This came out of the discussions at #23559 (comment) and #23559 (comment)

The AMP codebase contains hundreds of casts from Object to JsonObject.

• I wonder if there's a way to modify the declaration of JsonObject in amp.extern.js to make most of the casts unnecessary
  

  amphtml/build-system/amp.extern.js

  Lines 57 to 65 in 6b86000

  	/**
  	* A type for Objects that can be JSON serialized or that come from
  	* JSON serialization. Requires the objects fields to be accessed with
  	* bracket notation object['name'] to make sure the fields do not get
  	* obfuscated.
  	* @constructor
  	* @dict
  	*/
  	function JsonObject() {}

• For converting to JsonObject, there is parseJson() from src/json.js, but it doesn't work for cases where a non-null JsonObject is expected
  

  amphtml/src/json.js

  Lines 104 to 113 in c4a663d

  	/**
  	* Simple wrapper around JSON.parse that casts the return value
  	* to JsonObject.
  	* Create a new wrapper if an array return value is desired.
  	* @param {*} json JSON string to parse
  	* @return {?JsonObject} May be extend to parse arrays.
  	*/
  	export function parseJson(json) {
  	return /** @type {?JsonObject} */ (JSON.parse(/** @type {string} */ (json)));
  	}

• For checking if a variable is an Object, there is isObject() from src/types.js, but it doesn't work when a JsonObject is expected
  

  amphtml/src/types.js

  Lines 48 to 55 in c4a663d

  	/**
  	* Determines if value is actually an Object.
  	* @param {*} value
  	* @return {boolean}
  	*/
  	export function isObject(value) {
  	return toString(value) === '[object Object]';
  	}

Related to #23425