JavaScript: Arrays vs. Sets - The difference

The Set object type was introduced in the 2015 ECMAScript specification and is ready to be used in Node.js and most browsers.

Sets are a lot like Arrays, but a bit different. This article explores these differences and explains when one should be used over another. Let’s take a look.

Sets, the New Kid on the Block

Sets are a special object type available in ES6. You can create an empty set like this:

const characters = new Set()

Or, you can pass an iterable into the Set’s constructor to populate it. An iterable is just something that can be looped through, like an Array or a String.

const characters = new Set(['Rod', 'Todd', 'Sherri', 'Terri'])

Arrays, the Trusty Workhorse

Arrays are a major building block in most JavaScript applications, both old and new. If you have written JavaScript before, you are probably already familiar with them. You can create an array like this:

const characters = ['Rod', 'Todd', 'Sherri', 'Terri']

So What’s the Point?

These two data types are similar but are meant to do slightly different things. A set is designed to represent a collection of unique items whereas an array is a bit more general purpose.

A good example of something that could be represented as a Set would be the courses that a college student takes in a single semester. They can take one or more courses, but they can’t take the same course more than once in a semester.

const courses = new Set(['English', 'Science', 'Lego Robotics'])

On the other hand, a collection of Pokemon cards would not be a good use case for a Set because it could contain duplicates. In this case, an Array would be a better way to represent the data.

const cards = [

Duplicates can be passed into a Set, but they won’t be preserved. Copy the following code into your browser console and see for yourself:

new Set([
// Set(4) {"Machop", "Diglett", "Charmeleon", "Squirtle"}

The array passed into the set contained two Machops, but the set only retains one single copy. This behavior is subtle, but very useful.

How Can This Be Used?

Imagine that you are developing a blog and want to create a feature that allows visitors to search for posts that match one or more categories. Each category should only be applied once.

If you are using an Array to represent the list of active categories, you need to take care to avoid duplicates. This could be done by checking that the list does not already contain the category being added.

The indexOf or includes methods could be used to do this:

// If our list does not include the category
if (!list.includes(category)) {
  // Then add the new category to the list

I used to find myself writing this kind of code all the time, but Sets can be used to handle this problem automatically. You can simply use the add method and the Set will always remain unique.

// Just add the category to the list
// No need to perform any checks in advance!

Converting a Set Back to an Array

We already saw that an Array can be converted to a Set by passing the Array into the Set’s constructor, but how can a Set be converted to an Array?

One option is to call the Array from method statically:

const set = new Set(['Casablanca', 'The Wizard of Oz', 'Jaws'])
const arr = Array.from(set)
// (3) ["Casablanca", "The Wizard of Oz", "Jaws"]

The ES6 spread operator is another option:

const set = new Set(['Casablanca', 'The Wizard of Oz', 'Jaws'])
const arr = [...set]
// (3) ["Casablanca", "The Wizard of Oz", "Jaws"]

Sets do not support functional programming methods like map, filter, and reduce so it’s often convenient to convert them to Arrays for processing.

Using Sets to Remove Duplicates From an Array

Even if you prefer to hold your data in Arrays, Sets can still be helpful. A handy trick for removing duplicates from an Array is to convert it to a Set and then convert it back.

const cards = [
const uniqueCards = [ Set(cards)]
// (4) ["Machop", "Diglett", "Charmeleon", "Squirtle"]

How Do Sets Know Which Values Are Unique?

So far we’ve seen how Sets only hold unique values, but how exactly are unique values determined? Let’s play around a bit and find out.

First, let’s add the value 3 to a set twice:

new Set([1, 2, 3, 4, 3])
// Set(4) {1, 2, 3, 4}

The second 3 disappears. This is consistent with what we’ve learned so far, but what if that last 3 is added as a string instead?

new Set([1, 2, 3, 4, '3'])
// Set(5) {1, 2, 3, 4, "3"}

Interesting. The Set considers 3 to be different than '3'. What if we add matching arrays to a set?

new Set([['Jesse Pinkman'], ['Jesse Pinkman']])
// Set(4) {['Jesse Pinkman'], ['Jesse Pinkman']}

In this case, the Set retains two Arrays that have the same contents… What about objects?

new Set([{name: 'Ron Burgundy'}, {name: 'Ron Burgundy'}])
// Set(2) {{name: 'Ron Burgundy'}, {name: 'Ron Burgundy'}}

Object literals with matching keys and values are considered to be different as well…

How Can All This Be Explained?

Sets use strict equality (===) to determine which values are unique. This explains why the set maintains a copy of both 3 (the number) and '3' (the string).

It also explains why Arrays and object literals that have the same contents are found to be unique. JavaScript compares objects by their reference, not their contents, and Arrays are just one particular kind of object.


