How to tell JavaScript that my variable is an array and not a string?

The Array.isArray() method determines whether the passed value is an Array or not.

var someNumbers = [1,2,3];
console.log( Array.isArray( someNumbers ) );

and if it is an array it will return

true

Check this link to know more about Array.isArray

If your response is a string, you can use JSON.parse to turn it into an JS Object/Array.

var response = someAjaxStuff(...);
// response is a string
response = JSON.parse(response);
// now reponse is an Object/Array

If you use AJAX with jQuery, you can also use dataType:

$.ajax({
    url: 'https://example.com/api/somejson',
    dataType: 'json',
    method: 'get',
    success: function(res) {
        // console.log res will be an object/array here.
    }

});

But if you're writing your own API, I suggest you have to add the right header to tell browser that it is a JSON format instead of string.

For flask you can use jsonify (reference):

return jsonify(somedict)