Why is there an exclamation point when declaring a TS variable?

Question

typescript

I was reading this article and came across this code:

let exampleFunc!: (instance: Example) => number;

class Example {
  static #accessCount = 0;
  #initial: number;
  constructor(input: number) {
    this.#initial = input * 2;
  }

  static {
    exampleFunc = (instance: Example) => {
      Example.#accessCount++;

      return instance.#initial
    }
  }
}

if (exampleFunc) {
  exampleFunc(new Example(2)); // 4
}

and I can not understand why you need "!" when declaring exampleFunc:
let exampleFunc →!← : (instance: Example) => number;

since it's clearly not part of the variable name, I also don't think it says that the variable can't be null | undefined:

Answer 1

It tells the TS compiler not to swear at an uninitialized variable. That in fact it will be initialized in some way that TS cannot track. For example, in this case, they are initialized inside a static block, and TS apparently does not understand this yet.