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Last active December 14, 2015 00:58
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JS lazy evaluation and storing patterns
var SomeObject = {
views: {
firstPage: _.memoize(function(){
return new FirstPageView();
}) // Here all we have done is declare a function
// This function was never executed
},
renderFirstPage: function() {
var v = this.views.firstPage(); // Either the function is executed
// Or it had already been called before
// and then the result (our view) is just
// returned from memory.
v.render();
}
}
// Other way to do it with underscore:
function ViewStore() {
this.views = {};
}
ViewStore.prototype.add = function(name, view) {
this.views[name] = view;
}
ViewStore.prototype.get = function(name) {
return _.result(this.views, name);
// _.result returns the value if it's a value
// or the return value if it's a function
}
var vs = new ViewStore;
vs.add('FirstPage', new FirstPageView()); // This is NOT lazy evaluation
// Here we have already created the view.
vs.add('SecondPage', function() { return new SecondPageView();});
vs.add('ThirdPage', _.memoize(function() { return new ThirdPageView();}));
vs.get('FirstPage'); // => returns an object of type FirstPage
vs.get('FirstPage'); // => returns the same object object of type FirstPage
vs.get('SecondPage'); // => returns an object of type SecondPage
// This was lazily evaluated as we setup 'SecondPage' as a closure and never
// created the actual object before
vs.get('SecondPage'); // => returns a different object of type SecondPage
vs.get('SecondPage') === vs.get('SecondPage'); // => false
// It creates a new object every time.
vs.get('ThirdPage'); // => Returns an object of type ThirdPage, lazily evaluated too
// But this time:
vs.get('ThirdPage') === vs.get('ThirdPage'); // => true
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