push() instead of Array.splice(). This not only simplifies the syntax but also should be faster (see this jsperf).
For performance reasons it would be wise to use a for...of loop instead of a forEach() to iterate over the items in source - especially for large arrays. Functional programming is nice but it has drawbacks - especially when performance is concerned. Consider that each iteration has a call to cmp so calling an extra function for each element in source could lead to a lot of extra overhead. With such a change the variable insertAtEnd could likely be removed if a label outside the outer for loop was added and that label was used with a continue statement instead of the break.