Hire the Best JavaScript Developers In 72 Hours

JavaScript was invented around 30 years ago by Brendan Eich. Since then, many frameworks and libraries have been built on top of it. Today, it is the most widely used programming language across the world. It is supported across platforms - laptops, phones and tablets. JavaScript can be used on the client side as well as server side.

JavaScript developers are highly versatile and a great addition to your developer team. If you are looking to hire JavaScript developers, you’re at the right place. Below, we have put together all the details you need to know about hiring a JS dev.

• 1. Let's introduce JavaScript development to you
• 2. Why is JavaScript development widespread?
• 3. What are the tools used by JavaScript developers?
• 4. Writing the job description to hire JavaScript developers
• 5. Interview questions to hire JavaScript developers
• Basic Questions
• Advanced Questions
• Data Structures/Algo Questions

1. Let's introduce JavaScript development to you

• Origin Story: Brendan Eich invented JavaScript close to 30 years ago, in 1995. Initially, JavaScript was developed for Netscape 2. Later, in 1997, it became the ECMA-262 standard. Then, Mozilla worked on developing JavaScript to be used in the Firefox browser. In the same year, Internet Explorer was the first browser to support the first version of JavaScript, ECMA-262 Edition 1.
• Widely popular: According to Statista, JavaScript is the most used programming language in the world. More than 64% of all developers globally make use of this language.
• Versatility: JavaScript is a highly versatile programming language. Knowledge of JavaScript makes it easier to pick up popular front-end as well as back-end frameworks like Angular, React and NodeJS.

2. Why is JavaScript widespread?

• Completely customizable: JavaScript alone can be used in combination with HTML/CSS to build a simple, yet fully functioning application.
• Open community and standards: Having been around for so many years, the JavaScript community is huge. Clear documentation is available which makes it easy to solve challenges.

3. What are the tools used by JavaScript developers?

These are some of the commonly used JavaScript developer tools:

1. Code Editors

• Atom: Atom is a code editor with a lot of intuitive features that make writing JavaScript code easy. Developers can download a variety of plugins to improve their experience.
• Visual Studio Code: This tool includes inbuilt JS debugging, code navigation, formatting, and other useful features which make the JavaScript coding experience easy and fruitful.

2. Package Manager - npm

npm or Node Package Manager is a tool that puts modules in order so that these become easy to find and any dependency conflicts are easily managed.

3. Task Runner - Gulp

This is a cross-platform streaming task runner using which developers can automate a lot of tasks, reducing development time.

4. Module Bundler - Webpack

Webpack is a free, open-source module bundler. It bundles JavaScript files for usage in browsers. It generates a single or few files from a large number of files that are used to then run the app.

5. Code Analysis - ESLint

ESLint is a JavaScript linter that statically analyzes the written code to find any problems present in it. This helps JavaScript developers to make the code more consistent and avoid any bugs.

6. Testing - Mocha

Mocha is a simple and easy-to-use JavaScript testing framework, mainly for Node.js programs. It has features like browser support, async testing, and coverage reports.

4. Writing the job description to hire JavaScript developers

Given below are key points that we at Flexiple have learned through trial and error - a process of filtering through over 15,000 developers. You can use these in your process to hire JavaScript developers.

• No. of years of software development experience: Be specific about the technology you’re looking for and the number of years of experience needed. The number of production apps worked on can also be used as a proxy to this parameter.
• Type of products built in the past: Based on the type of product you are trying to build - social network, SAAS product, etc. - you can prioritize certain developers over others
• Industry-specific exposure: JavaScript developers who have built products in certain industries have better knowledge of the key aspects of laying the foundation for a great product. This makes prediction of possible problems easier.
• Experience working remotely: Hiring someone who has worked remotely in the past is an added benefit as they have front-hand experience of everything right from how the contracts work to communication and from timelines to deliverables.
• Strong communication: A vital thing to look at when hiring a freelancer is excellent communication skills. Since all interactions happen online, a freelancer should be able to effectively communicate to ensure a streamlined execution of all work

5. Interview questions to hire JavaScript developers

Now that you have prepared a quality JD, it can still be tricky to evaluate the skills of your applicants when you hire JavaScript developers. To help you with that, we have created a pool of questions that a good JavaScript developer should be comfortable with.

