JavaScript __TOP__ Interview Questions

What is the most efficient way to sort 100 million numbers in JavaScript?

 

or larger data set?

Things to Consider

Internal vs. External memory sorting 

• Number double precision floating point = 64 bits​

• distributed problem, multiple files

CPU intensive

• JavaScript is single threaded synchronous execution, executes code sequentially
• defer the task to external workers
• Node.js processes & CPU Cores

Sorting Algorithms Speed? Complexity

•  bucket sort, quick sort
• external merge sort

You own a website with 100 pages. How do you find all the broken links using JavaScript?

 

Things to Consider

Would you start writing from scratch?

libraries available

programming approach:

• store external links in a data structure
• recursively look all pages for the internal links and store
• check each link, once only

page scrapping with Node.js, using modules: request and cheerio

Design an OOP Car Park?

Application Architecture

Things to Consider

Classes & Objects

Class: a template used to create multiple Objects

Inheritance

Polymorphism

 

Automobile:

Car

Motorcycle

Car Parks:

Level 

Location

Space

 You are expected to understand all the OOP concepts.

Functional JavaScript, First Class Functions?

 

Recommended read:

Professor Frisby's Mostly Adequate Guide to Functional Programming

https://drboolean.gitbooks.io/mostly-adequate-guide/

First Class Functions?

can treat functions like any other data type ... they may be stored in variables, arrays, passed around, composed...

a function can contain:

-code

-name (optional)

-primitive types

-objects

-other functions

-arguments

Functional? computation as the evaluation of mathematical functions - avoids changing-state and mutable data.

Another recommended read: JavaScript Allonge

https://leanpub.com/javascriptallongesix/read

What is the difference between == and === ?

 

== checks value

 type conversion (coerces type)

 

=== strict equal

checks value and type

reference:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

 

Why use ==?

coerces type

best to avoid!

Does JavaScript pass members by value or reference ?

 

Primitives by value

 

(the value of var1 gets copied to var2)

//changing var2 does not impact var 1

 

 

 

Objects by reference (copy of reference)

 

//change(mutate) of obj2 impacts obj1

 

Explain hoisting for variables & functions?

 

Functions are hoisted to top

Variables declarations are hoisted to top (not the assignment)

 

 

 

step 1: JavaScript parser reads code and set’s memory spaces 

//similar to reading an algorithm and setting aside the requirements for space (memory) 

 

step 2: then executes the program....meaning variables and functions were already declared in step 1

 

Function closures?

 

The inner function execution context closes in the outer (lexical scope) variables and can access them

 

why important?

• create design patterns / factory pattern - a function that creates (returns) many functions /closures, module pattern.
• create private members
• function currying  

 

What JavaScript frameworks do you know and explain differences?

 

JS frameworks

 

https://github.com/showcases/front-end-javascript-frameworks

 

Why use 'use strict' ?

 

• Can help prevent errors under some circumstances
• JavaScript engine implements extra more stringent rules

 

reference:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

 

• may not implemented same by different JavaScript engines
• may create issues when concatenating JavaScript files

 

OOP JavaScript?

 

prototypal inheritance

inheritance: objects gets access to another object methods

 

prototype is a reference to proto object

each object has a proto object

MAKE OBJECTS: 

1)Function Constructors

- a function that is used to construct objects

adding new changes behavior & don’t forget to add it: new!

2)Object.Create // prototypal inheritance creates objects from other objects

3)ES6 classes, class is still an object not a template!! 

3 ways to mimic OOP classes!! ...  ... stay functional!