Sets give JavaScript developers a new way to represent data. While Arraysremain the general-purpose workhorse of JavaScript applications, Setsare intended to represent a uniquecollection of values.

Converting between Sets and Arrays is easy. You can use a Set to ensure your data remains unique and then convert it to an Array to take advantage of functional methods like map, filter, and reduce.

Sets use _strict equality_to compare values and determine what is unique. Since JavaScript compares objects by reference, Arrays and object literals can be considered unique even when they contain the same contents.

This article focused on the conceptual side of Sets. You should now have a good idea of when a Set should be used, and when it would be better to stick to an Array.

Thank you !

#JavaScript #Nodejs #Programming #Coding

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JavaScript: Arrays vs. Sets - The difference
Hermann  Frami

Hermann Frami


A Simple Wrapper Around Amplify AppSync Simulator

This serverless plugin is a wrapper for amplify-appsync-simulator made for testing AppSync APIs built with serverless-appsync-plugin.


npm install serverless-appsync-simulator
# or
yarn add serverless-appsync-simulator


This plugin relies on your serverless yml file and on the serverless-offline plugin.

  - serverless-dynamodb-local # only if you need dynamodb resolvers and you don't have an external dynamodb
  - serverless-appsync-simulator
  - serverless-offline

Note: Order is important serverless-appsync-simulator must go before serverless-offline

To start the simulator, run the following command:

sls offline start

You should see in the logs something like:

Serverless: AppSync endpoint: http://localhost:20002/graphql
Serverless: GraphiQl: http://localhost:20002


Put options under custom.appsync-simulator in your serverless.yml file

| option | default | description | | ------------------------ | -------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------- | | apiKey | 0123456789 | When using API_KEY as authentication type, the key to authenticate to the endpoint. | | port | 20002 | AppSync operations port; if using multiple APIs, the value of this option will be used as a starting point, and each other API will have a port of lastPort + 10 (e.g. 20002, 20012, 20022, etc.) | | wsPort | 20003 | AppSync subscriptions port; if using multiple APIs, the value of this option will be used as a starting point, and each other API will have a port of lastPort + 10 (e.g. 20003, 20013, 20023, etc.) | | location | . (base directory) | Location of the lambda functions handlers. | | refMap | {} | A mapping of resource resolutions for the Ref function | | getAttMap | {} | A mapping of resource resolutions for the GetAtt function | | importValueMap | {} | A mapping of resource resolutions for the ImportValue function | | functions | {} | A mapping of external functions for providing invoke url for external fucntions | | dynamoDb.endpoint | http://localhost:8000 | Dynamodb endpoint. Specify it if you're not using serverless-dynamodb-local. Otherwise, port is taken from dynamodb-local conf | | dynamoDb.region | localhost | Dynamodb region. Specify it if you're connecting to a remote Dynamodb intance. | | dynamoDb.accessKeyId | DEFAULT_ACCESS_KEY | AWS Access Key ID to access DynamoDB | | dynamoDb.secretAccessKey | DEFAULT_SECRET | AWS Secret Key to access DynamoDB | | dynamoDb.sessionToken | DEFAULT_ACCESS_TOKEEN | AWS Session Token to access DynamoDB, only if you have temporary security credentials configured on AWS | | dynamoDb.* | | You can add every configuration accepted by DynamoDB SDK | | rds.dbName | | Name of the database | | rds.dbHost | | Database host | | rds.dbDialect | | Database dialect. Possible values (mysql | postgres) | | rds.dbUsername | | Database username | | rds.dbPassword | | Database password | | rds.dbPort | | Database port | | watch | - *.graphql
- *.vtl | Array of glob patterns to watch for hot-reloading. |


    location: '.webpack/service' # use webpack build directory
      endpoint: 'http://my-custom-dynamo:8000'


By default, the simulator will hot-relad when changes to *.graphql or *.vtl files are detected. Changes to *.yml files are not supported (yet? - this is a Serverless Framework limitation). You will need to restart the simulator each time you change yml files.

Hot-reloading relies on watchman. Make sure it is installed on your system.

You can change the files being watched with the watch option, which is then passed to watchman as the match expression.


      - ["match", "handlers/**/*.vtl", "wholename"] # => array is interpreted as the literal match expression
      - "*.graphql"                                 # => string like this is equivalent to `["match", "*.graphql"]`

Or you can opt-out by leaving an empty array or set the option to false

Note: Functions should not require hot-reloading, unless you are using a transpiler or a bundler (such as webpack, babel or typescript), un which case you should delegate hot-reloading to that instead.

Resource CloudFormation functions resolution

This plugin supports some resources resolution from the Ref, Fn::GetAtt and Fn::ImportValue functions in your yaml file. It also supports some other Cfn functions such as Fn::Join, Fb::Sub, etc.

