building a multidimensional array in javascript

As stated by Steven, Javascript Arrays are not associative. However, you can parse an Associative PHP Array as a JSON Object which will led to the same structure when passing from or to Client Side.

P.E In PHP you'd have

$myArray = [
   "tag"=>"property",
   "properties"=>[
        "color"=>"blue",
        "name"=>"A Name"
    ]
];

Then, you return this array as JSON

return json_encode($myArray);

And when you receive it on Javascript you do a JSON Parse with the response

function getMyObject()
{
   // Code for retrieving the JSON From Server

   //End of Code for JSON Retrieval

   const myObject = JSON.parse(response);
}

This will led to an structure such as this in Javascript

console.log(myObject)

>>>>
{
  tag:"property",
  properties:{
    color:"blue",
    name:"A Name"
  }
}

Which you can access with the object.key notation or object[key] in Javascript.

Most of frameworks do this in a transparent way. For example, if you're using Laravel when you return the Array as a response in an API route, it will automatically parse it as a JSON. And then, in a framework such as Angular, the HttpClient will automatically parse the JSON response as a JavaScript object.

Then, in the opposite direction, if you send a object to an Endpoint. It will automatically be converted to a JSON body in the request and parsed by the backend framework.