why is my variable elecfirebase always equal to [ ]?

I have i do a request, after this my variable (elecfirebase) is always [ ]

function a(){
let elecfirebase=[]
    firebase
        .database()
        .ref("/fournisseurs")
        .child(0)
        .child("data")
        .on("value", data => {
          elecfirebase=data.val();    
      });
console.log("elecfirebase ",elecfirebase)
}