JavaScript Interview

Question 1

What is Scope?

Answer 1

Refers to the availability of variables in our code

Question 2

What is the scope of Var?

Answer 2

function scoped

Question 3

What is the scope of let and const ?

Answer 3

block scoped

Question 4

How are var declarations hoisted?

Answer 4

var declarations when hoisted they are initialized with undefined.

Question 5

How are let and cont declarations hoisted?

Answer 5

when hoisted they are not initialized with undefined.

So, this would trigger a reference error if accessing before declaration.

Question 6

What is Hoisting?

Answer 6

Hoisting means moving variable declarations to the top of their scope.

Question 7

Can you redeclare variables with the let keyword

Answer 7

No, this is not possible.

Error: Syntax Error: Identifier ‘name’ has already been declared

Question 8

What is Lexical Scoping?

Answer 8

means that a variable defined outside a function can be accessible inside another function defined after the variable declaration.

‘this’ refers to it’s current surrounding scope and no further.

Question 9

What is important to remember about Scope and nested functions?

Answer 9

Nested function have access to variables declared in their own scope as well as variables declared in the outer scope

Question 10

What are Closures?

Answer 10

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment).

In JavaScript, closures are created every time a function is created, at function creation time.

Question 11

What is Function Currying?

Answer 11

Currying is a process in functional programming in which we transform a function with multiple arguments into a sequence of nesting functions that take one argument at a time

function fn(a,b,c) transformed into fn(a)(b)(c)

Question 12

What is ‘this’?

Answer 12

The JavaScript this keyword refers to the object it belongs to. You can set it with bind() among others.

‘this’ keyword is the execution context for a function call

It allows you to introduce reusability by allowing this value to be dynamic based on how a function is invoked

Question 13

What is default binding

Answer 13

If call, apply, bind are not set, JS will default to the global scope and set ‘this’ keyword to the window object.

Question 14

Using implicit Binding, how do we know what ‘this’ is referencing?

Answer 14

When the function in invoked with the dot notation, the object to the left if that dot is what the this keyword is referencing.

Ex. person.sayMyName( )
‘this’ keyword is referencing the ‘person’ object

Question 15

What is the difference between call and bind methods

Answer 15

instead of invoking the function, Bind returns a new function that you can invoke whenever you wish

Question 16

What do you know about arrow functions?

Answer 16

Introduced in ES6
Concise Function syntax
Provides Lexical binding
Can’t be called with new keyword

Arrow functions do not bind their own “this” , instead, they inherit the one from the parent scope, which is called “lexical scoping”

Question 17

What are Higher Order Functions?

Answer 17

This is a function that can take in another function

Ex. The .map() function is a higher order function that will run a callback on every item in the array and will push each result into a new array.

Question 18

What are first class functions?

Answer 18

function that was built with the intention of being passed around to other functions. It does one specific thing, does not have side effects, and is not intended to be called directly, but rather, to be used by ‘other functions.’

Question 19

What are Callbacks?

Answer 19

is a function passed into another function as an argument to be executed later.

Question 20

What does the new keyword do?

Answer 20

• creates a new empty object { }
• sets the value of ‘this’ to be the new empty object
• calls the constructor method

Question 21

What is a promise

Answer 21

A promise is an object that may produce a single value some time in the future : either a resolved value, or a reason that it’s not resolved
Pending, initial state, fulfilled, rejected

Question 22

What are the differences between == and === ?

Answer 22

a===b means that a and b are equal in both value and type.

a==b means that they are equal in value only.

Question 23

What is the difference between null and undefined.

Answer 23

Undefined means a variable has been initialized but not been assigned a value;
//var initialized
let a;
//this log will give undefined
console.log(a)

null is an assignment

//var initialized and assigned to null
let a = null;
//log will give null
console.log(a)

Question 24

Why use Promises?

Answer 24

Promises help us deal with asynch code in a far more simpler way compared to callbacks

Question 25

What two methods/functions does the Promise Object provide access to?

Answer 25

.then() - called with promise.then()
.catch() - called with promise.catch()

And you would chain these together

Question 26

What is Promise.all()

Answer 26

Allows us to query multiple APIs and preform some actions but only after all the APIS have finished loading

If one promise rejects, promise.all() will reject with an error message.

Question 27

What are Classes?

Answer 27

Classes are like a blue print. Ex. Class could be an email for several users.

Every user has a different value for an email and email is the class.

Question 28

What is a constructor function?

Answer 28

Creates new objects based upon the Class