why this resolves to undefined not window/global-env in react component method if it not bind this in constructor - reactjs

Question

Just described as title.
I got answers that how to fix the situation - this refers to undefined.
But no answer tells me why this refers to undefined.
In my opinion, this will refer to window/global-env if it isn't specified implicitly or explicitly.
Of course ,this will be undefined when in strict-mode.

Answer 1

This is determined at runtime and depending on the code, it can be something different.
this is
determined at runtime, when a function is envoked
determined by how a function is invoked, not where the function is
defined
a reference to an object.
will always be an object
global (this) not available in strict mode
Example 1: this = window
var name = 'Global';
var callName1 = function() {
var name = 'Peter';
console.log('--- From callName1 ----');
console.log(this.name);
//console.log(this);
callName2();
}
var callName2 = function() {
var name = 'Jane';
console.log('--- From callName2 ----');
console.log(this.name);
//console.log(this);
}
callName1();
var execute = function(fn) {
var name = 'Mary';
console.log('--- From execute ----');
console.log(this.name);
//console.log(this);
}
execute(callName2);
Example 2: not available in strict mode
'use strict';
var name = 'Global';
var callName1 = function() {
var name = 'Peter';
console.log('--- From callName1 ----');
console.log(this.name);
console.log(this);
}
callName1();
Example 3: examining this with method invocation
var name = 'global';
var obj = {
name: 'James Obj1',
func: function() {
console.log('--- From func ----');
console.log(this.name);
console.log(this); // this reference obj1
}
}
obj.func()
var obj2 = {
name: 'Jame Obj2',
func: obj.func // this reference obj2, but the function is defined in obj1
}
obj2.func()
var obj3 = {
name: 'Kane Obj3',
obj4: {
name: 'Mary Obj4',
func: function () {
console.log('--- From obj4 ----');
console.log(this.name);
console.log(this); // this reference obj4
}
}
}
obj3.obj4.func()
With () => {} function this - is lexically bound. It means that it uses this from the code that contains the arrow function.

Answer 2

use code below to verify method is in strict mode or not.
Include code below in method.
Got a type error, that means , method is in strict mode, which resolve the confusion.
var obj2 = { get x() { return 17; } };
obj2.x = 5;