Quiz-02

04-20-17

Time Limit: 25min

Q1:

Which of the following methods removes the last element from an array and returns that element?

1. get()
2. pop()
3. last()
4. delete()

Answer: 1 Reference: MDN

var a = [1, 2, 3];
a.pop();

console.log(a); // [1, 2]

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Q2:

var a = 6
function test() {
    var a = 7
    function again() {
        var a = 8
        console.log(a)  // A
    }
    again()
    console.log(a)  // B
}
test()
console.log(a) // C

When executed, the below JavaScript code will pop up three alerts. Identify the corret answer?

1. 8; 7; 6
2. 6; 7; 8
3. 8; 6; 7
4. 7; 6; 8

Answer: 1

In the snippet, first var a = 6 is defined globally, then test() is defined but not called yet. At the end of it's definition the function is called.

• var a is now assigned the value 7, locally scoped within test()
• again() is defined and then called
• var a is now assigned the value 8, locally scoped within again()
• The console.log(a) // A gets executed, which outputs 8
• again() is now finished executing and then console.log(a) // B executes, which outputs 7
• test() finished executing and finally console.log(a) // C is executed, which outputs 6

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Q3:

var fs = require('fs')
fs.readFile('questions.txt', 'utf8', (err, data) => {
	if (!err) {
		console.log(data)
	}
})
console.log('Done!')

What will be the output? Explain

Answer:

The code reads the file questions.txt asynchronously that takes some time to return the file data in a callback. Hence, Done! is printed out first and then the file contents console.log(data).

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Q4:

var fs = require('fs')
var content = null
fs.readFile('questions.txt', 'utf8', (err, data) => {
	if (!err) {
		content = data
	}
})
console.log(content)
console.log('Done!')

What will be the output? Explain

Answer:

The code reads the file questions.txt asynchronously that takes some time to return the file data in a callback. While fs.readFile() executes, it runs console.log(content). However, the content is null, because fs.readFile() hasn't finished executing. It'll output undefined, then Done! and file contents are never logged in console.

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Q5:

var fs = require('fs')
var fileContent = fs.readFileSync('sample.json', 'utf8')
console.log(fileContent)
console.log('Done!')

What will be the output? Explain The code reads the file questions.txt synchronously.

• It then, prints out file content console.log(fileContent)
• and then print console.log('Done!')

---

Q6:

What is the difference between for loop and foreach statement?

Reference: Zsolt Fabók Blog

Consider this array: var elements = ["element1", "element2", "element3"]

For an asynhronous function being executed within a forEach loop

elements.forEach(function(element) {
  // The asynchronous action simulator
  setTimeout(function() {
    console.log(element)
  }, 1000)
})

It will out output all the elements of the array after 1000 ms.

element1
element2
element3

However, in case of for loop

for (var i = 0; i < elements.length; i++) {
  setTimeout(function() {
     console.log(elements[i])
  }, 100)
}

It will output undefined for all the elements

undefined
undefined
undefined

Javascript always passes the arguments of a function as references, there is no block scope in Javascript for loop finishes before the asynchronous methods start, and the value of i is 3 at that time, so when console.log() is finally executed, it prints the value of elements[3] - undefined - three times.

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Q7:

Write a dummy function to show how callbacks work.

// define the function with the callback argument
function doSomething(a, b, callback) {
  var my_number = a+b // Add a and b parameters
  callback(my_number) // Call the callback and pass our result
}

// Call the doSomething function
doSomething(5, 15, function(num) {		// a = 5 ; b = 15 ; callback is function(num)
  console.log("callback called! " + num) // this anonymous function will run when the callback is called
})

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Q8:

var deferred = Promise.defer()
var promise = deferred.promise

promise
	.then((value) => {
		console.log(value * 10)
	})
	.catch((value) => {
		console.log(value * 2)
	})

setTimeout(x, 3000)
setTimeout(y, 1000)

function x() {
	deferred.reject(5)
	console.log('Promise Status: ', promise)
}

function y() {
	deferred.resolve(10)
	console.log('Promise Status: ', promise)
}

What will be the output? Explain

Answer:

Promise Status:  Promise { 10 }
100
Promise Status:  Promise { 10 }

We create a deferred object and its promise is stored in var promise. It can either be resolved or rejected. The timing of which depends on the two setTimeout() statements. setTimeout(y, 1000) executes after 1 second and setTimeout(x, 3000) executes after 3 seconds. So y() is called before x(). y() resolves the promise with a value of 10 and outputs Promise { 10 }. Once that's done, the then() statement is called and console.log(value * 10) gets executed with the resolved value. So the output is 10x10 = 100. Next, x() gets executed. Since the promise is already resolved it can't be rejected. Next, it outputs Promise { 10 } with the resolved value. Lastly, catch statement never executes.

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Q9:

var io = require('socket.io').listen(server)

io.sockets.on('connection', function(socket){
	// Difference between following?
	socket.emit(identifier, data)             
	socket.broadcast.emit(identifier, data)   
	io.sockets.emit(identifier,data)
})

Explain the difference.

Answer:

• socket.emit(identifier, data) : Emits data back to the client socket itself
• socket.broadcast.emit(identifier, data) : Emits data to all connected clients except the socket client itself
• io.sockets.emit(identifier,data) : Emits data to everyone

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Q10: What does the following query do when performed on the posts collection?

db.posts.update({_id:1},{$set:{Author:"Tom"}})

1. Sets the complete document with _id as 1 with the document specified in second parameter by replacing it completely
2. Adds a new field Author in the searched collection if not already present
3. Updates only the Author field of the document with _id as 1
4. Both 2 and 3

Answer: 4

Reference