JSFiddle - React, Tailwind, and code Playground

JavaScript

function checkRegion(departement) {
    var
       regions = [alsace, aquitaine, auvergne, basseNormandie, iledefrance],
       regionsNames = ["alsace", "aquitaine", "auvergne", "basseNormandie", "iledefrance"];

    for (var i = 0; i < regions.length; i++) {
        if (~regions[i].indexOf(departement)) {
            return regionsNames[i];
        }
    }
    return "undennnnnfined"; // If the 'for' loop doesn't return anything, it's false.
};

var
   alsace = ["Strasbourg", "Colmar"],
   aquitaine = ["Gironde", "Landes", "Dordogne", "Pyrenees-Atlantiques", "Lot-et-Garonne"],
   auvergne = [],
   basseNormandie = [],
   iledefrance = [];
   
alert(checkRegion("Strasbourg"));