Polymer custom element attributes as boolean typed values

Question

While Demo A works, I would like to understand if it is possible to make it work like Demo B. Basically, to for the custom element to resolve happyFlag attribute as boolean type.

// Demo A - works. bob smiles. cat frowns.
<x-smiley name="Bob" happyFlag="true"></x-smiley>
<x-smiley name="Cat" happyFlag="false"></x-smiley>
<polymer-element name="x-smiley" attributes="name, happyFlag" noscript>
    <template>
        {{name}} is
        <span hidden?="{{happyFlag == 'true'}}">:-)</span>
        <span hidden?="{{happyFlag == 'false'}}">:-(</span>
        <hr>
    </template>
</polymer-element>

// Demo B - does not work (span always hidden)
<x-smiley name="Bob" happyFlag="true"></x-smiley>
<x-smiley name="Cat" happyFlag="false"></x-smiley>
<polymer-element name="x-smiley" attributes="name, happyFlag" noscript>
    <template>
        {{name}} is
        <span hidden?="{{happyFlag}}">:-)</span>
        <span hidden?="{{happyFlag}}">:-(</span>
        <hr>
    </template>
</polymer-element>

Answer 1

You can give Polymer hints as to what type of input an attribute is. See the following example where I added the Polymer script and specifically the this.happyFlag = false in the created method. This line gives Polymer a hint that this value should be treated as a boolean. See Polymer's Documentation about hinting types.

<polymer-element name="x-smiley" attributes="name, happyFlag">
<template>
    {{name}} is {{happyFlag}}
    <span hidden?="{{happyFlag}}">:-)</span>
    <span hidden?="{{happyFlag}}">:-(</span>
    <hr>
</template>
<script>
  Polymer('x-smiley', {
    created: function() {
        this.happyFlag = false;
    },
    ready: function() {
    }
  })
</script>