21 Essential JavaScript Interview Questions

Question 1
1. What is the difference between undefined and not defined in JavaScript?
In JavaScript, if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error var name is not defined and script will stop executing. However, if you use typeof undeclared_variable, then it will return undefined.

Before getting further into this, let's first understand the difference between declaration and definition.

Let's say var x is a declaration because you have not defined what value it holds yet, but you have declared its existence and the need for memory allocation.

> var x; // declaring x
> console.log(x); //output: undefined
Here var x = 1 is both a declaration and definition (also we can say we are doing an initialisation). In the example above, the declaration and assignment of value happen inline for variable x. In JavaScript, every variable or function declaration you bring to the top of its current scope is called hoisting.

The assignment happens in order, so when we try to access a variable that is declared but not defined yet, we will get the result undefined.

var x; // Declaration
if(typeof x === 'undefined') // Will return true
If a variable that is neither declared nor defined, when we try to reference such a variable we'd get the result not defined.

> console.log(y);  // Output: ReferenceError: y is not defined
Question 2
What will be the output of the code below?
var y = 1;
  if (function f(){}) {
    y += typeof f;
  }
  console.log(y);
The output would be 1undefined. The if condition statement evaluates using eval, so eval(function f(){}) returns function f(){} (which is true). Therefore, inside the if statement, executing typeof f returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.

var k = 1;
  if (1) {
    eval(function foo(){});
    k += typeof foo;
  }
  console.log(k);
The code above will also output 1undefined.

var k = 1;
  if (1) {
    function foo(){};
    k += typeof foo;
  }
  console.log(k); // output 1function
Question 3
What is the drawback of creating true private methods in JavaScript?
One of the drawbacks of creating true private methods in JavaScript is that they are very memory-inefficient, as a new copy of the method would be created for each instance.

var Employee = function (name, company, salary) {
    this.name = name || "";       //Public attribute default value is null
    this.company = company || ""; //Public attribute default value is null
    this.salary = salary || 5000; //Public attribute default value is null

    // Private method
    var increaseSalary = function () {
        this.salary = this.salary + 1000;
    };

    // Public method
    this.dispalyIncreasedSalary = function() {
        increaseSlary();
        console.log(this.salary);
    };
};

// Create Employee class object
var emp1 = new Employee("John","Pluto",3000);
// Create Employee class object
var emp2 = new Employee("Merry","Pluto",2000);
// Create Employee class object
var emp3 = new Employee("Ren","Pluto",2500);
Here each instance variable emp1, emp2, emp3 has its own copy of the increaseSalary private method.

So, as a recommendation, don’t use private methods unless it’s necessary.



Question 4
What is a “closure” in JavaScript? Provide an example
A closure is a function defined inside another function (called the parent function), and has access to variables that are declared and defined in the parent function scope.

The closure has access to variables in three scopes:

Variables declared in their own scope
Variables declared in a parent function scope
Variables declared in the global namespace
var globalVar = "abc";

// Parent self invoking function
(function outerFunction (outerArg) { // begin of scope outerFunction
    // Variable declared in outerFunction function scope
    var outerFuncVar = 'x'; 
    // Closure self-invoking function
    (function innerFunction (innerArg) { // begin of scope innerFunction
        // variable declared in innerFunction function scope
        var innerFuncVar = "y";
        console.log(       
            "outerArg = " + outerArg + "\n" +
            "outerFuncVar = " + outerFuncVar + "\n" +
            "innerArg = " + innerArg + "\n" +
            "innerFuncVar = " + innerFuncVar + "\n" +
            "globalVar = " + globalVar);

    }// end of scope innerFunction)(5); // Pass 5 as parameter
}// end of scope outerFunction )(7); // Pass 7 as parameter
innerFunction is closure that is defined inside outerFunction and has access to all variables declared and defined in the outerFunction scope. In addition, the function defined inside another function as a closure will have access to variables declared in the global namespace.

