What is this in JavaScript?

In JavaScript, the this keyword operates a little differently than in other languages. There are also some distinctions between stringent and non-strict modes.
The this keyword refers to different objects depending on how it is used:

• In an object method, this refers to the object.

• Alone, this refers to the global object.

• In a function, this refers to the global object.

• In a function, in strict mode, this is undefined.

• In an event, this refers to the element that received the event.

this in a Method:
When used in an object method, this refers to the object.

const person = {
  firstName: "John",
  lastName: "Doe",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};
console.log(person.fullName()) //output : John Doe

this Alone:
When used alone, this refers to the global object.
Because this is running in the global scope.
In a browser window the global object is [object Window]

let x = this;
console.log(x) //output is :[object Window]

In strict mode, when used alone, this also refers to the global object:

"use strict:";
let x = this;
console.log(x) //output is :[object Window]

this in a Function(Default):
In a function, the global object is the default binding for this.
In a browser window the global object is [object Window]:

function myFunction() {
  return this;
}
myFunction() //output is : [object Window]

this in a Function (Strict):
JavaScript strict mode does not allow default binding.
So, when used in a function, in strict mode, this is undefined.

"use strict";
function myFunction() {
  return this;
}
myFunction() //output is : undefined

this in Event Handlers:
In HTML event handlers, this refers to the HTML element that received the event:

<button onclick="this.style.display='none'">
  Click to Remove Me!
</button>