In most cases, the value of this is determined by how a function is called. It can't be set by assignment during execution, and it may be different each time the function is called You can change the this context through .call(), .apply() and .bind() Value of this is equal to the value of the object which invokes the function this is not assigned a value until an object invokes the function where this is defined

When functions are executed in the global scope, value of this is windows boject In strict mode, value of this in the global context will undefined. Consider the below example:

var myFunciton = function(){
  console.log(this);
  console.log(this=== window);
}

myFunction();// Output: Window object. In strict mode value will be undefined 

The above function was not executed inside any other function or object hence, by default the myFunction was called on the global oject Hence the value of this is 'window'. In strict, value would be undefined.

In strict mode, value of this will be undefined

When a function is called as a method of an object, its this is set to the object the method is called on.

To understand this, consider the below example:

var val = 37;
var myObj = {
	val : 10,
	someFunction : function(){
		console.log(this.val);//Output: 10 since tha value of this is equal to myObj
		console.log(window.val); //Output: 37
		console.log(this === myObj) // true
		console.log(this)//Output: myObj object
	}
}

myObj.someFunction();

Let's consider an another example to understand this

//Variable defined in global scope
var val = 37;

var myObj = {
	val : 10
}

var someFunction = function(){
		console.log(this.val);
		console.log(window.val);
		console.log(this === myObj);
		console.log(this);
		console.log(this === window) 
}

myObj.objectFunc = someFunction;
myObj.objectFunc();  
/*
Output:
	1. 10
	2. 37
	3. true
	4. myObj
	5. false
*/

someFunction();
/*
Output:
	1. 37
	2. 37
	3. false
	4. window
	5. true
*/

In the above example, first we call someFunction() in the context of myObj. Later we called the function in the global context. This shows that the value of this was determined dynamically on the basis of execution context.

When a function is used as a constructor (with the new keyword), its this is bound to the new object being constructed.

function ConstructorFunc(value){
	this.someValue = value;
}

var obj1 = new ConstructorFunction(20);
console.log(obj1.someValue)//Output: 20