If you're trying to uses indexes to select properties from objects, you shouldn't be, unless there is a very specific reason to do so.
Fortunately that's fine, you don't need to know the order. I took two of the objects from your JSON array, scrambled up the properties, and wrote a function that returns any object that contains the key/val you specify.
Your question is a little hard to follow, but I think this will give you the idea.
<script type="text/javascript">
let arr = [
{
"attributes":{
"friendly_name":"door"
},
"entity_id":"sensor.frontdoor",
"last_changed":"2016-12-31T11:15:59.395808+00:00",
"last_updated":"2016-12-31T11:15:59.395808+00:00",
"state":"closed"
},
{
"last_changed":"2016-12-31T11:15:59.395808+00:00",
"state":"closed",
"attributes":{
"friendly_name":"door"
},
"entity_id":"sensor.backdoor",
"last_updated":"2016-12-31T11:15:59.395808+00:00"
}
];
function findKey ( theKey, theVal ) {
let reduced = arr.filter ( d => {
return d [ theKey ] === theVal;
});
return reduced;
}
let targets = findKey ( 'entity_id', 'sensor.backdoor' );
targets.forEach ( d => {
// This check is a little naive, but should give you the idea
if ( 'state' in d ) {
console.log ( d.state );
}
} );
</script>