The JavaScript Array Methods (Merging an Array with Values) example doesn't use array in it's example, it still uses an array like in the examples above it.
The square brackets in the argument to concat should be removed, leaving 3 separate string arguments.
I think example could be improved to something like:
var arr1 = ["Cecilie", "Lone"];
var nephew = "Nick";
var myChildren = arr1.concat("Emil", "Tobias", "Linus", nephew);