Note: Under the hood, this features relies on the cfn-resolver-lib package. For more info on supported cfn functions, refer to the documentation

Basic usage

You can reference resources in your functions' environment variables (that will be accessible from your lambda functions) or datasource definitions. The plugin will automatically resolve them for you.

      Ref: MyBucket # resolves to `my-bucket-name`

      Type: AWS::DynamoDB::Table
        TableName: myTable
      Type: AWS::S3::Bucket
        BucketName: my-bucket-name

# in your appsync config
    name: dynamosource
        Ref: MyDbTable # resolves to `myTable`

Override (or mock) values

Sometimes, some references cannot be resolved, as they come from an Output from Cloudformation; or you might want to use mocked values in your local environment.

In those cases, you can define (or override) those values using the refMap, getAttMap and importValueMap options.

  • refMap takes a mapping of resource name to value pairs
  • getAttMap takes a mapping of resource name to attribute/values pairs
  • importValueMap takes a mapping of import name to values pairs


      # Override `MyDbTable` resolution from the previous example.
      MyDbTable: 'mock-myTable'
      # define ElasticSearchInstance DomainName
        DomainEndpoint: 'localhost:9200'
      other-service-api-url: ''

# in your appsync config
    name: elasticsource
      # endpoint resolves as 'http://localhost:9200'
          - ''
          - - https://
            - Fn::GetAtt:
                - ElasticSearchInstance
                - DomainEndpoint

Key-value mock notation

In some special cases you will need to use key-value mock nottation. Good example can be case when you need to include serverless stage value (${self:provider.stage}) in the import name.

This notation can be used with all mocks - refMap, getAttMap and importValueMap

      Fn::ImportValue: other-service-api-${self:provider.stage}-url

      - key: other-service-api-${self:provider.stage}-url
        value: ''


This plugin only tries to resolve the following parts of the yml tree:

  • provider.environment
  • functions[*].environment
  • custom.appSync

If you have the need of resolving others, feel free to open an issue and explain your use case.

For now, the supported resources to be automatically resovled by Ref: are:

  • DynamoDb tables
  • S3 Buckets

Feel free to open a PR or an issue to extend them as well.

External functions

When a function is not defined withing the current serverless file you can still call it by providing an invoke url which should point to a REST method. Make sure you specify "get" or "post" for the method. Default is "get", but you probably want "post".

        url: http://localhost:3016/2015-03-31/functions/addUser/invocations
        method: post
        method: post

Supported Resolver types

This plugin supports resolvers implemented by amplify-appsync-simulator, as well as custom resolvers.

From Aws Amplify:

  • NONE

Implemented by this plugin

  • HTTP

Relational Database

Sample VTL for a create mutation

#set( $cols = [] )
#set( $vals = [] )
#foreach( $entry in $ctx.args.input.keySet() )
  #set( $regex = "([a-z])([A-Z]+)")
  #set( $replacement = "$1_$2")
  #set( $toSnake = $entry.replaceAll($regex, $replacement).toLowerCase() )
  #set( $discard = $cols.add("$toSnake") )
  #if( $util.isBoolean($ctx.args.input[$entry]) )
      #if( $ctx.args.input[$entry] )
        #set( $discard = $vals.add("1") )
        #set( $discard = $vals.add("0") )
      #set( $discard = $vals.add("'$ctx.args.input[$entry]'") )
#set( $valStr = $vals.toString().replace("[","(").replace("]",")") )
#set( $colStr = $cols.toString().replace("[","(").replace("]",")") )
#if ( $valStr.substring(0, 1) != '(' )
  #set( $valStr = "($valStr)" )
#if ( $colStr.substring(0, 1) != '(' )
  #set( $colStr = "($colStr)" )
  "version": "2018-05-29",
  "statements":   ["INSERT INTO <name-of-table> $colStr VALUES $valStr", "SELECT * FROM    <name-of-table> ORDER BY id DESC LIMIT 1"]

Sample VTL for an update mutation

#set( $update = "" )
#set( $equals = "=" )
#foreach( $entry in $ctx.args.input.keySet() )
  #set( $cur = $ctx.args.input[$entry] )
  #set( $regex = "([a-z])([A-Z]+)")
  #set( $replacement = "$1_$2")
  #set( $toSnake = $entry.replaceAll($regex, $replacement).toLowerCase() )
  #if( $util.isBoolean($cur) )
      #if( $cur )
        #set ( $cur = "1" )
        #set ( $cur = "0" )
  #if ( $util.isNullOrEmpty($update) )
      #set($update = "$toSnake$equals'$cur'" )
      #set($update = "$update,$toSnake$equals'$cur'" )
  "version": "2018-05-29",
  "statements":   ["UPDATE <name-of-table> SET $update WHERE id=$", "SELECT * FROM <name-of-table> WHERE id=$"]

