Empty string in an array coming back undefined [closed]

I have narrowed my issue down to the following code. I don’t understand what weird javascript thing is happening here. An explanation would be greatly appreciated.

let test = ["Test"];
test.concat([""]);
console.log(test); //> ["Test", ""]
console.log(test[1]); //> "undefined"  <-Confused by this
test[0] += "1"; //> ["Test1", ""]
test[1] += "1"; //> ["Test1", "undefined1"]  <-How I found it.

As pointed out in the comments, I don’t get why the concatenation says it worked, but then I get an “undefined” instead of an empty “string”?