this inside the object points to the problem

1.

const obj2 = {fn: () => {console.log(this);} }}obj2.fn(); // window

The arrow function in the above code does not have its own this, so it looks up the this object (there is no scope in the object braces?), so it finally points to the window.

2.

const obj2 = {fn: function() {console.log(this); // obj2} }};obj2.fn();

The above code fn function is called by the obj2 object, this points to obj2

Second, the function in the object returns a function
1.

const obj2 = {fn: function() {console.log("1", this);return function() {console.log("2", this);} }} }}obj2.fn()(); // 1 obj2 2 window

The above code fn returns a function. This function is not obviously called by anyone, that is, it is called by window.

2.

const obj2 = {fn: function() {console.log("1", this);return () => {console.log("2", this);} }} }}obj2.fn()(); // 1 obj2 2 obj2

The above code fn returns an arrow function. Although there is no clear caller, the arrow function may involve searching for this in the upper scope. The upper scope is the scope of the fn function, becauseCalled by obj2, so this points to obj2.

Three, there is an immediate execution function in the function
1.

const obj2 = {fn: function() {console.log("1", this);(function() {console.log("2", this);} })();return () => {console.log("3", this);} }} }}obj2.fn()(); // 1 obj2 2 window 3 obj2

Here, an immediate execution function is executed, there is no explicit caller, this points to window.

2.

const obj2 = {fn: function() {console.log("1", this);(() => {console.log("2", this);} })();return () => {console.log("3", this);} }} }}obj2.fn()(); // 1 obj2 2 obj2 3 obj2

This is basically the same as the above example, except that the immediate execution function is an arrow function. The arrow function looks for this in the upper scope, so it finds this in fn,

Fourth, the situation with timer
1.

const obj2 = {fn: function() {console.log("1", this);(() => {console.log("2", this);})();return () => {console.log("3", this);setTimeout(() => {console.log("4", this);} }, 0);} }} }}obj2.fn()(); // 1 obj2 2 obj2 3 obj2 4 obj2

Is the timer callback function here an arrow function, or it involves searching for this in the upper-level scope.

2.

const obj2 = {fn: function() {console.log("1", this);(() => {console.log("2", this);})();return () => {console.log("3", this);setTimeout(function() {console.log("4", this);} }, 0);} }} }}obj2.fn()(); // 1 obj2 2 obj2 3 obj2 4 window

The timer callback function here does not have a clear caller, and this points to window.

Additional about arguments in arrow functions
In arrow functions, similar to this, arguments also point to arguments in the previous function.

function f1(x, y) {return (z) => {console.log(arguments);} }}const tempFn = f1(1, 2);tempFn(3);/*[Arguments] { '0': 1, '1': 2 }