Great JavaScript Interview Questions


19 Great JavaScript Interview Questions

copied from here


What is the significance of, and reason for, wrapping the entire content of a JavaScript source file in a function block?

A: This is an increasingly common practice, employed by many popular JavaScript libraries (jQuery, Node.js, etc.). This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries.

Another feature of this technique is to allow for an easily referenceable (presumably shorter) alias for a global variable. This is often used, for example, in jQuery plugins. jQuery allows you to disable the $ reference to the jQuery namespace, using jQuery.noConflict(). If this has been done, your code can still use $ employing this closure technique, as follows:

(function($) { /* jQuery plugin code referencing $ */ } )(jQuery);

The this key word in function

What will the code below output to the console and why?

var myObject = {
    foo: "bar",
    func: function() {
        var self = this;
        console.log("outer func: = " +;
        console.log("outer func: = " +;
        (function() {
            console.log("inner func: = " +;
            console.log("inner func: = " +;

The above code will output the following to the console:

outer func: = bar
outer func: = bar
inner func: = undefined
inner func: = bar

In the outer function, both this and self refer to myObject and therefore both can properly reference and access foo.

In the inner function, though, this no longer refers to myObject. As a result, is undefined in the inner function, whereas the reference to the local variable self remains in scope and is accessible there. (Prior to ECMA 5, this in the inner function would refer to the global window object; whereas, as of ECMA 5, this in the inner function would be undefined.)

use strict

Q: What is the significance, and what are the benefits, of including 'use strict' at the beginning of a JavaScript source file?

A: the short and most important answer here is that use strict is a way to voluntarily enforce stricter parsing and error handling on your JavaScript code at runtime. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions. In general, it is a good practice.

Some of the key benefits of strict mode include:

  • Makes debugging easier. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions, alerting you sooner to problems in your code and directing you more quickly to their source.
  • Prevents accidental globals. Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common errors in JavaScript. In strict mode, attempting to do so throws an error.
  • Eliminates this coercion. Without strict mode, a reference to a this value of null or undefined is automatically coerced to the global. This can cause many headfakes and pull-out-your-hair kind of bugs. In strict mode, referencing a this value of null or undefined throws an error.
  • Disallows duplicate property names or parameter values. Strict mode throws an error when it detects a duplicate named property in an object (e.g., var object = {foo: "bar", foo: "baz"};) or a duplicate named argument for a function (e.g., function foo(val1, val2, val1){}), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
  • Makes eval() safer. There are some differences in the way eval() behaves in strict mode and in non-strict mode. Most significantly, in strict mode, variables and functions declared inside of an eval() statement are not created in the containing scope (they are created in the containing scope in non-strict mode, which can also be a common source of problems).
  • Throws error on invalid usage of delete. The delete operator (used to remove properties from objects) cannot be used on non-configurable properties of the object. Non-strict code will fail silently when an attempt is made to delete a non-configurable property, whereas strict mode will throw an error in such a case.

semi-colon appended at the end of line

Consider the two functions below. Will they both return the same thing? Why or why not?

function foo1(){
  return {
      bar: "hello"

function foo2()
      bar: "hello"

Surprisingly, these two functions will not return the same thing. Rather:

console.log("foo1 returns:");
console.log("foo2 returns:");

will yield:

foo1 returns:
Object {bar: "hello"}
foo2 returns:

Not only is this surprising, but what makes this particularly gnarly is that foo2() returns undefined without any error being thrown.

The reason for this has to do with the fact that semicolons are technically optional in JavaScript (although omitting them is generally really bad form). As a result, when the line containing the return statement (with nothing else on the line) is encountered in foo2(), a semicolon is automatically inserted immediately after the return statement.

No error is thrown since the remainder of the code is perfectly valid, even though it doesn’t ever get invoked or do anything (it is simply an unused code block that defines a property bar which is equal to the string "hello").

This behavior also argues for following the convention of placing an opening curly brace at the end of a line in JavaScript, rather than on the beginning of a new line. As shown here, this becomes more than just a stylistic preference in JavaScript.

inaccurate float number

What will the code below output? Explain your answer.

console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 0.3);

An educated answer to this question would simply be: “You can’t be sure. it might print out “0.3” and “true”, or it might not. Numbers in JavaScript are all treated with floating point precision, and as such, may not always yield the expected results.”

The example provided above is classic case that demonstrates this issue. Surprisingly, it will print out:


Discuss possible ways to write a function isInteger(x) that determines if x is an integer.

This may sound trivial and, in fact, it is trivial with ECMAscript 6 which introduces a new Number.isInteger() function for precisely this purpose. However, prior to ECMAScript 6, this is a bit more complicated, since no equivalent of the Number.isInteger() method is provided.