Note that the ability to answer these questions doesn't imply that you have a top quality candidate. But, it definitely is a big step in that direction.

To help you navigate through these questions, we’ve categorized the interview questions in 3 parts:

A. Basic concepts: Includes all basic concepts used across languages. This will give you an understanding of how strong their programming foundation is.

B. Advanced concepts: Includes all concepts that someone with higher expertise should know.

C. DS/Algorithm questions: To test the logical capability of the candidate.

A. Basic concepts

class ValidParenthesesFunc {
    func isValid(_ s: String) -> Bool {
        var sta = [Character]()

        for char in s {
            if char == "(" || char == "[" || char == "{" {
                sta.append(char)
            } else if char == ")" {
                guard sta.count != 0 && sta.removeLast() == "(" else {
                    return false
                }
            } else if char == "]" {
                guard sta.count != 0 && sta.removeLast() == "[" else {
                    return false
                }
            } else if char == "}" {
                guard sta.count != 0 && sta.removeLast() == "{" else {
                    return false
                }
            }
        }
        return sta.isEmpty
    }
}

The above code will input 0(false). 

It is JSON which stands for Javascript Object Notation. It is a lightweight data interchange format. It is easy for humans to read. Even though it has Javascript in its full form, it is language independent.

It’s built on 2 structures:

• Collection of key-value pairs. This is called an object, dictionary, hashmap, associative array, in different languages
• Ordered list of values, This is called an array, list, vector, in different languages

These are universal data structures which is important for a file format which is language independent.

Example of JSON:

{
  "flexiple": {
      "top": "1%",
      "location": "remote"
  },
  "blog": {
    "topic": "engineering",
    "title": "What are stable coins and how do they work?"
  }
}

This in XML would look like this:

<?xml version="1.0" encoding="UTF-8"?>
<root>
   <blog>
      <title>What are stable coins and how do they work?</title>
      <topic>engineering</topic>
   </blog>
   <flexiple>
      <location>"remote"</location>
      <top>1%</top>
   </flexiple>
</root> 

Valid JSON

Few of the rules to remember while handling JSON are that:

• Empty objects and arrays are okay
• Strings can contain any unicode character, this includes object properties
• null is a valid JSON value on its own
• All object properties should always be double quoted
• Object property values must be one of the following: String, Number, Boolean, Object, Array, null
• Number values must be in decimal format, no octal or hex representations
• Trailing commas on arrays are not allowed

These are all examples of valid JSON: -

{"name":"John Doe","age":32,"title":"Vice President of JavaScript"}

["one", "two", "three"]

// nesting valid values is okay
{"names": ["John Doe", "Jane Doe"] }

[ { "name": "John Doe"}, {"name": "Jane Doe"} ]

{} // empty hash

[] // empty list

null

{ "key": "\uFDD0" } // unicode escape codes

These are all examples of bad JSON formatting:-

{ name: "John Doe", 'age': 32 } // name and age should be in double quotes

[32, 64, 128, 0xFFF] // hex numbers are not allowed

{ "name": "John Doe", "age": undefined } // undefined is an invalid value

// functions and dates are not allowed
{ "name": "John Doe",
  "birthday": new Date('Fri, 26 Jan 2019 07:13:10 GMT'),
  "getName": function() {
      return this.name;
  }
}

Javascript provides 2 methods to encode data structures to JSON and to convert JSON to objects or arrays. These methods are JSON.stringify() and JSON.parse().

JSON.stringify()

It takes a Javascript object or array as input and returns a serialised string form of it.

const obj = {
    name: "Flexiple",
    city: "Bengaluru"
}

const jsonStr = JSON.stringify(obj)
console.log(jsonStr)
// output is {"name":"Flexiple","city":"Bengaluru"}

JSON.stringify() can take 3 arguments in total, the first one being the data structure.