Thus, the output of the code above would be:

outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abc
Question 5
Write a mul function which will produce the following outputs when invoked:
console.log(mul(2)(3)(4)); // output : 24
console.log(mul(4)(3)(4)); // output : 48
Below is the answer followed by an explanation to how it works:

function mul (x) {
    return function (y) { // anonymous function
        return function (z) { // anonymous function
            return x * y * z;
        };
    };
}
Here the mul function accepts the first argument and returns an anonymous function, which takes the second parameter and returns another anonymous function that will take the third parameter and return the multiplication of the arguments that have been passed.

In JavaScript, a function defined inside another one has access to the outer function's variables. Therefore, a function is a first-class object that can be returned by other functions as well and be passed as an argument in another function.

A function is an instance of the Object type
A function can have properties and has a link back to its constructor method
A function can be stored as a variable
A function can be pass as a parameter to another function
A function can be returned from another function


Question 6
How to empty an array in JavaScript?
For instance,

 var arrayList =  ['a','b','c','d','e','f'];
How can we empty the array above?

There are a couple ways we can use to empty an array, so let's discuss them all.

Method 1
arrayList = []
Above code will set the variable arrayList to a new empty array. This is recommended if you don't have references to the original array arrayList anywhere else, because it will actually create a new, empty array. You should be careful with this method of emptying the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged.

For Instance,

var arrayList = ['a','b','c','d','e','f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList = []; // Empty the array
console.log(anotherArrayList); // Output ['a','b','c','d','e','f']
Method 2
arrayList.length = 0;
The code above will clear the existing array by setting its length to 0. This way of emptying the array also updates all the reference variables that point to the original array. Therefore, this method is useful when you want to update all reference variables pointing to arrayList.

For Instance,

var arrayList = ['a','b','c','d','e','f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList.length = 0; // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []
Method 3
arrayList.splice(0, arrayList.length);
The implementation above will also work perfectly. This way of emptying the array will also update all the references to the original array.

var arrayList = ['a','b','c','d','e','f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList.splice(0, arrayList.length); // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []
Method 4
while(arrayList.length){
  arrayList.pop();
}
The implementation above can also empty arrays, but it is usually not recommended to use this method often.

Question 7
How do you check if an object is an array or not?
The best way to find out whether or not an object is an instance of a particular class is to use the toString method from Object.prototype:

  var arrayList = [1,2,3];
One of the best use cases of type-checking an object is when we do method overloading in JavaScript. For example, let's say we have a method called greet, which takes one single string and also a list of strings. To make our greet method workable in both situations, we need to know what kind of parameter is being passed. Is it a single value or a list of values?

function greet(param){
  if(){ // here have to check whether param is array or not
  }else{
  }
}
However, as the implementation above might not necessarily check the type for arrays, we can check for a single value string and put some array logic code in the else block. For example:

function greet(param){
  if(typeof param === 'string'){
  }else{
    // If param is of type array then this block of code would execute
  }
}
Now it's fine we can go with either of the aforementioned two implementations, but when we have a situation where the parameter can be single value, array, and object type, we will be in trouble.

Coming back to checking the type of an object, as mentioned previously we can use
Object.prototype.toString

if( Object.prototype.toString.call( arrayList ) === '[object Array]' ) {
    console.log('Array!');
}
If you are using jQuery, then you can also use the jQuery isArray method:

if($.isArray(arrayList)){
    console.log('Array');
}else{
  console.log('Not an array');
}
FYI, jQuery uses Object.prototype.toString.call internally to check whether an object is an array or not.

In modern browsers, you can also use

Array.isArray(arrayList);
Array.isArray is supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5

Question 8
What will be the output of the following code?
var output = (function(x){
    delete x;
    return x;
  })(0);

  console.log(output);
The output would be 0. The delete operator is used to delete properties from an object. Here x is not an object but a local variable. delete operators don't affect local variables.

Question 9
What will be the output of the following code?
var x = 1;
var output = (function(){
    delete x;
    return x;
  })();

  console.log(output);
The output would be 1. The delete operator is used to delete the property of an object. Here x is not an object, but rather it's the global variable of type number.

