How Do You Make Sure He Selects One?

I want him to select one instead of all.

if(input == ["1", "2"]) {

}

I expect the output is '1 or 2' but the actual output is '1,2'. He goes only further in the code if it is '1,2' as input.

You can use includes to check if the array contains a certain element

var input = "2";

if ( ["1", "2"].includes(input) ) {
  console.log("Included");
}

Another option is using indexOf. This method returns the first index at which a given element can be found in the array, or -1 if it is not present.

var input = "2";

if ( ["1", "2"].indexOf(input) !== -1 ) {
  console.log("Included");
}