The issue is that, in the ECMAScript specification, integers only exist conceptually; i.e., numeric values are always stored as floating point values.

With that in mind, the simplest and cleanest pre-ECMAScript-6 solution (which is also sufficiently robust to return false even if a non-numeric value such as a string or null is passed to the function) would be the following:

function isInteger(x) { return (x^0) === x; }

The following solution would also work, although not as elegant as the one above:

function isInteger(x) { return Math.round(x) === x; }

Note that Math.ceil() or Math.floor() could be used equally well (instead of Math.round()) in the above implementation.

Or alternatively:

function isInteger(x) { return parseInt(x, 10) === x; }

While this parseInt-based approach will work well for many values of x, once x becomes quite large, it will fail to work properly. The problem is that parseInt() coerces its first parameter to a string before parsing digits. Therefore, once the number becomes sufficiently large, its string representation will be presented in exponential form (e.g., 1e+21). Accordingly, parseInt() will then try to parse 1e+21, but will stop parsing when it reaches the e character and will therefore return a value of 1. Observe:

> String(1000000000000000000000)

> parseInt(1000000000000000000000, 10)

> parseInt(1000000000000000000000, 10) === 1000000000000000000000

String to Array

Write a one-line function (less than 80 characters) that returns a boolean indicating whether or not a string is a palindrome.

The following one line function will return true if str is a palindrome; otherwise, it returns false.

function isPalindrome(str) { return str.split('').reverse().join('') === str; }

For example:

console.log(isPalindrome("level"));      // logs 'true'
console.log(isPalindrome("levels"));     // logs 'false'

setTimeout and thread

In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?

(function() {
    setTimeout(function(){console.log(2)}, 1000);
    setTimeout(function(){console.log(3)}, 0);

The values will be logged in the following order:


Let’s first explain the parts of this that are presumably more obvious:

  • 1 and 4 are displayed first since they are logged by simple calls to console.log() without any delay
  • 2 is displayed after 3 because 2 is being logged after a delay of 1000 msecs (i.e., 1 second) whereas 3 is being logged after a delay of 0 msecs. OK, fine. But if 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away? And, if so, shouldn’t it be logged before 4, since 4 is being logged by a later line of code?

The answer has to do with properly understanding JavaScript events and timing.

The browser has an event loop which checks the event queue and processes pending events. For example, if an event happens in the background (e.g., a script onload event) while the browser is busy (e.g., processing an onclick), the event gets appended to the queue. When the onclick handler is complete, the queue is checked and the event is then handled (e.g., the onload script is executed).

Similarly, setTimeout() also puts execution of its referenced function into the event queue if the browser is busy.

When a value of zero is passed as the second argument to setTimeout(), it attempts to execute the specified function “as soon as possible”. Specifically, execution of the function is placed on the event queue to occur on the next timer tick. Note, though, that this is not immediate; the function is not executed until the next tick. That’s why in the above example, the call to console.log(4) occurs before the call to console.log(3) (since the call to console.log(3) is invoked via setTimeout, so it is slightly delayed).

Closure captures variable by reference

Q: Consider the following code snippet:

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', function(){ console.log(i); });

(a) What gets logged to the console when the user clicks on “Button 4” and why? (b) Provide one or more alternate implementations that will work as expected.

A: (a) No matter what button the user clicks the number 5 will always be logged to the console. This is because, at the point that the onclick method is invoked (for any of the buttons), the for loop has already completed and the variable i already has a value of 5. (Bonus points for the interviewee if they know enough to talk about how execution contexts, variable objects, activation objects, and the internal “scope” property contribute to the closure behavior.)

(b) The key to making this work is to capture the value of i at each pass through the for loop by passing it into a newly created function object. Here are three possible ways to accomplish this:

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', (function(i) {
    return function() { console.log(i); };

Or, we could replace the for loop with a call to the array object’s native forEach method:

['a', 'b', 'c', 'd', 'e'].forEach(function (value, i) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', function() { console.log(i); });

Type Conversion

What will the code below output to the console and why ?

console.log(1 +  "2" + "2");
console.log(1 +  +"2" + "2");
console.log(1 +  -"1" + "2");
console.log(+"1" +  "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);

The above code will output the following to the console:


Here’s why…

The fundamental issue here is that JavaScript (ECMAScript) is a loosely typed language and it performs automatic type conversion on values to accommodate the operation being performed. Let’s see how this plays out with each of the above examples.

Example 1: 1 + "2" + "2" Outputs: "122" Explanation: The first operation to be performed in 1 + "2". Since one of the operands ("2") is a string, JavaScript assumes it needs to perform string concatenation and therefore converts the type of 1 to "1", 1 + "2" yields "12". Then, "12" + "2" yields "122".

