added construct event

[rauchg]

Not sure about this one.

[ianstormtaylor]

based on these two:

• https://github.com/component/model/issues/23
• https://github.com/component/model/issues/24#issuecomment-10343199

but feel free to push back, and i can change as needed. for naming i debated init, initialize and construct, but figured construct was the least ambiguous since that is only ever going to go in the constructor.

i'm using it for the same case as #24:

var clone = require('clone')
  , each = require('each')
  , type = require('type');

/**
 * Mixin our plugin.
 */

module.exports = function (Model) {
  Model.on('construct', function (model, attrs) {
    each(Model.attrs, function (key, options) {
      if (options.default !== undefined) def(model, key, options.default);
    });
  });
};

/**
 * Default a `model` with a `value` for a `key` if it doesn't exist. Use a clone
 * of the value, so that they it's easy to declare objects and arrays without
 * worrying about copying by reference.
 *
 * @param {Model}          model  The model.
 * @param {String}         key    The key to back by a default.
 * @param {Mixed|Function} value  The default value to use.
 */

function def (model, key, value) {
  if ('function' === type(value)) value = value();
  if (!model.attrs[key]) model.attrs[key] = clone(value);
}

[ianstormtaylor]

any word on this one? here's the component i'd like it for: https://github.com/segmentio/model-defaults