# Get value at index `key` on the array or map `self`
	#
	# require: `self.is_array or self.is_map`
	# require: `self.is_array implies key isa Int`
	#
	#     assert """{"a": 123}""".to_json_value["a"].to_i == 123
	#     assert """{"123": "a"}""".to_json_value[123].to_s == "a"
	#     assert """{"John Smith": 1980}""".to_json_value["John Smith"].to_i == 1980
	#     assert """{"a": 123}""".to_json_value["b"].is_error
	#
	#     assert """["a", "b", "c"]""".to_json_value[0].to_s == "a"
	#     assert """["a", "b", "c"]""".to_json_value[3].is_error
	fun [](key: Object): JsonValue do
		var value = value
		var result: nullable Object
		if is_error then
			return self
		else if value isa MapRead[String, nullable Object] then
			key = key.to_s
			if value.has_key(key) then
				result = value[key]
			else
				result = new JsonKeyError("Key `{key}` not found.", self, key)
			end
		else if value isa SequenceRead[nullable Object] then
			if key isa Int then
				if key < value.length and key >= 0 then
					result = value[key]
				else
					result = new JsonKeyError("Index `{key}` out of bounds.",
							self, key)
				end
			else
				result = new JsonKeyError("Invalid key type. Expecting `Int`. Got `{key.class_name}`.",
						self, key)
			end
		else
			result = new JsonKeyError("Invalid `[]` access on a `{json_type}` JsonValue.",
					self, key)
		end
		return new JsonValue(result)
	end