/js-interview

js-interview

A list of questions that I have asked throughout the process of learning js.

Constructors

new

What does new do?

Question:

What does new do in the following example.

var Vehicle = function Vehicle() {
  // ...
}
var vehicle = new Vehicle();

Answer :

When new Vehicle() is called, JavaScript does four things:

  1. It creates a new object.
  2. It sets the constructor property of the object to Vehicle.
  3. It sets up the object to delegate to Vehicle.prototype.
  4. It calls Vehicle() in the context of the new object.

Question:

Will new Date run without throwing a runtime exception?

Answer:

Yes

Follow up Question:

Is there a functional difference between the following two statements?

var date1 = new Date;
var date2 = new Date();

Follow up Answer:

No

Question:

What is the value of this when invoking privateBirthday while initializing daniel?

function Person(age, name)
{

	this.age  = age;
	this.name = name;

	privateBirthday();
	this.birthday();
}

Person.prototype.birthday = birthday;

function birthday()
{
	this.age++;
}

function privateBirthday()
{
	this.age++;
}

var daniel = new Person(24, 'Daniel');

Answer:

window

Follow up Question:

Follow up Answer:

#Operators

Precedence

Question:

Given the following snippet of code, what is the value of url once the code has finished running?

var flag = true;

url = '/root/server/system';
url += '/endpoint/' + flag ? 'true' : 'false';

Answer:

/root/server/systemtrue

Follow up Question:

How would you fix this code to work as you would initially expect it to? Why does this fix the problem? Alternatively you could answer why the problem was happening.

Follow up Answer:

var flag = true;

url = '/root/server/system';
url += '/endpoint/' + (flag ? 'true' : 'false';)

Mind Benders

Syntax

Question :

Write out the expected console.log.

(function(arg1)
 {
  console.log("func1");
     
     return function(arg)
     {
         console.log('func3');
         console.log(arg);
     }
 })()

(function nextFunc()
{
   console.log('func2'); 
})();

Answer :

2015-03-04 11:03:34.469 (index):24 func1
2015-03-04 11:03:34.469 (index):28 func3
2015-03-04 11:03:34.469 (index):29 function nextFunc(){ console.log('func2'); }
2015-03-04 11:03:34.469 (index):34 Uncaught TypeError: (intermediate value)(intermediate value)(...)(...) is not a function

Why :

Whitespace

(function(arg1)
 {
  console.log("func1");
     
     return function(arg)
     {
         console.log('func3');
         console.log(arg);
     }
 })()

(function nextFunc()
{
   console.log('func2'); 
})();

After func1 is invoked.

function(arg)
{
 console.log('func3');
 console.log(arg);
}(function nextFunc(){console.log('func2'); })();

After func3 is invoked

// implicitly return so undefined is left
undefined();