JSFiddle - React, Tailwind, and code Playground

JavaScript

function checkRegion(departement){
    var region = '';

    if(~alsace.indexOf(departement)) {
        region = "alsace";
    }
    else if(~aquitaine.indexOf(departement)) {
        region = "aquitaine";
    }
    else if(~auvergne.indexOf(departement)) {
        region = "auvergne";
    }
    else if(~basseNormandie.indexOf(departement)) {
       region = "basse-normandie";
    }
    else if(~iledefrance.indexOf(departement)) {
        region = "ile-de-france";
		}
    else{
        region = 'undennnnnfined';
    }
    return region;
};

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