Javascript anonymous function in settimeout function

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Javascript

Please refer to the below code. This code works correctly.

let user = { 
  firstName: "John", 
  sayHi() {   
  alert(`Hello, ${this.firstName}!`);  
 } 
};  
 //using anonymous function 
  setTimeout(function() { 
  user.sayHi(); // Hello, John!
 }, 1000);

But when I code

setTimeout(user.sayHi, 1000); 

instead of anonymous function, it doen't work(I know why. it's because "this" of "this.firstName" loses its context ). But I can't understand, why does it work when you use anonymous function?

What I have tried:

I know I can use "bind", instead of anonymous function. But I wanna know, the reason why it works when you use anonymous function.