Assign value to global variable after $.ajax and behave like a const

Question

how you doing? I wondered, there is a way to declare an immutable variable (like the const keyword) in js, but assign the value just one time after maybe an ajax request or a fetch?

My problem is quite simple: I've 3 _id that I need to retrieve from my server when the document is on load and I need to store them inside some global variable. I need these 3 _ids to be unmutable, because all the next ajax requests on the site rely on these, and I cannot let someone change the value inside those variables.

Const keywords are perfect, but I don´t have the scope to use them or I cannot declare the variable before the ajax request.

There's a workaround?

const _id1 = null; // Correct, but this means nothing to me
const _id2; // Error: Missing initializer in const declaration
let _id_3 = null; // Formally correct, I can assign a new value to the variable
                  // but everybody can modify it from the console...

// What I need to do is
// 1) declare the variable
let magic_id;
// 2) do my ajax calls to my server
$.ajax({
    url, data, type, ...,
    success(payload){
        // 3) Assign the value to my id
        let aPay = JSON.parse(payload);
        magic_id = aPay._id;
    }
});

// 4) If someone tries to assign a new value i need to prevent it
magic_id = new_value; // Uncaught TypeError: Assignment to constant variable.