25+ JavaScript Coding Interview Questions (SOLVED with CODE)

A callback function is a function that is passed to another function as an argument and is executed after some operation has been completed. Below is an example of a simple callback function that logs to the console after some operations have been completed.

function modifyArray(arr, callback) {
  // do something to arr here
  arr.push(100);
  // then execute the callback function that was passed
  callback();
}

var arr = [1, 2, 3, 4, 5];

modifyArray(arr, function() {
  console.log("array has been modified", arr);
});

var string = "Welcome to this Javascript Guide!";

// Output becomes !ediuG tpircsavaJ siht ot emocleW
var reverseEntireSentence = reverseBySeparator(string, "");

// Output becomes emocleW ot siht tpircsavaJ !ediuG
var reverseEachWord = reverseBySeparator(reverseEntireSentence, " ");

function reverseBySeparator(string, separator) {
  return string.split(separator).reverse().join(separator);
}

The best way to find whether an object is instance of a particular class or not using toString method from Object.prototype

var arrayList = [1 , 2, 3];

One of the best use cases of type checking of an object is when we do method overloading in JavaScript. For understanding this let say we have a method called greet which take one single string and also a list of string, so making our greet method workable in both situation we need to know what kind of parameter is being passed, is it single value or list of value?

function greet(param) {
  if() {
    // here have to check whether param is array or not
  }
  else {
  }
}

However, in above implementation it might not necessary to check type for array, we can check for single value string and put array logic code in else block, let see below code for the same.

 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 above two implementations, but when we have a situation like a parameter can be single value, array, and object type then we will be in trouble.

Coming back to checking type of object, As we mentioned that 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 used 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 browser, 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

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 way of empty the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged, Only use this way if you have only referenced the array by its original variable arrayList.

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;

Above code will clear the existing array by setting its length to 0. This way of empty the array also update all the reference variable which pointing to the original array. This way of empty the array is useful when you want to update all the another reference variable which 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);

Above implementation will also work perfectly. This way of empty the array will also update all the references of 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();
}

Above implementation can also empty the array. But not recommended to use often.

A very simply way to check if a number is a decimal or integer is to see if there is a remainder left when you divide by 1.

function isInt(num) {
  return num % 1 === 0;
}

console.log(isInt(4)); // true
console.log(isInt(12.2)); // false
console.log(isInt(0.3)); // false

Enqueue means to add an element, dequeue to remove an element.

var inputStack = []; // First stack
var outputStack = []; // Second stack

// For enqueue, just push the item into the first stack
function enqueue(stackInput, item) {
  return stackInput.push(item);
}

function dequeue(stackInput, stackOutput) {
  // Reverse the stack such that the first element of the output stack is the
  // last element of the input stack. After that, pop the top of the output to
  // get the first element that was ever pushed into the input stack
  if (stackOutput.length <= 0) {
    while(stackInput.length > 0) {
      var elementToOutput = stackInput.pop();
      stackOutput.push(elementToOutput);
    }
  }

  return stackOutput.pop();
}

function duplicate(arr) {
  return arr.concat(arr);
}

duplicate([1, 2, 3, 4, 5]); // [1,2,3,4,5,1,2,3,4,5]

function mul (x) {
  return function (y) { // anonymous function
    return function (z) { // anonymous function
      return x * y * z;
    };
  };
}

Here mul function accept the first argument and return anonymous function which take the second parameter and return anonymous function which take the third parameter and return multiplication of arguments which is being passed in successive

In JavaScript function defined inside has access to outer function variable and function is the first class object so it can be returned by function as well and passed as 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
• Function can be stored as variable
• Function can be pass as a parameter to another function
• Function can be returned from function

You can create a closure to keep the value passed to the function createBase even after the inner function is returned. The inner function that is being returned is created within an outer function, making it a closure, and it has access to the variables within the outer function, in this case the variable baseNumber.

function createBase(baseNumber) {
  return function(N) {
    // we are referencing baseNumber here even though it was declared
    // outside of this function. Closures allow us to do this in JavaScript
    return baseNumber + N;
  }
}

var addSix = createBase(6);
addSix(10);
addSix(21);

Check out this version of FizzBuzz:

for (let i = 1; i <= 100; i++) {
  let f = i % 3 == 0,
    b = i % 5 == 0;
  console.log(f ? (b ? 'FizzBuzz' : 'Fizz') : b ? 'Buzz' : i);
}

var firstWord = "Mary";
var secondWord = "Army";

isAnagram(firstWord, secondWord); // true

function isAnagram(first, second) {
  // For case insensitivity, change both words to lowercase.
  var a = first.toLowerCase();
  var b = second.toLowerCase();

  // Sort the strings, and join the resulting array to a string. Compare the results
  a = a.split("").sort().join("");
  b = b.split("").sort().join("");

  return a === b;
}

You can create a function within an outer function (a closure) that allows you to update a private variable but the variable wouldn't be accessible from outside the function without the use of a helper function.

function counter() {
  var _counter = 0;
  // return an object with several functions that allow you
  // to modify the private _counter variable
  return {
    add: function(increment) { _counter += increment; },
    retrieve: function() { return 'The counter is currently at: ' + _counter; }
  }
}

// error if we try to access the private variable like below
// _counter;

// usage of our counter function
var c = counter();
c.add(5); 
c.add(9); 

// now we can access the private variable in the following way
c.retrieve(); // => The counter is currently at: 14

The question is when a non-boolean value is coerced to a boolean, does it become true or false, respectively?

The specific list of "falsy" values in JavaScript is as follows:

• "" (empty string)
• 0, -0, NaN (invalid number)
• null, undefined
• false

Any value that's not on this "falsy" list is "truthy." Here are some examples of those:

• "hello"
• 42
• true
• [ ], [ 1, "2", 3 ] (arrays)
• { }, { a: 42 } (objects)
• function foo() { .. } (functions)

Above code would give output 1undefined. If condition statement evaluate using eval so eval(function f() {}) which return function f() {} which is true so inside if statement code execute. typeof f return undefined because if statement code execute at run time, so statement inside if condition evaluated at run time.

var k = 1;
if (1) {
  eval(function foo() {});
  k += typeof foo;
}
console.log(k);

Above code will also output 1undefined.

var k = 1;
if (1) {
  function foo() {};
  k += typeof foo;
}
console.log(k); // output 1function

The code above will output 5 even though it seems as if the variable was declared within a function and can't be accessed outside of it. This is because

var a = b = 5;

is interpreted the following way:

var a = b;
b = 5;

But b is not declared anywhere in the function with var so it is set equal to 5 in the global scope.

You can create a closure to keep the value of a even after the inner function is returned. The inner function that is being returned is created within an outer function, making it a closure, and it has access to the variables within the outer function, in this case the variable a.

function multiply(a) {
  return function(b) {
    return a * b;
  }
}

multiply(5)(6);