JSFiddle - React, Tailwind, and code Playground

JavaScript

class PseudoArray {
  constructor(...els) {
    ['push', 'shift', 'unshift', 'pop', 'toString'].forEach(method => {
      this[method] = Array.prototype[method];
    });
    
    this.length = 0; // пока ничего не вставили

    if (els.length > 0) this.push(...els);
  }
};

var a = new PseudoArray();
alert(a.length);
a.push(1, 2, 3);
alert(a.length);

var b = new PseudoArray(1, 2, 3);
alert(b.length);
b.push(4, 5, 6);
alert(b.length);