What is this Javascript require

Question

I m trying to get Javascript to read write to a PostgreSQL database  I found this project on github  I was able to get the following sample code to run in node   var pg   require  pg      native libpq bindings    var pg   require  pg   native  var conString    tcp   postgres 1234 localhost postgres    var client   new pg Client conString   client connect       queries are queued and executed one after another once the connection becomes available client query  CREATE TEMP TABLE beatles name varchar 10   height integer  birthday timestamptz     client query  INSERT INTO beatles name  height  birthday  values  1   2   3      Ringo   67  new Date 1945  11  2     client query  INSERT INTO beatles name  height  birthday  values  1   2   3      John   68  new Date 1944  10  13        queries can be executed either via text parameter values passed as individual arguments   or by passing an options object containing text   optional  parameter values  and  optional  query name client query     name   insert beatle     text   INSERT INTO beatles name  height  birthday  values  1   2   3      values    George   70  new Date 1946  02  14          subsequent queries with the same name will be executed without re-parsing the query plan by postgres client query     name   insert beatle     values    Paul   63  new Date 1945  04  03       var query   client query  SELECT   FROM beatles WHERE name    1     John        can stream row results back 1 at a time query on  row   function row      console log row     console log  Beatle name   s   row name     Beatle name  John   console log  Beatle birth year   d   row birthday getYear       dates are returned as javascript dates   console log  Beatle height   d   d     Math floor row height 12   row height 12     integers are returned as javascript ints        fired after last row is emitted query on  end   function        client end          Next I tried to make it run on a webpage  but nothing seemed to happen  I checked on the Javascript console and it just says  require not defined    So what is this  require   Why does it work in node but not in a webpage   Also  before I got it to work in node  I had to do npm install pg  What s that about  I looked in the directory and didn t find a file pg  Where did it put it  and how does Javascript find it

User · Answer

I noticed that whilst the other answers explained what require is and that it is used to load modules in Node they did not give a full reply on how to load node modules when working in the Browser.

It is quite simple to do. Install your module using npm as you describe, and the module itself will be located in a folder usually called node_modules.

Now the simplest way to load it into your app is to reference it from your html with a script tag which points at this directory. i.e if your node_modules directory is in the root of the project at the same level as your index.html you would write this in your index.html:

<script src="node_modules/ng"></script>

That whole script will now be loaded into the page - so you can access its variables and methods directly.

There are other approaches which are more widely used in larger projects, such as a module loader like require.js. Of the two, I have not used Require myself, but I think it is considered by many people the way to go.

User · Answer

You know how when you are running JavaScript in the browser  you have access to variables like  window  or Math   You do not have to declare these variables  they have been written for you to use whenever you want   Well  when you are running a file in the Node js environment  there is a variable that you can use    It is called  module   It is an object   It has a property called  exports    And it works like this   In a file that we will name example js  you write   example js  module exports    some code     Now  you want this string  some code  in another file   We will name the other file  otherFile js  In this file  you write   otherFile js  let str   require    example js     That  require   statement goes to the file that you put inside of it  finds whatever data is stored on the module exports property     The let str       part of your code means that whatever that require statement returns is stored to the str variable   So  in this example  the end-result is that in otherFile js you now have this   let string    some code     or -      let str       example js   module exports  Note     the file-name that is written inside of the require statement   If it is a local file  it should be the file-path to example js   Also  the  js extension is added by default  so I didn t have to write it   You do something similar when requiring node js libraries  such as Express   In the express js file  there is an object named  module   with a property named  exports      So  it looks something like along these lines  under the hood  I am somewhat of a beginner so some of these details might not be exact  but it s to show the concept   express js  module exports   function           It returns an object with all of the server methods     return           listen  function port             get  function route  function req  res                  If you are requiring a module  it looks like this  const moduleName   require  module-name     If you are requiring a local file  it looks like this  const localFile   require    path to local-file      notice the    at the beginning of the file name     Also note that by default  the export is an object    eg module exports       So  you can write module exports myfunction            before assigning a value to the module exports    But you can also replace the object by writing module exports    I am not an object anymore

User · Answer

Alright  so let s first start with making the distinction between Javascript in a web browser  and Javascript on a server  CommonJS and Node    Javascript is a language traditionally confined to a web browser with a limited global context defined mostly by what came to be known as the Document Object Model  DOM  level 0  the Netscape Navigator Javascript API    Server-side Javascript eliminates that restriction and allows Javascript to call into various pieces of native code  like the Postgres library  and open sockets   Now require   is a special function call defined as part of the CommonJS spec  In node  it resolves libraries and modules in the Node search path  now usually defined as node modules in the same directory  or the directory of the invoked javascript file  or the system-wide search path   To try to answer the rest of your question  we need to use a proxy between the code running in the the browser and the database server   Since we are discussing Node and you are already familiar with how to run a query from there  it would make sense to use Node as that proxy   As a simple example  we re going to make a URL that returns a few facts about a Beatle  given a name  as JSON      your connection code     var express   require  express    var app   express createServer    app get   beatles  name   function req  res        var name   req params name            name   name replace    a-zA Z             if   name length            res send            else           var query   client query  SELECT   FROM BEATLES WHERE name      name     LIMIT 1            var data               query on  row   function row                data   row              res send data                          app listen 80   127 0 0 1