Question 10
What will be the output of the code below?
var x = { foo : 1};
var output = (function(){
    delete x.foo;
    return x.foo;
  })();

  console.log(output);
The output would be undefined. The delete operator is used to delete the property of an object. Here, x is an object which has the property foo, and as it is a self-invoking function, we will delete the foo property from object x. After doing so, when we try to reference a deleted property foo, the result isundefined.

Question 11
What will be the output of the code below?
var Employee = {
  company: 'xyz'
}
var emp1 = Object.create(Employee);
delete emp1.company
console.log(emp1.company);
The output would be xyz. Here, emp1 object has company as its prototype property. The delete operator doesn't delete prototype property.

emp1 object doesn't have company as its own property. You can test it console.log(emp1.hasOwnProperty('company')); //output : false. However, we can delete the company property directly from theEmployee object using delete Employee.company. Or, we can also delete the emp1 object using the __proto__ property delete emp1.__proto__.company.

Question 12
What is undefined x 1 in JavaScript?
var trees = ["redwood","bay","cedar","oak","maple"];
delete trees[3];
When you run the code above and type console.log(trees); into your Chrome developer console, you will get
["redwood", "bay", "cedar", undefined × 1, "maple"]. When you run the code in Firefox's browser console, you will get ["redwood", "bay", "cedar", undefined, "maple"]. Thus, it's clear that the Chrome browser has its own way of displaying uninitialised indexes in arrays. However, when you check trees[3] === undefined in both browsers, you will get similar output as true.

Note: Please remember you do not need to check for the uninitialised index of array in trees[3] === 'undefined × 1', as it will give you an error. 'undefined × 1' is just way of displaying an array's uninitialised index in Chrome.

Question 13
What will be the output of the code below?
var trees = ["xyz","xxxx","test","ryan","apple"];
delete trees[3];

  console.log(trees.length);
The output would be 5. When we use the delete operator to delete an array element, the array length is not affected from this. This holds even if you deleted all elements of an array using the delete operator.

In other words, when the delete operator removes an array element, that deleted element is not longer present in array. In place of value at deleted index undefined x 1 in chrome and undefined is placed at the index. If you do console.log(trees) output ["xyz", "xxxx", "test", undefined × 1, "apple"] in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"].

Question 14
What will be the output of the code below?
var bar = true;
console.log(bar + 0); 
console.log(bar + "xyz");
console.log(bar + true);
console.log(bar + false); 
The code will output 1, "truexyz", 2, 1. Here's a general guideline for addition operators:

Number + Number -> Addition
Boolean + Number -> Addition
Boolean + Number -> Addition
Number + String -> Concatenation
String + Boolean -> Concatenation
String + String -> Concatenation
Question 15
What will be the output of the code below?
var z = 1, y = z = typeof y;
console.log(y);
The output would be undefined. According to the associativity rule, operators with the same precedence are processed based on the associativity property of the operator. Here, the associativity of the assignment operator is Right to Left, so typeof y will evaluate first , which is undefined. It will be assigned to z, and then y would be assigned the value of z and then z would be assigned the value 1.

Question 16
What will be the output of the code below?
// NFE (Named Function Expression
var foo = function bar(){ return 12; };
typeof bar();
The output would be Reference Error. To make the code above work, you can re-write it as follows:

Sample 1

var bar = function(){ return 12; };
typeof bar();
or

Sample 2

function bar(){ return 12; };
typeof bar();
A function definition can have only one reference variable as its function name. In sample 1, bar's reference variable points to anonymous function. In sample 2, the function's definition is the name function.

var foo = function bar(){
    // foo is visible here
    // bar is visible here
  console.log(typeof bar()); // Work here :)
 };
// foo is visible here
// bar is undefined here
Question 17
What is the difference between the function declarations below?
var foo = function(){
    // Some code
};
 function bar(){
    // Some code
};
The main difference is the function foo is defined at run-time whereas function bar is defined at parse time. To understand this in better way, let's take a look at the code below:

Run-Time function declaration
<script>
foo(); // Calling foo function here will give an Error
  var foo = function(){
    console.log("Hi I am inside Foo");
 };
 </script>
 <script>
Parse-Time function declaration
bar(); // Calling foo function will not give an Error
 function bar(){
  console.log("Hi I am inside Foo");
 };
</script>
Another advantage of this first-one way of declaration is that you can declare functions based on certain conditions. For example:

