this refers to the object that is calling the function.

It depends on how the function is invoked, not where it is defined.

Where You Use this What It Refers To
console.log(this) global context window in browser
Inside an object method The object calling the method.
Inside a regular function Depends on how the function is called (global or undefined in strict mode).
In a constructor function (new) The new object being created.
With call or apply Whatever object you explicitly set as this.
Inside an arrow function Inherits this from the surrounding scope.
Feature call apply bind
Invocation Invokes the function immediately Invokes the function immediately Returns a new function (not invoked)
Arguments Passed individually Passed as an array Can pre-fill arguments (partial application)
Use Case When you know arguments beforehand When arguments are in an array When you want to create a reusable function

Examples: Practical Use Cases

1. Borrowing Methods

You can use call, apply, or bind to borrow methods from one object to use on another.

Example with call:

const person1 = { name: 'Alice' };
const person2 = { name: 'Bob' };

function introduce() {
    console.log(`Hi, I'm ${this.name}`);
}

introduce.call(person1); // Output: Hi, I'm Alice
introduce.call(person2); // Output: Hi, I'm Bob

2. Using apply for Array Manipulation

Math.max doesn’t work directly with arrays, but apply can help.

const numbers = [3, 7, 2, 8];
const max = Math.max.apply(null, numbers);
console.log(max); // Output: 8

Here:thisArg is null because Math.max doesn’t use this.

3. Creating Partially Applied Functions with bind

You can use bind to create a function with pre-filled arguments.

function multiply(a, b) {
    return a * b;
}

const double = multiply.bind(null, 2); // Pre-filling `a` with 2
console.log(double(5)); // Output: 10

Still will continue to study more.