User · Answer

Two flavours of module exports   require    see here   Flavour 1 export file  misc js    var x   5  var addX   function value      return value   x     module exports x   x  module exports addX   addX    other file   var misc   require    misc    console log  Adding  d to 10 gives us  d   misc x  misc addX 10      Flavour 2 export file  user js    var User   function name  email      this name   name    this email   email     module exports   User    other file   var user   require    user    var u   new user

User · Answer

It s used to load modules  Let s use a simple example   In file circle object js   var Circle   function  radius        this radius   radius   Circle PI   3 14  Circle prototype         area  function              return Circle PI   this radius   this radius            We can use this via require  like   node gt  require  circle object      node gt  Circle    Function  PI  3 14   node gt  var c   new Circle 3    radius  3   node gt  c area     The require   method is used to load and cache JavaScript modules  So  if you want to load a local  relative JavaScript module into a Node js application  you can simply use the require   method   Example   var yourModule   require   your module name        js file extension is optional

User · Answer

Necromancing  IMHO  the existing answers leave much to be desired  It s very simple   Require is simply a  non-standard  function defined at global scope    window in browser  global in NodeJS   Now  as such  to answer the question  quot what is require quot   we  quot simply quot  need to know what this function does  This is perhaps best explained with code   Here s a simple implementation by Michele Nasti  the code you can find on his github page  Basically  let s call our minimalisc require function myRequire  function myRequire name         console log  Evaluating file   name         if    name in myRequire cache             console log    name  is not in cache  reading from disk            let code   fs readFileSync name   utf8            let module     exports                myRequire cache name    module          let wrapper   Function  quot require  exports  module quot   code           wrapper myRequire  module exports  module             console log    name  is in cache  Returning it           return myRequire cache name  exports    myRequire cache   Object create null   window require   myRequire  const stuff   window require    main js    console log stuff    Now you notice  the object  quot fs quot  is used here   For simplicity s sake  Michele just imported the NodeJS fs module  const fs   require  fs     Which wouldn t be necessary  So in the browser  you could make a simple implementation of require with a SYNCHRONOUS XmlHttpRequest  const fs         file           module exports     quot Hello World  quot                module exports   function    return 5 3                     getFile fileName  string  encoding  string   string                  https   developer mozilla org en-US docs Web API XMLHttpRequest Synchronous and Asynchronous Requests         let client   new XMLHttpRequest               client setRequestHeader  quot Content-Type quot    quot text plain charset UTF-8 quot                open method  url  async          client open  quot GET quot   fileName  false           client send            if  client status     200              return client responseText           return null                readFileSync  function  fileName  string  encoding  string   string                  this getFile fileName  encoding           return this file     Example  getFile would fetch this file            Basically  what require thus does  is download a JavaScript-file  eval it in an anonymous namespace  aka Function   with the global parameters  quot require quot    quot exports quot  and  quot module quot   and return the exports  meaning an object s public functions and properties  Note that this evaluation is recursive  you require files  which themselfs can require files  This way  all  quot global quot  variables used in your module are variables in the require-wrapper-function namespace  and don t pollute the global scope with unwanted variables  Also  this way  you can reuse code without depending on namespaces  so you get  quot modularity quot  in JavaScript   quot modularity quot  in quotes  because this is not exactly true  though  because you can still write window bla  and hence still pollute the global scope    Also  this establishes a separation between private and public functions  the public functions being the exports  Now instead of saying module exports   function    return 5 3     You can also say  function privateSomething         return 42      function privateSomething2         return 21      module exports           getRandomNumber  privateSomething       getHalfRandomNumber  privateSomething2     and return an object  Also  because your modules get evaluated in a function with parameters  quot require quot    quot exports quot  and  quot module quot   your modules can use the undeclared variables  quot require quot    quot exports quot  and  quot module quot   which might be startling at first  The require parameter there is of course a ByVal pointer to the require function saved into a variable   Cool  right    Seen this way  require looses its magic  and becomes simple   Now  the real require-function will do a few more checks and quirks  of course  but this is the essence of what that boils down to  Also  in 2020  you should use the ECMA implementations instead of require  import defaultExport from  quot module-name quot   import   as name from  quot module-name quot   import   export1   from  quot module-name quot   import   export1 as alias1   from  quot module-name quot   import   export1   export2   from  quot module-name quot   import   foo   bar   from  quot module-name path to specific un-exported file quot   import   export1   export2 as alias2           from  quot module-name quot   import defaultExport    export1               from  quot module-name quot   import defaultExport    as name from  quot module-name quot   import  quot module-name quot    And if you need a dynamic non-static import  e g  load a polyfill based on browser-type    there is the ECMA-import function keyword  var promise   import  quot module-name quot     note that import is not synchronous like require   Instead  import is a promise  so var something   require  quot something quot     becomes var something   await import  quot something quot     because import returns a promise  asynchronous   So basically  unlike require  import replaces fs readFileSync with fs readFileAsync  async readFileAsync fileName  encoding         const textDecoder   new TextDecoder encoding          textDecoder ignoreBOM   true      const response   await fetch fileName       console log response ok       console log response status       console log response statusText          let json   await response json           let txt   await response text           let blo Blob   response blob           let ab ArrayBuffer   await response arrayBuffer           let fd   await response formData          Read file almost by line        https   developer mozilla org en-US docs Web API ReadableStreamDefaultReader read Example 2 - handling text line by line     let buffer   await response arrayBuffer        let file   textDecoder decode buffer       return file       End Function readFileAsync  This of course requires the import-function to be async as well   quot use strict quot   async function myRequireAsync name        console log  Evaluating file   name         if    name in myRequireAsync cache             console log    name  is not in cache  reading from disk            let code   await fs readFileAsync name   utf8            let module     exports                myRequireAsync cache name    module          let wrapper   Function  quot asyncRequire  exports  module quot   code           await wrapper myRequireAsync  module exports  module             console log    name  is in cache  Returning it           return myRequireAsync cache name  exports    myRequireAsync cache   Object create null   window asyncRequire   myRequireAsync  async      gt        const asyncStuff   await window asyncRequire    main js        console log asyncStuff       Even better  right   Well yea  except that there is no ECMA-way to dynamically import synchronously  without promise   Now  to understand the repercussions  you absolutely might want to read up on promises async-await here  if you don t know what that is  But very simply put  if a function returns a promise  it can be  quot awaited quot   function sleep  fn  par       return new Promise  resolve    gt           wait 3s before calling fn par      setTimeout      gt  resolve fn par    3000           var fileList   await sleep listFiles  nextPageToken   Which is nice way to make asynchronous code look synchronous   Note that if you want to use async await in a function  that function must be declared async  async function doSomethingAsync         var fileList   await sleep listFiles  nextPageToken     And also please note that in JavaScript  there is no way to call an async function  blockingly  from a synchronous one  the ones you know   So if you want to use await  aka ECMA-import   all your code needs to be async  which most likely is a problem  if everything isn t already async    An example of where this simplified implementation of require fails  is when you require a file that is not valid JavaScript  e g  when you require css  html  txt  svg and images or other binary files   And it s easy to see why   If you e g  put HTML into a JavaScript function body  you of course rightfully get SyntaxError  Unexpected token   lt    because of Function  quot bla quot    quot  lt doctype    quot   Now  if you wanted to extend this to for example include non-modules  you could just check the downloaded file-contents with for code indexOf  quot module exports quot      -1  and then e g  eval  quot jquery content quot   instead of Func  which works fine as long as you re in the browser   Since downloads with Fetch XmlHttpRequests are subject to the same-origin-policy  and integrity is ensured by SSL TLS  the use of eval here is rather harmless  provided you checked the JS files before you added them to your site  but that much should be standard-operating-procedure  Note that there are several implementations of require-like functionality   the CommonJS  CJS  format  used in Node js  uses a require function and module exports to define dependencies and modules  The npm ecosystem is built upon this format   this is what is implemented above  the Asynchronous Module Definition  AMD  format  used in browsers  uses a define function to define modules  the ES Module  ESM  format  As of ES6  ES2015   JavaScript supports a native module format  It uses an export keyword to export a module   s public API and an import keyword to import it  the System register format  designed to support ES6 modules within ES5  the Universal Module Definition  UMD  format  compatible to all the above mentioned formats  used both in the browser and in Node js  It   s especially useful if you write modules that can be used in both NodeJS and the browser

