Why after defining some method from object to variable context lost?

please help to get answer i can't understand why the context lost after defination

    class A {
    	func() {
    		console.log(this)
    	}
    }
    
    let a = new A();
    let b = a.func;
    b();

You can refer to this in order to clarify your doubt.

Simple this short if you call a function on object javascript considers that object as its this/context.

eg.

let obj = {
   key : 'value',
   fun : function(){

   }
}
// if called like obj.fun() --> obj is going to be this
// if called like let ffun = obj.fun; ffun();  ---> window/global object is going to be this

If you call that method by using call/apply/bind you need to specify custom context as first parameter to these methods.

//if call like obj.fun.call(someObject, p1, p2)
// Or obj.fun.apply(someObject, [p1, p2])
// Or let ffun = obj.fun.bind(someObject, [p1, p2]);   ffun();
// someObject is going to be this in all these 3 cases

In otherwise cases where a function is directly called it takes window/global object as its context.