0.8 property declarations documentation?

[mich...@chromium.org]

I'm confused about properties. Is there precise documentation about how different ways of declaring properties affects the created elements?

If I have this properties block:

properties: {
  string1: String,

  string2: {
    type: String
  },

  string3:{
    type: String,
    value: ''
  },

  string4: {
    type: String,
    reflectToAttribute: true,
  },

  string5: {
    type: String,
    value: '',
    reflectToAttribute: true
  }
}

It seems an instance of my element will only have one attribute: <element string5="">.

Furthermore:

element.hasOwnProperty('string1') === false;
element.hasOwnProperty('string2') === false;
element.hasOwnProperty('string3') === true; element.string3 === '';
element.hasOwnProperty('string4') === true; element.string4 === undefined;
element.hasOwnProperty('string5') === true; element.string5 === '';

The behavior of element.string4 is particularly baffling.

[Scott Miles]

string1 and string2 are the same, string1 uses a compact syntax available if you want only to express type. Neither property is ever assigned any value, so they are not visible as properties (hasOwnProperty is false).

string3 is the same as 1 and 2, except for being initialized to '' at create-time. Because it's assigned a value, it's now visible as an own-property.

string4 will reflectToAttribute if it is assigned a value, but since it never is assigned a value, it never reflects and no attribute is created. Because the property has a side-effect (reflection) it's prepared as accessor pair (getter/setter) on the element _prototype_ so it can be observed. For this reason, it's not an own-property.

string5 is just like string4, except it's assigned the value '' at create-time, and consequently reflects that value to it's attribute.

Also, fwiw, in all cases `type: String` can be omitted, since this is the default.

Much of this behavior is explained by the simple principle that Polymer strives to do as little work as possible, to optimize performance.

HTH,

Scott

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