Namespace: masseuse/ComputedProperty

A ComputedProperty is a property that depends on one or more other model values. Each time one of the other values is changed, the ComputeProperty callback is called with the values of all the dependent fields. See the tests for example usage.

Computed Properties are properties on models that are computed from an array of other properties on the same model. Any time one of the dependent properties are chenged, the computed property is recalculated.

Computed Properties are set, optionally, when the computed property is defined, and subsequently anytime one of the dependent properties are chnaged.

For example if there is a PurchasedItem model with properties of price, taxes, discounts then a Computed Property could be created like this:

var item = new PurchasedItem({
                           'price': 10,
                           'taxes': 0.8,
                           'discounts': 3,
                           'total':
                               ComputedProperty(['price', 'taxes', 'discounts'],
                                   function (price, taxes, discounts) {
                                                    return price + taxes - discounts;
                                   })
                       });

console.log(7.8 == item.get('total'));

Computed Properties can be set like other Backbone properties. This is they can be set on initialization or after, and they can be set using a key, value or as part of a larger attributes object.

An optional thired parameter of a Computed Property can be set to truthy to skip the initial Computed Property calculation:

var item = new PurchasedItem({
                           'total': ComputedProperty(['price'], function(price) { return ++price; }, true)
                       });

console.log(undefined === item.get('total'));

item.set('price', 1);
console.log(2 === item.get('total'));

Since Computed Properties depend on other properties, the should not be set directly.

Computed Properties are tested in masseuseModelTests.js.