Sample resolver for delete mutation

  "version": "2018-05-29",
  "statements":   ["UPDATE <name-of-table> set deleted_at=NOW() WHERE id=$", "SELECT * FROM <name-of-table> WHERE id=$"]

Sample mutation response VTL with support for handling AWSDateTime

#set ( $index = -1)
#set ( $result = $util.parseJson($ctx.result) )
#set ( $meta = $result.sqlStatementResults[1].columnMetadata)
#foreach ($column in $meta)
    #set ($index = $index + 1)
    #if ( $column["typeName"] == "timestamptz" )
        #set ($time = $result["sqlStatementResults"][1]["records"][0][$index]["stringValue"] )
        #set ( $nowEpochMillis = $util.time.parseFormattedToEpochMilliSeconds("$time.substring(0,19)+0000", "yyyy-MM-dd HH:mm:ssZ") )
        #set ( $isoDateTime = $util.time.epochMilliSecondsToISO8601($nowEpochMillis) )
        $util.qr( $result["sqlStatementResults"][1]["records"][0][$index].put("stringValue", "$isoDateTime") )
#set ( $res = $util.parseJson($util.rds.toJsonString($util.toJson($result)))[1][0] )
#set ( $response = {} )
#foreach($mapKey in $res.keySet())
    #set ( $s = $mapKey.split("_") )
    #set ( $camelCase="" )
    #set ( $isFirst=true )
    #foreach($entry in $s)
        #if ( $isFirst )
          #set ( $first = $entry.substring(0,1) )
          #set ( $first = $entry.substring(0,1).toUpperCase() )
        #set ( $isFirst=false )
        #set ( $stringLength = $entry.length() )
        #set ( $remaining = $entry.substring(1, $stringLength) )
        #set ( $camelCase = "$camelCase$first$remaining" )
    $util.qr( $response.put("$camelCase", $res[$mapKey]) )

Using Variable Map

Variable map support is limited and does not differentiate numbers and strings data types, please inject them directly if needed.

Will be escaped properly: null, true, and false values.

  "version": "2018-05-29",
  "statements":   [
    "UPDATE <name-of-table> set deleted_at=NOW() WHERE id=:ID",
    "SELECT * FROM <name-of-table> WHERE id=:ID and unix_timestamp > $ctx.args.newerThan"
  variableMap: {
    ":ID": $,
##    ":TIMESTAMP": $ctx.args.newerThan -- This will be handled as a string!!!


Author: Serverless-appsync
Source Code: 
License: MIT License

#serverless #sync #graphql 

Lowa Alice

Lowa Alice


JavaScript Arrays Tutorial. DO NOT MISS!!!

Learn JavaScript Arrays

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#arrays #javascript #javascript arrays #javascript arrays tutorial

Samanta  Moore

Samanta Moore


Java Vs. JavaScript: Know the Difference

What a mess it could be to share the same name — especially if you are a programming language. Even if you are already over 20 years old, IT newbies periodically confuse you with your namesake. This happens all the time with Java and JavaScript, although they are not related at all! As someone on the internet said. They correlate in much the same way as a car and a carpet.

Why do these two languages have such similar names? How do they differ from each other, and what else do they have in common? This article will provide the answers to these questions.

In the Beginning, It Was Java

The Same Year, A Little Bit Later: Meet JavaScript!

Technical Differences Between Java and JavaScript

What Can You Build in Java and JavaScript?

#java #javascript #javascript-development #java-development #learn-to-code-java #learn-javascript #programming #java-vs-javascript

Terry  Tremblay

Terry Tremblay


Fill and Filter in Array in JavaScript

By the word Array methods, I mean the inbuilt array functions, which might be helpful for us in so many ways. So why not just explore and make use of them, to boost our productivity.

Let’s see them together one by one with some amazing examples.


The _fill()_ method changes all elements in an array to a static value, from a start index (default _0_) to an end index (default _array.length_). It returns the modified array.

In simple words, it’s gonna fill the elements of the array with whatever sets of params, you pass in it. Mostly we pass three params, each param stands with some meaning. The first param value: what value you want to fill, second value: start range of index(inclusive), and third value: end range of index(exclusive). Imagine you are going to apply this method on some date, so that how its gonna look like eg: array.fill(‘Some date’, start date, end date).

NOTE: Start range is inclusive and end range is exclusive.

Let’s understand this in the below example-

//declare array
var testArray = [2,4,6,8,10,12,14];


When you run this code, you gonna see all the elements of testArray will be replaced by 'A' like [“A”,"A","A","A","A","A","A"].

#javascript-tips #array-methods #javascript-development #javascript #arrays

Mahipal Nehra

Mahipal Nehra


Java vs. JavaScript: Know The Difference

Java vs. JavaScript

#java #javascript #Java vs. JavaScript #Java vs JavaScript #programming