Any way to define getters for lazy variables in Javascript arrays?

Question

I'm trying to add elements to an array that are lazy-evaluated. This means that the value for them will not be calculated or known until they are accessed. This is like a previous question I asked but for objects.

What I ended up doing for objects was

Object.prototype.lazy = function(var_name, value_function) {
  this.__defineGetter__(var_name, function() {
    var saved_value = value_function();
    this.__defineGetter__(var_name, function() {
      return saved_value;
    });
    return saved_value;
  });
}

lazy('exampleField', function() {
  // the code that returns the value I want
});

But I haven't figured out a way to do it for real Arrays. Arrays don't have setters like that. You could push a function to an array, but you'd have to call it as a function for it to return the object you really want. What I'm doing right now is I created an object that I treat as an array.

Object.prototype.lazy_push = function(value_function) {
  if(!this.length)
    this.length = 0;
  this.lazy(this.length++, value_function);
}

So what I want to know is, is there a way to do this while still doing it on an array and not a fake array?

UPDATE: The following function works only if the value_function returns a primitive data type.

Array.prototype.lazy_push = function(value_function) {
  var a = this,
  i = this.length;
  this.push({
    toString: function() {
      return a[i] = value_function();
    }
  });
}

If you try to push say an object that has properties in it, you won't be able to access the properties until you access the object directly. This doesn't happen with setters, that's why I want some kind of setting syntax for Javascript. For now I will be using the fake array, which is enough for what I'm doing.

Answer 1

In 2019, you could use a Proxy.

Here is an example which assumes any value which is a function should be evaluated when accessed.

function LazyArray() {
  return new Proxy([], {
    get: (obj, prop) => {
      if (typeof obj[prop] === 'function') {
        // replace the function with the result
        obj[prop] = obj[prop]()
      }
      return obj[prop]
    },
  })
}

const a = LazyArray()

a[0] = () => {
  console.log('calculating...')
  return 42
}

console.log(a[0]) // lazy evaluated
console.log(a[0])

console.log(a.length) // accessing other properties

Answer 2

There is not. Unfortunately this is a big deal.