Example 2: 1 + +"2" + "2" Outputs: "32" Explanation: Based on order of operations, the first operation to be performed is +"2" (the extra + before the first "2" is treated as a unary operator). Thus, JavaScript converts the type of "2" to numeric and then applies the unary + sign to it (i.e., treats it as a positive number). As a result, the next operation is now 1 + 2 which of course yields 3. But then, we have an operation between a number and a string (i.e., 3 and "2"), so once again JavaScript converts the type of the numeric value to a string and performs string concatenation, yielding "32".

Example 3: 1 + -"1" + "2" Outputs: "02" Explanation: The explanation here is identical to the prior example, except the unary operator is - rather than +. So "1" becomes 1, which then becomes -1 when the - is applied, which is then added to 1 yielding 0, which is then converted to a string and concatenated with the final "2" operand, yielding "02".

Example 4: +"1" + "1" + "2" Outputs: "112" Explanation: Although the first "1" operand is typecast to a numeric value based on the unary + operator that precedes it, it is then immediately converted back to a string when it is concatenated with the second "1" operand, which is then concatenated with the final "2" operand, yielding the string "112".

Example 5: "A" - "B" + "2" Outputs: "NaN2" Explanation: Since the - operator can not be applied to strings, and since neither "A" nor "B" can be converted to numeric values, "A" - "B" yields NaN which is then concatenated with the string "2" to yield “NaN2”.

Example 6: "A" - "B" + 2 Outputs: NaN Explanation: As exlained in the previous example, "A" - "B" yields NaN. But any operator applied to NaN with any other numeric operand will still yield NaN.

Array reverse & push & concat & slice

What will the code below output to the console and why?

var arr1 = "john".split('');
var arr2 = arr1.reverse();
var arr3 = "jones".split('');
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));

The logged output will be:

"array 1: length=5 last=j,o,n,e,s"
"array 2: length=5 last=j,o,n,e,s"

arr1 and arr2 are the same after the above code is executed for the following reasons:

  • Calling an array object’s reverse() method doesn’t only return the array in reverse order, it also reverses the order of the array itself (i.e., in this case, arr1).

  • The reverse() method returns a reference to the array itself (i.e., in this case, arr1). As a result, arr2 is simply a reference to (rather than a copy of) arr1. Therefore, when anything is done to arr2 (i.e., when we invoke arr2.push(arr3);), arr1 will be affected as well since arr1 and arr2 are simply references to the same object.

And a couple of side points here that can sometimes trip someone up in answering this question:

  • Passing an array to the push() method of another array pushes that entire array as a single element onto the end of the array. As a result, the statement arr2.push(arr3); adds arr3 in its entirety as a single element to the end of arr2 (i.e., it does not concatenate the two arrays, that’s what the concat() method is for).

  • Like Python, JavaScript honors negative subscripts in calls to array methods like slice() as a way of referencing elements at the end of the array; e.g., a subscript of -1 indicates the last element in the array, and so on.

How to determine if a variable is an object?

Q: What is a potential pitfall with using typeof bar === "object" to determine if bar is an object? How can this pitfall be avoided?

A: Although typeof bar === "object" is a reliable way of checking if bar is an object, the surprising gotcha in JavaScript is that null is also considered an object!

Therefore, the following code will, to the surprise of most developers, log true (not false) to the console:

var bar = null;
console.log(typeof bar === "object");  // logs true!

As long as one is aware of this, the problem can easily be avoided by also checking if bar is null:

console.log((bar !== null) && (typeof bar === "object"));  // logs false

To be entirely thorough in our answer, there are two other things worth noting:

First, the above solution will return false if bar is a function. In most cases, this is the desired behavior, but in situations where you want to also return true for functions, you could amend the above solution to be:

console.log((bar !== null) && ((typeof bar === "object") || (typeof bar === "function")));

Second, the above solution will return true if bar is an array (e.g., if var bar = [];). In most cases, this is the desired behavior, since arrays are indeed objects, but in situations where you want to also false for arrays, you could amend the above solution to be:

console.log((bar !== null) && (typeof bar === "object") && ( !== "[object Array]"));

Or, if you’re using jQuery:

console.log((bar !== null) && (typeof bar === "object") && (! $.isArray(bar)));

Inadvertent global variable

What will the code below output to the console and why?

  var a = b = 3;

console.log("a undefined? " + (typeof a === 'undefined'));
console.log("b undefined? " + (typeof b === 'undefined'));

Since both a and b are defined within the enclosing scope of the function, and since the line they are on is preceded by the var keyword, most JavaScript developers would expect typeof a and typeof b to both be undefined in the above example.

However, the above code will actually output:

a undefined? true
b undefined? false

Why isn’t b undefined outside of the scope of the enclosing function?

