Lazy object properties

lazy.js

/////////////////////////////////////////////////
//// Lazy object properties
/////////////////////////////////////////////////
// Sets a property that computes its value on first access
// (see a test below for usage example)

function lazy(obj, prop, fn){

  if(typeof prop === 'string') set(prop, fn)
  else Object.keys(prop).forEach(function(k){ set(k, prop[k]) })

  function set(name, fn){

    // Set getter
    Object.defineProperty(obj, name, {
      get: function(){
        
        // Calculate value on first access
        var value = fn(name, obj)

        // Set value
        Object.defineProperty(obj, name, {
          value: value,
          enumerable: true,
          configurable: true,
          writable: true
        })

        return value
      },

      enumerable: true,
      configurable: true
    })
  }

  return obj
}

/*
(function test(){
  var obj = lazy({}, {test: function(){ return Math.random() }})
  
  // Test
  var calculated = obj.test
  var predefined = obj.test
  console.log(calculated === predefined, calculated, predefined)
})()
*/