The second argument, called the replacer parameter, can either be a function or an array. It is used to manipulate the properties (keys and values) in JSON.

let cost = {
    candy: 5,
    bread: 20,
    cheese: 100,
    milk: 15
}

let func = (key, value) => {
    if (value < 15) {
        return undefined
    }
    return value
}

let arr = ['candy', 'bread']

let jsonStrWithFunc = JSON.stringify(cost, func)
console.log(jsonStrWithFunc)
// Output is {"bread":20,"cheese":100,"milk":15}

let jsonStrWithArr = JSON.stringify(cost, arr)
console.log(jsonStrWithArr)
// Output is {"candy":5,"bread":20}

The third argument, called the space parameter, takes in either a number or a string. It is used to control the spacing of the final string by deciding the size of indentation.

let cost = {
    candy: 5,
    bread: 20,
    cheese: 100,
    milk: 15
}

let func = (key, value) => {
    if (value < 15) {
        return undefined
    }
    return value
}

let arr = ['candy', 'bread']

let jsonStrWithFunc = JSON.stringify(cost, func)
console.log(jsonStrWithFunc)
// Output is {"bread":20,"cheese":100,"milk":15}

let jsonStrWithArr = JSON.stringify(cost, arr)
console.log(jsonStrWithArr)
// Output is {"candy":5,"bread":20}

JSON.stringify() is also popularly used for debugging, as trying to see a JSON object using console.log() isn’t always successful

const obj = {
    "nest1": {
        "ex": "ex",
        "nest2": {
            "nest3": {
                "ex": "ex",
            }
        }
    }
}
  
console.log(obj)
// Output is { nest1: { ex: 'ex', nest2: { nest3: [Object] } } }

As we can see that after a certain level of nesting the log just displays ‘[Object]’. Therefore, JSON.stringify() can be used to make debugging easier.

const obj = {
    "nest1": {
        "ex": "ex",
        "nest2": {
            "nest3": {
                "ex": "ex",
            }
        }
    }
}
  
console.log(JSON.stringify(obj,null,2))
// Output is 
// {
//     "nest1": {
//       "ex": "ex",
//       "nest2": {
//         "nest3": {
//           "ex": "ex"
//         }
//       }
//     }
// }

JSON.parse()

It takes a JSON string and converts it into its respective data structure

 const jsonStr = '{"name":"Flexiple","city":"Bengaluru"}'
const obj = JSON.parse(jsonStr)

console.log(obj)
// output is { name: 'Flexiple', city: 'Bengaluru' }

Calling JSON.parse() on invalid JSON string will throw a SyntaxError.

Cloning Objects in Javascript

These 2 functions of JSON can be used together to clone javascript objects.

const obj = {
    "prop1": {
        "ex": "ex",
        "prop2": {
            "ex": "ex"
        }
    }
}

const objCopy = JSON.parse(JSON.stringify(obj))
  
console.log(objCopy)
// Output is { prop1: { ex: 'ex', prop2: { ex: 'ex' } } }

But this is not recommended, because of multiple reasons:

• It is slower than alternatives
• This works properly only for valid JSON, but not all javascript objects fall in that category

A better alternative for deep cloning objects would be to either use an npm library called ‘lodash’ or write a custom recursive function to do the same. The spread operator can be used for shallow cloning of objects.

B. Advanced concepts

Linting is the automatic checking of source code for programmatic and stylistic errors. Programmatic errors are the errors such as compilation and run time errors. Each company decides to write their code in a certain style and format, this is done for easy maintenance and to provide further structure to the code.

Stylistic errors are those which might compile without an issue but it does not meet the standards set by the company. Apart from stylistic checks, it is able to identify certain types of bugs like those related to variable scope, undeclared variables, global variables and so on.

Linting is done using a lint tool or linter, which is basically a static code analyzer. It is important as it reduces errors and increases the standard of the code. It can greatly benefit early stages of development and can save time during code reviews and code maintenance.

