<default-value> is a a web-component for handling default values with firebase
data property is equal to input property (usually bound to data property of <firebase-document>) when different from null or {}. Otherwise, it returns default-value.
A default-type allows to properly serialize default-value.
Example Usage:
<firebase-document
path="/settings/termsVersion"
data="{{fbTermsVersion}}"></firebase-document>
<!-- termsVersion will be equal to "" when fbTermsVersion is null or {}. Otherwise it is "v0" (primitive, data stored at "settings/termsVersion")-->
<default-value
data="{{termsVersion}}"
input="{{fbTermsVersion}}"
default-value=""></default-value>
<firebase-document
path="/userData/terms/[[user.uid]]/[[termsVersion]]"
data="{{userTerms}}"></firebase-document>
<!-- userTimersSigned is false whenever userTerms id null or {}. Note "object" default-type so that "false" is parsed to false -->
<default-value
data="{{userTermsSigned}}"
input="{{userTerms}}"
default-type="object"
default-value="false"></default-value>