Asynchronous JS and Error Handling JavaScript Interview Questions

 

1. What is JSON and its common operations?

JSON stands for JavaScript Object Notation.

It is a data representation format used for:

• Storing data (Client/Server)
• Exchanging data between Client and Server

It is just a text file with an extension of

.json

Common JSON operations:

Parsing: It parses a JSON string and returns a corresponding value (object, etc.).

Example:

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let profile = {

name: "Rahul",

age: 29,

designation: "Web Developer"

};

console.log(JSON.parse(profile)) // {"name":"Rahul","age":29,"designation":"Web Developer"}

 

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Stringification: It converts the given value into JSON string.

Example:

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let profile = {

name: "Rahul",

age: 29,

designation: "Web Developer"

};

console.log(JSON.stringify(profile)) // '{"name":"Rahul","age":29,"designation":"Web Developer"}'

 

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2. What is the syntax of a JSON object?

In JSON, all keys in an object must be enclosed with double-quotes. While in JS, this is not necessary.

So, keys must be strings, written with double quotes in JSON.

JS:

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let profile = {

name: "Rahul",

age: 29,

designation: "Web Developer"

};

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• JSON:

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{

"name": "Rahul",

"age": 29,

"designation": "Web Developer"

}

JSON

3. How do you access keys from JSON string?

First, parse the stringified JSON using the

JSON.parse

method and then access keys from the returned object.

Example:

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const myJSON = '{"name":"John", "age":30, "car":null}';

const myObj = JSON.parse(myJSON);

const name = myObj.name;

console.log(name); // Output: John

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4. Explain about async and await in JavaScript?

• Async: The keyword

  async
  before a function makes the function return a promise.

• Await: The keyword

  await
  before a function makes the function wait for a promise.

Note

The await keyword can only be used inside an async function.

5. Why do we use Async and Await?

The Async/Await is a modern way to consume promises. The

await

ensures processing completes before the next statement executes.

• We can avoid chaining promise altogether using async/await.
• It allows asynchronous execution while maintaining a regular, synchronous feel and readability.

6. Which one would you prefer between Promises or async/await?

The async/await simply gives you a synchronous feel to asynchronous code. It's a very elegant form of syntactical sugar and is more readable.

For simple queries and data manipulation, Promises can be preferred otherwise we will prefer async/await.

7. What is an Asynchronous Execution?

In Asynchronous execution, the second statement won't wait until the first statement execution.

Example: Fetch

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const url = "https://apis.ccbp.in/jokes/random";

fetch(url)

.then((response) => {

return response.json();

})

.then((jsonData) => {

//statement-1

console.log(jsonData);

});

//statement-2

console.log("fetching done");

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Output:

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fetching done

Object{ value:"The computer tired when it got home because it had a hard drive" }

8. What is a fetch method in JS?

The

fetch

method is used to fetch resources across the Internet. It returns a promise.

Syntax:

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fetch(URL, OPTIONS)

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• URL: URL of the resource
• OPTIONS: Request Configuration

Request Configuration:

We can configure a request by passing an options object with required properties like,

• Request Method
• Headers
• Body
• Credentials
• Cache, etc.

Example:

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let options = {

method: "GET",

headers: {

"Content-Type": "application/json"

},

body: JSON.stringify(data)

};

fetch("https://gorest.co.in/public-api/users", options);

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9. How to send the data to the server using the Fetch method?

We use the HTTP POST method to send the data to the server.

Example:

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let data = {

name: "Rahul",

email: "rahul@gmail.com",

};

let options = {

method: "POST",

headers: {

"Content-Type": "application/json",

Accept: "application/json",

Authorization: "Bearer ACCESS-TOKEN",

},

body: JSON.stringify(data),

};

fetch("https://gorest.co.in/public-api/users", options)

.then(function (response) {

return response.json();

})

.then(function (jsonData) {

console.log(jsonData);

});

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10. What are HTTP request methods?

HTTP Methods indicate the desired action to be performed for a given resource.

Name	Description
GET (Read)	Request for a resource(s) from the server
POST (Create)	Submit data to the server
PUT (Update)	The data within the request must be stored at the URL supplied, replacing any existing data
DELETE (Delete)	Delete a resource(s)

11. What is the difference between the HTTP GET method and the HTTP POST method?

The HTTP GET method is used to retrieve (get) data from a specified resource while the HTTP POST method is used to send data to the server.