ESLint

ESLint is one the most popular lints available for Javascript. Let us go into the basics of using ESLint. This is going to the most basic version of ESLint, remember you can install it with different code style plugins as per your choice.

• After setting up your project (that is running ‘npm init’), run ‘npm install eslint --save-dev’.
• After that we run ‘npx eslint --init’ this is used to generate the ‘.eslintrc.json’ file which is the config file for eslint.
• When we are generating a the config file, it asks a bunch of questions to generate it, these are what I’ve chosen:

√ How would you like to use ESLint? · style
√ What type of modules does your project use? · commonjs
√ Which framework does your project use? · none
√ Does your project use TypeScript? · No / Yes
√ Where does your code run? · node
√ How would you like to define a style for your project? · guide
√ Which style guide do you want to follow? · standard
√ What format do you want your config file to be in? · JSON 

• It might have some peer dependencies that it wants you to install - install them too.
• Now we come to using the linter. We run ‘npx eslint app.js’ (here app.js is the source js file).

My app.js file looks like this:

const obj = {
    "prop1": {
        "ex": "ex",
        "prop2": {
            "ex": "ex"
        }
    }
}

const objCopy = JSON.parse(JSON.stringify(obj))
  
console.log(objCopy)

And the output after running ‘npx eslint app.js’ is:

2:1   error  Expected indentation of 2 spaces but found 4   indent
   2:5   error  Strings must use singlequote                   quotes
   2:5   error  Unnecessarily quoted property 'prop1' found    quote-props
   3:1   error  Expected indentation of 4 spaces but found 8   indent
   3:9   error  Unnecessarily quoted property 'ex' found       quote-props
   3:9   error  Strings must use singlequote                   quotes
   3:15  error  Strings must use singlequote                   quotes
   4:1   error  Expected indentation of 4 spaces but found 8   indent
   4:9   error  Strings must use singlequote                   quotes
   4:9   error  Unnecessarily quoted property 'prop2' found    quote-props
   5:1   error  Expected indentation of 6 spaces but found 12  indent
   5:13  error  Strings must use singlequote                   quotes
   5:13  error  Unnecessarily quoted property 'ex' found       quote-props
   5:19  error  Strings must use singlequote                   quotes
   6:1   error  Expected indentation of 4 spaces but found 8   indent
   7:1   error  Expected indentation of 2 spaces but found 4   indent
  11:1   error  Trailing spaces not allowed                    no-trailing-spaces
  12:21  error  Newline required at end of file but not found  eol-last

✖ 18 problems (18 errors, 0 warnings)
  18 errors and 0 warnings potentially fixable with the `--fix` option.

Some of the errors can be fixed using the ‘--fix’ flag, so run ‘npx eslint app.js --fix’. This is how my app.js changed after that:

const obj = {
  prop1: {
    ex: 'ex',
    prop2: {
      ex: 'ex'
    }
  }
}

const objCopy = JSON.parse(JSON.stringify(obj))

console.log(objCopy)

Now it does not show any errors on running ‘npx eslint app.js’.

Similarly ESLint can be used to either check syntax, fix bugs, enforce code style and it has a lot of different options to choose from. It supports plugins and allows custom rules also. Apart from ESLint, some of the other popular lints for Javascript are JSLint and JSHint.

Cookies and local storage are primarily used for session management in web application development. What this means is that once the user is authenticated, there needs to be a way for the application to remember the user for a period of time without asking the user to login again.

In an architecture design it is standard to keep the server stateless, but this would mean that the user information cannot be stored on the server. Therefore, the decision was taken to store it on the client. This was originally done using cookies. Cookies are basically a storage facility with the browser, which the client side javascript or the server (using headers) can interact with.

Each item stored in cookies is called a cookie, and each cookie has an expiration time which can be manually set too. The cookie persists in the browser for that duration and is not removed by page refreshes or window being shut. When a client interacts with a server using HTTP requests, then the cookies are also sent along in headers.

