An Illustrative Tale of JavaScript Behaviours

method_loading.js

function setLoader(store) {
  // Now `store` is imported into the closure scope
  return function set(val) {
    /**
     * To update a value, reassign a property, not a variable
     * For example, `store = { value: 'new value' };` would NOT
     * work. It just reassigns the scope variable `store` to 
     * point to something else, rather than going to the memory
     * spot of what `store` points to and alter the value there
     */ 
    store.value = val;
  };
}

function Thing() {
  var _store = { value: '' };
  this.set = setLoader(_store);
  this.get = function() { 
    return _store.value; 
  };
}

var a = new Thing();
var b = new Thing();

a.set(10); 
console.log(a.get()); // 10

// Each instance gets its own separate `store`
b.set(20);
console.log(a.get()); // 10
console.log(b.get()); // 20

/** 
 * Use an expression, not declaration, lest it be hoisted!
 * If it were a declaration and hoisted, the first definition
 * above would get overridden, before `a` and `b` even get a
 * chance to use the old behaviour
 */
var setLoader = function(store) {
  return function set(val) {
    store.value = 'New Behaviour! ' + val;
  };
}

console.log(a.get());

var c = new Thing();
c.set(42);

console.log(c.get()); // New Behaviour! 42