User · Answer

So what is this  require      require   is not part of the standard JavaScript API  But in Node js  it s a built-in function with a special purpose  to load modules   Modules are a way to split an application into separate files instead of having all of your application in one file  This concept is also present in other languages with minor differences in syntax and behavior  like C s include  Python s import  and so on   One big difference between Node js modules and browser JavaScript is how one script s code is accessed from another script s code    In browser JavaScript  scripts are added via the  lt script gt  element  When they execute  they all have direct access to the global scope  a  shared space  among all scripts  Any script can freely define modify remove call anything on the global scope  In Node js  each module has its own scope  A module cannot directly access things defined in another module unless it chooses to expose them  To expose things from a module  they must be assigned to exports or module exports  For a module to access another module s exports or module exports  it must use require      In your code  var pg   require  pg    loads the pg module  a PostgreSQL client for Node js  This allows your code to access functionality of the PostgreSQL client s APIs via the pg variable      Why does it work in node but not in a webpage    require    module exports and exports are APIs of a module system that is specific to Node js  Browsers do not implement this module system       Also  before I got it to work in node  I had to do npm install pg  What s that about    NPM is a package repository service that hosts published JavaScript modules  npm install is a command that lets you download packages from their repository      Where did it put it  and how does Javascript find it    The npm cli puts all the downloaded modules in a node modules directory  where you ran npm install  Node js has very detailed documentation on how modules find other modules which includes finding a node modules directory

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