The data which is stored in a cookie is generally a token such as a JSON Web Token (JWT). JWT consists of a payload which is used to fetch information about the user. JWT tokens are signed on the server using a secret key. The routes allow only authorized users to check whether the token is present in the cookies. This is the outline of the architecture followed for session management.

Difference between Cookie and Local Storage 

Primarily both function in similar ways - i.e. both involve persistent storage in the browser. The differences come in slight nuances of their functioning:

• Local storage does not have the concept of expiration time. So, the developer dealing with it needs to handle the expiration of tokens stored in it. Whereas, the expiration time for cookies can be set while storing the cookie and the browser handles the rest.
• Local storage is not sent with every HTTP request, like a cookie is sent. This reduces load on especially those HTTP requests which are public and do not require to use the token stored.
• The server can directly interact with cookies, whereas only the client side script can interact with local storage.
• Local storage has a much larger storage capacity of 5mb compared to 4kb of cookie. This is primarily because cookies are meant to be read by the server, whereas local storage is meant for the client side to be able to persistently store data.

Vulnerabilities

Local storage

Local storage is accessible only by client-side javascript code. This makes it particularly vulnerable to XSS attacks. XSS (Cross-Site Scripting) attacks are a type of injections in which malicious scripts are injected into otherwise trusted websites due to a vulnerability.

This script then executes on the client’s browser and can access the local storage quite easily. XSS attacks are mainly used to steal cookies, session tokens and other information. After obtaining session tokens they can access protected routes using it. Therefore, storing authentication tokens on local storage is very risky.

Cookies

Cookies function similarly as they can also be accessed by client-side javascript code. They are also vulnerable to XSS attacks, but there can be a further layer of protection added to prevent this.

If the ‘httpOnly’ flag is marked as true while setting cookies then client side javascript cannot access that cookie. This allows only the server side code to interact with it. So this protects it from XSS attacks. However, due to its property of sending a cookie on every request, it gets vulnerable to CSRF/XSRF attacks.

CSRF/XSRF (Cross-Site Request Forgery) attacks are when malicious web apps can influence the interaction between a client browser and a web server that trusts the browser. Therefore, further measures need to be taken like using CSRF tokens and proper use of the Same-Site cookie attribute.

C. Data Structure/ Algorithm

class ValidParenthesesF {
    func isValid(_ s: String) -> Bool {
        var stg = [Character]()

        for char in s {
            if char == "(" || char == "[" || char == "{" {
                stg.append(char)
            } else if char == ")" {
                guard stg.count != 0 && stg.removeLast() == "(" else {
                    return false
                }
            } else if char == "]" {
                guard stg.count != 0 && stg.removeLast() == "[" else {
                    return false
                }
            } else if char == "}" {
                guard stg.count != 0 && stg.removeLast() == "{" else {
                    return false
                }
            }
        }
        return stg.isEmpty
    }
}

The above code will input 0(false). 

var p = 2;
var q = 4;
var r = 6;
if (p > q > r)
     document.write("true");
else
    document.write("false");

The answer is False. It may look like the output can be true because 6 > 4 > 2 is true, but PHP evaluates $z > $y 
first, which returns a boolean value of 1 or true. This value (true or 1) is compared to the 
next integer in the chain, bool(1) > $z, which will result in NULL and echo “false.”

Hire JavaScript developers - parting thoughts

This is everything you need to know about hiring a freelance JavaScript developer. As mentioned, it isn't easy to find a quality freelance JavaScript developer but this guide makes the process easier for you. To offload the entire hiring process, reach out to us at Flexiple. We've designed a high-quality, 6-step screening process to find the top 1%, freelance developers. You can find the best freelance JavaScript developers here. We've already served over a hundred clients, earning great reviews for the quality of service.

Lastly, to quickly summarize the process to hire JavaScript developers for you:

• Note your project requirements and hire accordingly. Do not go for the lowest or the highest-paid developer.
• Don’t hire without vetting- consider asking questions right from the basics to advanced to logical questions.
• Look for companies like Flexiple that help you find the perfect fit.

Happy hiring! :)