从对象列表中获取属性

Question

By using underscore or jQuery I need to take the maximum id value incremented by one contained in a list of objects.

The javascript object is a Backbone.Collection and it looks like this:

this.collection.models = [{attributes: {id: 1, ....}}, {}];

I wrote the following code which works, but I would like to know if there are any change to improve it.

Thanks.

getId: function () {
  return _.max(
          _.map(
           _.pluck(this.collection.models, 'attributes'), function (attributes) {
              return attributes.id;
    })) + 1;
},

Answer 1

_.max accepts an callback iterator function:

var next = _.max(list, function(i) { return i.attributes.id; }).attributes.id + 1;

I knew it was true for the lodash library, didn't know it was true for underscore.

Cheers!

Answer 2

One way is to soert your collection by id, using the comparator method.

collection.comparator = function(model) {
    return model.id;
}

When you set that, your last model is guaranteed to have the largets id.

collection.next = function(){
    return this.last().id + 1;
}

It would probably better to define these when defining the collection:

var Collection = Backbone.Collection.extend({
  comparator: function(model) {
    return model.id;
  },
  next: function(){
    return this.last().id + 1;
  }
});

Answer 3

Live demo here

How about this:

var result = _.chain(this.collection.models)
    .pluck('attributes')
    .max(function(value) {
        return value.id;
    })
    .value();

Answer 4

There's nothing wrong with simple loops, something like this inside the collection is perfectly acceptable:

var max = 0;
for(var i = 0; i < this.models.length; ++i)
    max = this.models[i].id > max ? this.models[i].id : max;
return max + 1;

Or like this if you're compelled to use Underscore:

var max = 0;
this.each(function(m) {
    max = m.id > max ? m.id : max;
});
return max + 1;

Both of those would go inside the collection, touching a collection's models array outside the collection is bad manners.

Just because you have all the jQuery and Underscore machinery kicking around doesn't mean that you have to use it everywhere.

Answer 5

var max = _.max(this.collection.pluck('id')) + 1;

And don't use this:

this.collection.models = [{attributes: {id: 1, ....}}, {}];

Here is proper way:

this.collection.reset([{id: 1}, {id: 2}])