What does var that this mean in JavaScript

Question

In a JavaScript file I saw    function Somefunction       var that   this               What is the purpose of declaring that and assigning this this to it

User · Answer

The use of that is not really necessary if you make a workaround with the use of call   or apply     var car       car starter        car start   function        this starter active   false       var activateStarter   function                this  now points to our main object         this starter active   true              activateStarter apply this

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Here is an example       document  ready function             var lastItem   null              our-work-group  gt  p  gt  a   click function e                e preventDefault                 var item     this  html      Here value of  this  is   our-work-group  gt  p  gt  a              if  item    lastItem                    lastItem   null                      our-work-single-page   show                  else                   lastItem   item                      our-work-single-page   each function                         var imgAlt     this  find  img   attr  alt      Here value of  this  is   our-work-single-page                        if  imgAlt    item                              this  hide                          else                             this  show                                                                                    So you can see that value of this is two different values depending on the DOM element you target but when you add  that  to the code above you change the value of  this  you are targeting       document  ready function             var lastItem   null              our-work-group  gt  p  gt  a   click function e                e preventDefault                var item     this  html      Here value of  this  is   our-work-group  gt  p  gt  a              if  item    lastItem                    lastItem   null                  var that   this                      our-work-single-page   show                  else                   lastItem   item                      our-work-single-page   each function                             that  css  background-color     ffe700         Here value of  that  is   our-work-group  gt  p  gt  a                          var imgAlt     this  find  img   attr  alt                         if  imgAlt    item                              this  hide                          else                             this  show                                                                                           that  css  background-color     ffe700      Here value of  that  is   our-work-group   p   a  because the value of var that   this  so even though we are at  this     our-work-single-page   still we can use  that  to manipulate previous DOM element

User · Answer

From Crockford     By convention  we make a private that   variable  This is used to make the   object available to the private   methods  This is a workaround for an   error in the ECMAScript Language   Specification which causes this to be   set incorrectly for inner functions    JS Fiddle  function usesThis name        this myName   name       function returnMe             return this           scope is lost because of the inner function            return           returnMe   returnMe          function usesThat name        var that   this      this myName   name       function returnMe             return that               scope is baked in with  that  to the  class             return           returnMe   returnMe          var usesthat   new usesThat  Dave    var usesthis   new usesThis  John    alert  UsesThat thinks it s called     usesthat returnMe   myName     r n           UsesThis thinks it s called     usesthis returnMe   myName     This alerts        UsesThat thinks it s called Dave      UsesThis thinks it s called undefined

User · Answer

This is a hack to make inner functions  functions defined inside other functions  work more like they should  In javascript when you define one function inside another this automatically gets set to the global scope  This can be confusing because you expect this to have the same value as in the outer function   var car       car starter        car start   function        var that   this          you can access car starter inside this method with  this      this starter active   false       var activateStarter   function                this  now points to the global scope             this starter  is undefined  so we use  that  instead          that starter active   true              you could also use car starter  but using  that  gives            us more consistency and flexibility             activateStarter          This is specifically a problem when you create a function as a method of an object  like car start in the example  then create a function inside that method  like activateStarter   In the top level method this points to the object it is a method of  in this case  car  but in the inner function this now points to the global scope  This is a pain   Creating a variable to use by convention in both scopes is a solution for this very general problem with javascript  though it s useful in jquery functions  too   This is why the very general sounding name that is used  It s an easily recognizable convention for overcoming a shortcoming in the language   Like El Ronnoco hints at Douglas Crockford thinks this is a good idea

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Sometimes this can refer to another scope and refer to something else  for example suppose you want to call a constructor method inside a DOM event  in this case this will refer to the DOM element not the created object   HTML   lt button id  button  gt Alert Name lt  button gt    JS  var Person   function name      this name   name    var that   this    this sayHi   function         alert that name            var ahmad   new Person  Ahmad    var element   document getElementById  button    element addEventListener  click   ahmad sayHi        gt  Ahmad   Demo  The solution above will assing this to that then we can and access the name property inside the sayHi method from that  so this can be called without issues inside the DOM call   Another solution is to assign an empty that object and add properties and methods to it and then return it  But with this solution you lost the prototype of the constructor   var Person   function name      var that         that name   name    that sayHi   function         alert that name          return that

User · Answer

I m going to begin this answer with an illustration   var colours     red    green    blue    document getElementById  element   addEventListener  click   function            this is a reference to the element clicked on      var that   this       colours forEach function                this is undefined            that is a reference to the element clicked on               My answer originally demonstrated this with jQuery  which is only very slightly different       element   click function           this is a reference to the element clicked on      var that   this           elements   each function               this is a reference to the current element in the loop            that is still a reference to the element clicked on               Because this frequently changes when you change the scope by calling a new function  you can t access the original value by using it   Aliasing it to that allows you still to access the original value of this   Personally  I dislike the use of that as the alias   It is rarely obvious what it is referring to  especially if the functions are longer than a couple of lines  I always use a more descriptive alias   In my examples above  I d probably use clickedEl

[javascript] What does 'var that = this;' mean in JavaScript?

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