Javascript trick questions

There’s lot tricky test question if you go through interview, here I put some tricky and might be simple but we might not conscience that basic method and lost the point while interview:

Basic concept questions:

What does “use strict” do?

The “use strict” literal is entered at the top of a JavaScript program or at the top of a function and it helps you write safer JavaScript code by throwing an error if a global variable is created by mistake. For example, the following program will throw an error:

function doSomething(val) {
  "use strict"; 
  x = val + 10;

once you add “use strict”, it will through an error: x is not declare as a variable even though this will not cause code error once it is show undefined, and follow is correct version:

function doSomething(val) {
  "use strict"; 
  var x = val + 10;

What is the difference between == and ===?
The difference is that == performs implicit type conversion to check if values are equal to each other. So it will convert the values to true or false boolean values and then compare them. Below are some weird results of this implicit type conversion using the double equal operator:

"1" == 1 // true
true == 1 // true
[] == false // true
"" == false // true
undefined == null // true

This can really cause some problems when you’re writing JavaScript, so the safe thing to do is use the triple equal operator instead, which checks that the two values are of the same type and does not perform type conversion.

How would you check if a number is an integer?

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

What’s the difference between undefined and not defined in JavaScript

You declare JavaScript variables with the var keyword: var carname;
After the declaration, the variable is empty (it has no value).
To assign a value to the variable, use the equal sign var carname=”Volvo”;
In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input. Variable declared without a value will have the value undefined.
The variable carname will have the value undefined after the execution of the following statement: var carname;
var hoisting
In JavaScript, a variable can be declared after being used.

var x; // Declaration
typeof x === 'undefined'; // Will return true

What is “closure” in javascript? Can you provide an example?
A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.

The closure has access to the variable in three scopes:

Variable declared in his own scope
Variable declared in parent function scope
Variable declared in the global namespace

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


How to check if an object is an array or not?

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

if( === '[object Array]') {

what is function hoisting in JavaScript?
In JavaScript var-declared variables and functions are hoisted. Let’s take function hoisting first. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it’s declared.

foo(); // Here foo is still undefined
var foo = function foo() {
  return 12;
var foo = undefined;
foo = function foo() {
  // Some code stuff
foo(); // Now foo is defined here

Explain what a callback function is and provide a simple example.
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
  // then execute the callback function that was passed

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

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

And also there’s lot test javascript question quiz on line

ECMAScript 6 new features compare ECMAScript 5

This is to compare the difference between ECMAScript 6 and ECMAScript 5

Use ECMAScript 6 you could reduced lots code , but the core problem is not all the browser support, to resolve this ECMAScript 6 need to use a compiler to transform your ECMAScript 6 code to ECMAScript 5 compatible code, in this purpose you could use Babel which allowed you run in current browsers

As follow is introduced some syntax that I used often in ES5 and it’s completely shorten in ES6

Before you should know the difference between var, let and const

compare to var, let will be more strict but it also changeable in the block scope,

const in the word means it is immutable, try in browser to defined between then you could find the error notice once you had declared with let and const, this could be more easy to debug since it’s more strict compare var

you could get more introduction here

1 Constants


//  only in ES5 through the help of object properties
//  and only in global context and not in a block scope
Object.defineProperty(typeof global === "object" ? global : window, "PI", {
    value:        3.141593,
    enumerable:   true,
    writable:     false,
    configurable: false
PI > 3.0;


const PI = 3.141593
PI > 3.0

In ES6 you could defined the constant, variables which cannot be re-assigned new content, but in case the content is an object, this means the object itself can still be altered: ( follow as resign value and it show error, but property in object can be added or removed or be changed) :

2  Scoping: Block-scoped Variables

without hoisting, so you do not need to declare variable to the top of the current script or the current function


var i, x, y;
for (i = 0; i < a.length; i++) {
    x = a[i];
for (i = 0; i < b.length; i++) {
    y = b[i];

var callbacks = [];
for (var i = 0; i <= 2; i++) {
    (function (i) {
        callbacks[i] = function() { return i * 2; };
callbacks[0]() === 0;
callbacks[1]() === 2;
callbacks[2]() === 4;


for (let i = 0; i < a.length; i++) {
    let x = a[i]
for (let i = 0; i < b.length; i++) {
    let y = b[i]

let callbacks = []
for (let i = 0; i <= 2; i++) {
    callbacks[i] = function () { return i * 2 }
callbacks[0]() === 0
callbacks[1]() === 2
callbacks[2]() === 4

3 Arrow Functions, which reduced funtion expressive closure syntax


odds  = (v) { return v + 1; });
pairs = (v) { return { even: v, odd: v + 1 }; });
nums  = (v, i) { return v + i; });


odds  = => v + 1)
pairs = => ({ even: v, odd: v + 1 }))
nums  =, i) => v + i)

and so does statement bodies:

nums.forEach(function (v) {
   if (v % 5 === 0)


nums.forEach(v => {
   if (v % 5 === 0)