The issue here is that most developers understand the statement var a = b = 3; to be shorthand for:

var b = 3;
var a = b;

But in fact, var a = b = 3; is actually shorthand for:

b = 3;
var a = b;

Therefore, b ends up being a global variable (since it is not preceded by the var keyword) and is still in scope even outside of the enclosing function.

Note: The above behavior is what occurs if you are using use strict. Without use strict both a and b will be defined in the above example (which is yet another reason you should use use strict as a matter of course in your code!).


What is NaN? What is its type? How can you reliably test if a value is equal to NaN?

The NaN property represents a value that is “not a number”. This special value results from an operation that could not be performed either because one of the operands was non-numeric (e.g., "abc" / 4), or because the result of the operation is non-numeric (e.g., an attempt to divide by zero).

While this seems straightforward enough, there are a couple of somewhat surprising characteristics of NaN that can result in hair-pulling bugs if one is not aware of them.

For one thing, although NaN means “not a number”, its type is, believe it or not, Number:

console.log(typeof NaN === "number");  // logs "true"

Additionally, NaN compared to anything – even itself! – is false:

console.log(NaN === NaN);  // logs "false"

A semi-reliable way to test whether a number is equal to NaN is with the built-in function isNaN(), but even using isNaN() is an imperfect solution.

A better solution would either be to use value !== value, which would only produce true if the value is equal to NaN. Also, ES6 offers a new Number.isNaN() function, which is a different and more reliable than the old global isNaN() function.

Curry function

Write a sum method which will work properly when invoked using either syntax below.

console.log(sum(2,3));   // Outputs 5
console.log(sum(2)(3));  // Outputs 5

Here’s the answer, followed by an explanation of how it works:

function sum(x) {
  if (arguments.length == 2) {
    return arguments[0] + arguments[1];
  } else {
    return function(y) { return x + y; };

In JavaScript, functions provide access to an arguments object which provides access to the actual arguments passed to a function. This enables us to use the length property to determine at runtime the number of arguments passed to the function.

If two arguments are passed, we simply add them together and return.

Otherwise, we assume it was called in the form sum(2)(3), so we return an anonymous function that adds together the argument passed to sum() (in this case 2) and the argument passed to the anonymous function (in this case 3).

See here for more about curry function.

What is a “closure” in JavaScript? Provide an example.

A closure is an inner function that has access to the variables in the outer (enclosing) function’s scope chain. The closure has access to variables in three scopes; specifically: (1) variable in its own scope, (2) variables in the enclosing function’s scope, and (3) global variables.

Here is a simple example:

var globalVar = "xyz";

(function outerFunc(outerArg) {
  var outerVar = 'a';

  (function innerFunc(innerArg) {
    var innerVar = 'b';

      "outerArg = " + outerArg + "\n" +
      "innerArg = " + innerArg + "\n" +
      "outerVar = " + outerVar + "\n" +
      "innerVar = " + innerVar + "\n" +
      "globalVar = " + globalVar);


In the above example, variables from innerFunc, outerFunc, and the global namespace are all in scope in the innerFunc. The above code will therefore produce the following output:

outerArg = 123
innerArg = 456
outerVar = a
innerVar = b
globalVar = xyz

When, why, and how would you use a closure in JavaScript? Provide examples.

Probably the most famous example of the use case for closures is the following code snippet:

for (var i = 0; i < 5; i++) {
  setTimeout(function() { console.log(i); }, i * 1000 );

As expected, the above code will log 5 values to the console, one every 1,000 milliseconds (i.e., one every second). However, it will not display the values 0, 1, 2, 3, and 4 as might be expected; rather, it will display 5, 5, 5, 5, and 5.

The reason for this is that each function executed within the loop will be executed after the entire loop has completed and all will therefore reference the last value stored in i, which was 5.

Closures can be used to prevent this problem by creating a unique scope for each iteration, storing each unique value of the variable within its scope, as follows:

var createFunction = function(i) {
  return function() { console.log(i); };
for (var i = 0; i < 5; i++) {
  setTimeout(createFunction(i), i * 1000 );

This will produce the presumably desired result of logging 0, 1, 2, 3, and 4 to the console.

Closures can also be used to resolve issues with the this keyword, which is unique to each scope. For example:

// Using a closure to access inner and outer object instances simultaneously.
var outerObj = {
  myName: "outer",
  outerFunction: function() {
    // Provide a reference to outerObj through innerFunction's closure
    var self = this;
    var innerObj = {
      myName: "inner",
      innerFunction: function() {
        console.log( self.myName, this.myName ); // "outer inner"
  console.log( this.myName ); // "outer"

Use Object as key of dictionay

What is the output out of the following code? Explain your answer.

var a={},



The output of this code will be


When setting an object property, JavaScript will implicitly stringify the parameter value. In this case, b and c will both be converted to Object. Because of this, a[c] will override the a[b] and set the value to 456.

Written with StackEdit.