<script>
if(testCondition) {// If testCondition is true then
   var foo = function(){
    console.log("inside Foo with testCondition True value");
   };
 }else{
  var foo = function(){
    console.log("inside Foo with testCondition false value");
   };
}
</script>
However, if you try to run similar code using the format below, you'd get an error:

<script>
if(testCondition) {// If testCondition is true then
   function foo(){
    console.log("inside Foo with testCondition True value");
   };
 }else{
  function foo(){
    console.log("inside Foo with testCondition false value");
   };
}
</script>
Question 18
What is function hoisting in JavaScript?
Function Expression

var foo = function foo(){
  return 12;
};
In JavaScript, variable and functions are hoisted. Let's take function hoisting first. Basically, the JavaScript interpreter looks ahead to find all variable declarations and then hoists them to the top of the function where they're declared. For example:

foo(); // Here foo is still undefined
var foo = function foo(){
  return 12;
};
Behind the scene of the code above looks like this:

var foo = undefined;
    foo(); // Here foo is undefined
     foo = function foo(){
        / Some code stuff
      }
 var foo = undefined;
  foo = function foo(){
       / Some code stuff
    }
    foo(); // Now foo is defined here
Question 19
What will be the output of code below?
var salary = "1000$";

 (function () {
     console.log("Original salary was " + salary);

     var salary = "5000$";

     console.log("My New Salary " + salary);
})();
The output would be undefined, 5000$. Newbies often get tricked by JavaScript's hoisting concept. In the code above, you might be expecting salary to retain its value from the outer scope until the point that salary gets re-declared in the inner scope. However, due to hoisting, the salary value was undefined instead. To understand this better, have a look of the code below:

var salary = "1000$";

(function () {
    var salary = undefined;
    console.log("Original salary was " + salary);

    salary = "5000$";

    console.log("My New Salary " + salary);
})();
salary variable is hoisted and declared at the top in the function's scope. The console.log inside returns undefined. After the console.log, salary is redeclared and assigned 5000$.

Question 20
What is the instanceof operator in JavaScript? What would be the output of the code below?
function foo(){
  return foo;
}
new foo() instanceof foo;
Here, instanceof operator checks the current object and returns true if the object is of the specified type.

For Example:

var dog = new Animal();
dog instanceof Animal // Output : true
Here dog instanceof Animal is true since dog inherits from Animal.prototype.

var name = new String("xyz");
name instanceof String // Output : true
Here name instanceof String is true since dog inherits from String.prototype. Now let's understand the code below:

function foo(){
  return foo;
}
new foo() instanceof foo;
Here function foo is returning foo, which again points to function foo.

function foo(){
  return foo;
}
var bar = new foo();
// here bar is pointer to function foo(){return foo}.
So the new foo() instanceof foo return false;

Ref Link

Question 21
If we have a JavaScript associative array
var counterArray = {
    A : 3,
  B : 4
};
counterArray["C"] = 1;
How can we calculate the length of the above associative array's counterArray?
There are no in-built functions and properties available to calculate the length of associative array object here. However, there are other ways by which we can calculate the length of an associative array object. In addition to this, we can also extend an Object by adding a method or property to the prototype in order to calculate length. However, extending an object might break enumeration in various libraries or might create cross-browser issues, so it's not recommended unless it's necessary. Again, there are various ways by which we can calculate length.

Object has the keys method which can be used to calculate the length of an object:


We can also calculate the length of an object by iterating through an object and by counting the object's own property.

```javascript
function getSize(object){
  var count = 0;
  for(key in object){
    // hasOwnProperty method check own property of object
    if(object.hasOwnProperty(key)) count++;
  }
  return count;
}
We can also add a length method directly on Object:

Object.length = function(){
  var count = 0;
  for(key in object){
    // hasOwnProperty method check own property of object
    if(object.hasOwnProperty(key)) count++;
  }
  return count;
}
//Get the size of any object using
console.log(Object.length(counterArray))
Bonus: We can also use Underscore (recommended, As it's lightweight) to calculate object length.