Lexical this


//  variant 1
var self = this;
this.nums.forEach(function (v) {
    if (v % 5 === 0)

//  variant 2
this.nums.forEach(function (v) {
    if (v % 5 === 0)
}, this);

//  variant 3 (since ECMAScript 5.1 only)
this.nums.forEach(function (v) {
    if (v % 5 === 0)


this.nums.forEach((v) => {
    if (v % 5 === 0)

3 Extended Parameter Handling
ES6 could be like other functional programme lauguage, you could simple and intuitive default values in your function parameters:


function f (x, y, z) {
    if (y === undefined)
        y = 7;
    if (z === undefined)
        z = 42;
    return x + y + z;
f(1) === 50;


function f (x, y = 7, z = 42) {
    return x + y + z
f(1) === 50

And you could use aggregation of remaining arguments into single parameter of variadic functions, aggregation is quiet useful in Reactjs


function f (x, y) {
    var a =, 2);
    return (x + y) * a.length;
f(1, 2, "hello", true, 7) === 9;


function f (x, y, ...a) {
    return (x + y) * a.length
f(1, 2, "hello", true, 7) === 9

Spreading of elements in an array or a string into both literal elements:


var params = [ "hello", true, 7 ];
var other = [ 1, 2 ].concat(params); // [ 1, 2, "hello", true, 7 ]


var params = [ "hello", true, 7 ]
var other = [ 1, 2, ...params ] // [ 1, 2, "hello", true, 7 ]

Template Literals
String Interpolation for output of defined variables, this is quiet useful once you had lots string to express, for example show error message string in JS:
instead of using “+ card.amount +” , used ${card.amount} is more readable

var customer = { name: "Foo" };
var card = { amount: 7, product: "Bar", unitprice: 42 };
var message = "Hello " + + ",\n" +
"want to buy " + card.amount + " " + card.product + " for\n" +
"a total of " + (card.amount * card.unitprice) + " bucks?";


var customer = { name: "Foo" }
var card = { amount: 7, product: "Bar", unitprice: 42 }
var message = `Hello ${},
want to buy ${card.amount} ${card.product} for
a total of ${card.amount * card.unitprice} bucks?`

Enhanced Object Properties

Property Shorthand:


var x = 0, y = 0;
obj = { x: x, y: y };


var x = 0, y = 0
obj = { x, y }

Computed Property Names


var obj = {
    foo: "bar"
obj[ "baz" + quux() ] = 42;


let obj = {
    foo: "bar",
    [ "baz" + quux() ]: 42

Class definition
like other OOP functional language, ES6 could be more intuitive, OOP-style and boilerplate-free classes, if you write php then you will love this point


var Shape = function (id, x, y) { = id;
    this.move(x, y);
Shape.prototype.move = function (x, y) {
    this.x = x;
    this.y = y;


class Shape {
    constructor (id, x, y) { = id
        this.move(x, y)
    move (x, y) {
        this.x = x
        this.y = y

New Built-In Methods

Object Property Assignment: assign()

this will return the value of assign target object

var dest = { quux: 0 };
var src1 = { foo: 1, bar: 2 };
var src2 = { foo: 3, baz: 4 };
Object.keys(src1).forEach(function(k) {
    dest[k] = src1[k];
Object.keys(src2).forEach(function(k) {
    dest[k] = src2[k];
dest.quux === 0;  === 3;  === 2;
dest.baz  === 4;


var dest = { quux: 0 }
var src1 = { foo: 1, bar: 2 }
var src2 = { foo: 3, baz: 4 }
Object.assign(dest, src1, src2)
dest.quux === 0  === 3  === 2
dest.baz  === 4

Array Element Finding
New function for finding an element in an array


[ 1, 3, 4, 2 ].filter(function (x) { return x > 3; })[0]; // 4
// no equivalent in ES5


[ 1, 3, 4, 2 ].find(x => x > 3) // 4
[ 1, 3, 4, 2 ].findIndex(x => x > 3) // 2

String Repeating


Array((4 * depth) + 1).join(" ");
Array(3 + 1).join("foo");


" ".repeat(4 * depth)

String Searching
New specific string functions to search for a sub-string


"hello".startsWith("ello", 1) // true
"hello".endsWith("hell", 4)   // true
"hello".includes("ell")       // true
"hello".includes("ell", 1)    // true
"hello".includes("ell", 2)    // false


"hello".indexOf("ello") === 1;    // true
"hello".indexOf("hell") === (4 - "hell".length); // true
"hello".indexOf("ell") !== -1;    // true
"hello".indexOf("ell", 1) !== -1; // true
"hello".indexOf("ell", 2) !== -1; // false

Number Truncation
Truncate a floating point number to its integral part, completely dropping the fractional part.

