Javascript Interview Questions




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.

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

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

3. 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

4. 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

5. 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.

6. 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.

7. 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"].

8. 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.

9. 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$.