Top 10 Node.js Interview Questions and Answers

Q1. What is an  error-first call back ?

Error-first callbacks are used to pass error and data. The first argument to these functions is an error object and the subsequent arguments represent associated data. So, you can check the first argument i.e, the error object to see if something went wrong and may be handle it. In case there is no error you can use the subsequent arguments and go ahead.

e.g.

var post = new Post({title: 'test'});

post.save(function(err,savedInstance){

  if(err){

     //handle error and return

  }

  //go ahead with `savedInstance`

});

The above snippet gives an example of error-first callback.

Q2. How can you avoid callback hells

As many of you probably know, all JavaScript really is underneath is a single-threaded event loop that processes queued events. If you were to execute a long-running task within a single thread then the process would block, causing other events to have to wait to be processed (i.e. UI hangs, data doesn't get saved, etc). This is exactly what you want to avoid in an event-driven system. Here is a great video explaining much more about the JavaScript event loop.

To solve this blocking problem, JavaScript heavily relies on callbacks, which are functions that run after a long-running process (IO, timer, etc) has finished, thus allowing the code execution to proceed past the long-running task.

downloadFile('example.com/weather.json', function(err, data) { 

    console.log('Got weather data:', data);

});

Q3. What are promises?

A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it’s not resolved (e.g., a network error occurred). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

Q4. Whats your favorite HTTP framework and why?

As far as Node.js alternatives, Express is a standout amongst the most develop, well-archived choices. It's steady, underpins a lot of modules and has an incredible directing API. The system has a moderate layout that enables the engineer to assume responsibility for each task and how it will be sorted out. This gives incredible adaptability however can be somewhat overpowering to the individuals who are new to Node.js.

Q5. What's a stub? Name a utilization case.

Stubs are capacities/programs that reproduce the practices of parts/modules. Stubs give canned responses to capacity calls made during experiments. Additionally, you can state on with what these stubs were called.

A utilization case can be a document perused when you would prefer not to peruse a real record:

var fs = require('fs');

var readFileStub = sinon.stub(fs, 'readFile', work (way, cb) {

return cb(null, 'filecontent');

});

expect(readFileStub).to.be.called;

readFileStub.restore();

Q6. Why use Node.js?

·         Node.js makes building scalable network programs easy. Some of its advantages include:

·         It is generally fast

·         It almost never blocks

·         It offers a unified programming language and data type

·         Everything is asynchronous 

·         It yields great concurrency

Q7. How else can the JavaScript code below be written using Node.Js to produce the same output?

console.log("first");
setTimeout(function() {
    console.log("second");
}, 0);
console.log("third");

Output:

first
third
second

In Node.js version 0.10 or higher, setImmediate(fn) will be used in place of setTimeout(fn,0) since it is faster. As such, the code can be written as follows:

console.log("first");
setImmediate(function(){
    console.log("second");
});
console.log("third");

Q8.How do you prevent/fix callback hell?

The three ways to prevent/fix callback hell are:

·         Handle every single error

·         Keep your code shallow

·         Modularize – split the callbacks into smaller, independent functions that can be called with some parameters then joining them to achieve desired results.

The first level of improving the code above might be:

var logError = function(error){
    if(!error){
      console.log("success!!");
    }else{
      console.log("error");
    }
  },
  updateTransaction = function(t){
    query("UPDATE transactions SET value = " + (t.value*0.1) + " WHERE id=" + t.id, logError);
  },
  handleTransactions = function(transactions){
    transactions.each(updateTransaction);
  },
  handleClient = function(id){
    query("SELECT * FROM transactions WHERE clientId=" + id, handleTransactions);
  };

query("SELECT clientId FROM clients WHERE clientName='picanteverde';",handleClient);

You can also use Promises, Generators and Async functions to fix callback hell.

Q9. What is the difference between AngularJS and Node.js?

Angular.JS is a web application development framework while Node.js is a runtime system. 

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Q10. Why is consistent style important and what tools can be used to assure it?

Consistent style helps team members modify projects easily without having to get used to a new style every time. Tools that can help include Standard and ESLint.