function mathTrunc (x) {
    return (x < 0 ? Math.ceil(x) : Math.floor(x));
console.log(mathTrunc(42.7)) // 42
console.log(mathTrunc( 0.1)) // 0
console.log(mathTrunc(-0.1)) // -0


console.log(Math.trunc(42.7)) // 42
console.log(Math.trunc( 0.1)) // 0
console.log(Math.trunc(-0.1)) // -0

lazy load js

what is lazy load, the basic concept is spped up page loading time especially it’s comes to lots images

There’s some library to use for, like echo.js
or beLazy.js

Here we introduced lazy-load-xt


Simply it does support: css animation like transitions affect, it is support scrollable containers like IFRAME contend, and more other media like videos, and even no script support ( disable js support)

well, it is just more powerful, but if you simply just want load images faster you could consider just use upper introduced library

But remember it is has to user jQuery 1.7

How to use?

1 Installation:

You could install by node js or bower

$ npm install lazyloadxt
$ bower install lazyloadxt

or simply use this cdn

2 add html tag to called the image src, or you could defined a variable to call image src


3 Then defined image size or any css style for layout

4 That’s it you should be able see the image load

and here has so API to play with

$.extend($.lazyLoadXT, {
  edgeY:  200,
  srcAttr: 'data-src'



code play here

ResfulAPI, what’s all about?

downloadWhat is ResfulAPI?

Actually it does get bit time for me to understand what is restful API, I mean I though I do understand it, since use post, get, update, delete action, but actually not at all. It’s not only about the action, it’s also about the whole react concept design pattern after those action. Haha, still not get it right? me too!!

Well, that’s better describe like this:


1.”Browser” == “client”,

2.”Server” == “shop”,


Client ask shop : ” please give me Black chocolate”,

shop receive from Client: “ok, got your message, I will send you Black chocolate”


shop send client Black chocolate

You could see through this scenario, there’s has roles, verb(ask, receive), result

those all play drama called ” Restful API”

Got it a bit more? in big view, Restful API is Representation State Transfer, it’s about the architectural style, there’s no standard of how you build your restful API ( like if you write your drama there should be no standard pattern, right?), it’s all about the action between Browser | Server, and how you communicate with for the object you want get.

Here is sample graphic from above drama:

CrunchifyGot more clarify? So RestfulAPI is this drama whole picture gathers by: Browser| Web Server, action by method: GET, POST, PUT, DELET etc, and Browser will react the result to the web page.

When I finally get this concept, then you will see the key point is to communicate between Browser | Web Server, so what is this tool? Mostly we use http this media to connect both. So we got to finger out what is http request


What is http request?

This is like how to design method of communication, we could send the specific request and get the specific product we want directly or we could send the the common request and retrieve one of the product in the whole list. Depend on your request and also depend on how complexity of list products. Well, like the way we want range book category in our bookshelf

But in terms we could look at this way:

Nouns: like whole collects of our books, our resource, in http, we said this is like URL, for example

http: //{list}

Verbs: this is the way we act to book from our bookshelf, you could get the book, remove the book, in http, we use GET, POST, PUT,DELET.

Content types: The book info like book author name, editor, publish date. in http, we show this information with XML, JSON, YAML format.

Here is the graphic to show the whole concept:


So if I want book 1 , the URL could be like this to get book 1: http: //,

If I want the author info, publish date from book 1, the URL could be like this: http: //

Here we could get authorInfo : “John”, and publish date: “2016”

in JSON format will be looks like this:



“authorInfo” : “John”,

“publishDate”: “2016”



In this URL, we could use action by GET or POST to get those information. And different between GET & POST? you could look at here.

And action should be return back status, right?

So http has status code to show return status:

200: ok,

404: Not Found,

500: internal Server Error

Here’s link you could look all the different status


How do you look those API through web browser?

Open web browser, F12, you could click Network and see the api run through web event binding:

CaptureFor example here, the resource(nouns) is :

method is get

content type resource is: 376?model=touareg&zip=55344, which is json format:

CaptureAnd how will this information show in our final destination: web browser?

You could use any language, here I use javascript:

  1. first load this resource
  2. second define the condition, if process 1 is success, we need to get offers list deal, and get title: “499/month for 36 months.”
  3. if process 1 is not success, then web browser show show status
  4. And final show the result in web site

The concept code will be like this:

$('#main-menu a').click(function(event) {
   $.ajax(this.href, {
      success: function(data) {
      error: function() {
         $('#notification-bar').text('An error occurred');
         //console.log("fn xhr.status: " + xhr.status);

So the final expectation is show the title: 499/month for 36 months on the browser web page.


This is just basic concept, it has also lots method to use, Rest data elements, connectors, components. This could be description in whole book, but I think the basic Restful API concept is keep simple, cause original ideal is reduce communication work between front & backend. So if not keep it simple, then what does make sense to use it?

But it does true, keep it simple could be the most difficult part!! I am new in this too, so any error or lack of sense part please feedback as much as you could. thank you!!