I want to add functions and define init but I don't know what is ahead of js.

with the others inheritances, I used for this inheritance :

odoo.define('pos_modification.DB', function (require) { 
"use strict";
var DB = require('point_of_sale.DB');
 DB= DB.extend({

and I tested :

odoo.define('pos_modification.DB', function (require) {
 "use strict"; 
var DB = require('point_of_sale.DB');
 DB.PosDB=DB.PosDB.extend({

if I change the module point of sale by adding the changes (in init etc.) it works

edit :

 init: function (options) { 
 this._super(options);
; this.myvar = [];}
Myfunction: function(options){
...
}...
});
return DB;
});