Oleksandr Gavenko's blog
2017-01-11 22:30 JavaScript interview questions 1

It's time for some common JS interview questions.

What is a difference between a function expression and a function declaration

function fdecl() { return -1; }

var fexpr = function() { return -1; };

Context and this keyword

var Obj = {
  field: 1,
  func: function() { return this.field; }

 console.log(Obj.func() === 1);  // this is Obj

 var f = Obj.func;
 console.log(f() === undefined); // this is Window

 var solution = Obj.func.bind(Obj);
 console.log(solution() === 1);   // this is Obj

Bind shim and currying

Function.prototype.bind = Function.prototype.bind || function(context) {
  var self = this;
  return function(){
    return self.apply(context, Array.prototype.slice.call(arguments));

Function.prototype.curry = function() {
  var self = this;
  var oldargs = Array.prototype.slice.call(arguments);
  return function() {
    var newargs = Array.prototype.slice.call(arguments);
    return self.apply(this, oldargs.concat(newargs));

Make a function that add a space between each character in a string

function spacify(str) {
  return str.split('').join(' ');

spacify('hello world');

For "inline" syntax:

String.prototype.spacify = function(){
  return this.split('').join(' ');

'hello world'.spacify();

Palindrome check function

function isPalindrome(str) {
  str = str.replace(/\W/g, '').toLowerCase();
  return str == str.split('').reverse().join('');

Example of usage:

console.log(isPalindrome("level"));                   // logs 'true'
console.log(isPalindrome("levels"));                  // logs 'false'
console.log(isPalindrome("A car, a man, a maraca"));  // logs 'true'

Where can you put your javascript script tags and why?

<html><head><script src="..."></script></head>...<html>

Page rendering stopped until script has been fully loaded, parsed and executed. Because there is no any page content loaded till this moment user will look to empty page during delay.

<html><head>...</head><body>...<script src="..."></script>...</body><html>

Page rendering stopped until script has been fully loaded, parsed and executed. Because some page content loaded and partially rendered user will look to partially rendered page during delay.

But if <script src="..."></script> is at the end of HTML document most of the HTML will be parsed and rendered.

use strict

  • Prevents accidental globals. For example function() { var a = b = 1; } fail because b is global...

  • Disallows duplicate property names or parameter value, like {foo: "bar", foo: "baz"}.

  • Throws error on delete on non-configurable properties of the object.

Implicit semicolon

Each operator ended with implicit semicolon. So this is:

function() {

equals to:

function() {

null vs undefined

undefined appear in cases:

  • Lookups of non-existent properties in an object.

  • Function parameters that have not passed.

  • A variable is declared without assigning any value to it.

  • Implicit returns of functions because of missing return statements.

  • Return statements that do not explicitly return anything.

  • Anything that has been set to the value undefined.

  • Any expression in the form of void(expression).

  • The value of the global variable undefined.

Except for cases of lookup of non-existent properties in object and missing parameters in function call encountering undefined usually represents an error in code.

On other hand null represents "missing value" semantic by desing.

== vs ===

== performed with type conversion of operands. === performed with respect of type of both operands.

Find max value in array

Math.max.apply(null, arr);
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