12. What is meant by try...catch statement?

• The

  try
  block includes the code that might generate an error.

• The

  catch
  includes the code that is executed when an error occurs inside the
  try
  block.

Syntax:

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try {

// Block of code to try

}

catch(err) {

// Block of code to handle errors

}

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13. Where do we use try...catch statements?

We use the try-catch statements when our code has a chance of throwing an exception which need to be handled.

14. Explain about SetInterval(), SetTimeout() ?

setTimeout()	setInterval()
setTimeout()  allows us to run a function once after the interval of time.	setInterval()  allows us to run a function repeatedly, starting after the interval of time, then repeating continuously at that interval.

15. How to stop setTimeout()?

• By using 
  clearTimeout()
   method we can stop 
  setTimeout()
  .
• A call to 
  setTimeout
   returns a timer identifier 
  timerId
   that we can use to cancel the execution.

Example:

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let timerId = setTimeout(function () {

alert("Hello");

}, 3000);

clearTimeout(timerId);

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16. Explain about setInterval and clearInterval methods in JavaScript?

setInterval():

The

setInterval

method allows us to run a function at the specified interval of time repeatedly.

Syntax:

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setInterval(function, delay);

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• function: A callback function that is called repeatedly at the specified interval of time (delay).
• delay: time in milliseconds. (1 second = 1000 milliseconds)

clearInterval():

• The 
  clearInterval()
   method cancels a schedule previously set up by calling 
  setInterval()
  .
• To execute 
  clearInterval()
   method, we need to pass the uniqueId returned by 
  setInterval()
   as an argument.

Syntax:

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clearInterval(uniqueId);

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17. Can you explain the below code, and How it works?

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console.log(1);

console.log(2);

setTimeout(() => {

console.log(3);

}, 6);

console.log(4);

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Output:

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3

Here,

setTimeout(()=>{console.log(3)},6)

The function in

setTimeout

executes after completing the time specified in the second parameter. So 4 is logged before 3.

18. What is the difference between Promises and Callback?

Promises	Callbacks
With promises, the executing function returns a special object to us and then we tell the promise what to do when the asynchronous task completes.	With callbacks, we tell the executing function what to do when the asynchronous task completes.

19. What is a callback function?

A callback function is a function passed into another function as an argument.

Example:

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function greeting(name) {

alert("Hello " + name);

}

function processUserInput(callback) {

let name = alert("Please enter your name.");

callback(name);

}

processUserInput(greeting);

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20. What are the uses of callback?

Callbacks are generally used when the function needs to perform events before the callback is executed.

It will be useful when we need to use the result of the first function into another function.

For Example,

setTimeout

,

setInterval

, etc.

21. Explain about JS promises?

JS Promises:

• Promise is a way to handle Asynchronous operations.

• A promise is an object that represents a result of an operation that will be returned at some point in the future.

• A promise will be in any one of the three states:

  • Pending: Neither fulfilled nor rejected
  • Fulfilled: Operation completed successfully
  • Rejected: Operation failed

Example:

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const url = "https://apis.ccbp.in/jokes/random";

let responseObject = fetch(url);

console.log(responseObject); // Output: Promise{ [[PromiseStatus]]:pending, [[PromiseValue]]:undefined }

console.log("fetching done"); // Output: fetching done

 

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• To know more, refer to JS Promises | Cheat Sheet

22. When to use JS Promises?

• Promises are used to handle asynchronous operations in JavaScript. They can handle multiple asynchronous operations easily and provide better error handling than callbacks and events.
• To know more, refer to JS Promises | Cheat Sheet in JavaScript Essentials course

23. What are the handling methods of a Promise?

The methods

promise.then()

,

promise.catch()

, and

promise.finally()

are used to handle the promise that becomes settled.

promise.then()

The

then

method is called after the Promise is resolved.

Example:

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const url = "https://apis.ccbp.in/jokes/random";

let responsePromise = fetch(url);

responsePromise.then((response) => {

console.log(response); // Output: Response{ … }

});

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promise.catch()

The

catch

method is called after the Promise is rejected.

Fetching a resource can be failed for various reasons like:

• The URL is spelt incorrectly
• Server is taking too long to respond
• Network failure error, etc.

Example:

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const url = "https://apis.ccbp.in/jokes/random";

let responsePromise = fetch(url);

responsePromise.then((response) => {

console.log(response);

});

responsePromise.catch((error) => {

console.log(error); // TypeError{ "Failed to fetch" }

});

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