1. Inversion of control:

• We are giving control of our code to something else.
• The main disadvantage of callbacks is an inversion of control.
• Inversion of control: if there is some part of code whose control we are passing to Someone else, and we don't know how that part will be executed, this is called inversion of control.
• You lose control of your own business logic; you are just giving it to someone else. How they are treating or using, you don't know.

2. Callback Hell :

• Code grows horizontally instead of vertically.
• It is also called as pyramid of dom. Code can be ugly, and code maintenance will be difficult.
• Callback hell makes your code unreadable.

tasks :

1. Write a function to download data from url

2. Write a function to save the downloaded data in a file and return the file name.

3. Write a function to upload the file, written in the previous step, to a new URL.

through callback :

function customeFetch(data, fn) {
	console.log("starting downloading..");
	setTimeout(() => {
		console.log("download completed.. ");
		let response = "data";
		fn(response);
		console.log("ending the customeFetch function");
	}, 4000);
}

function writeFile(data, fn) {
	console.log("starting writting data..", data);
	setTimeout(() => {
		console.log("writting completed.. ");
		let filename = "fileName.txt";
		fn(filename);
		console.log("writting ended..");
	}, 10000);
}

function uploadFile(filename, newUrl, fn) {
	console.log("upoading started..");
	setTimeout(() => {
		console.log(`file ${filename} uploaded succesful on ${newUrl}`);
		let uploadResponse = "success";
		fn(uploadResponse);
		console.log("upload ended..");
	}, 10000);
}

customeFetch("www.google.com", (response) => {
	console.log("downloaded response is ", response);
	writeFile(response, (filenameResponse) => {
		console.log("new file written is", filenameResponse);
		uploadFile(filenameResponse, "www.drive.google.com", (uploadResponse) => {
			console.log("successful uploaded ", uploadResponse);
		});
	});
});

Explanation: whatever the variables that are declared inside the customFetch or writeFile, or uploadFile functions, those function variables are accessible within the function calls(invoke) due to the concept of closure and lexical scope.