У вашего броузера проблема в совместимости с HTML5
In this episode I talk about strict null checks in TypeScript
Writing the example code:
00:09 Defining variables with different types
01:09 Assign strings to the variables
01:30 Assign null to the variables
01:56 Assign undefined to the variables
02:25 Configure TypeScript to use strict null checks
04:00 Pros and Cons of using strict null checks
05:56 One way of overcoming the 3rd party problem
You can see all the code here:
https://github.com/slavik57/typescript-playground
TypeScript - Union, Intersection & String Literal Types:
https://youtu.be/IpP6xbkojy4
Learn TypeScript:
https://www.youtube.com/playlist?list=PLtm06noQc9vmfoC4U0w90HTIRzpVKeXlC