How to access the correct this inside a callback

Question

I have a constructor function which registers an event handler    x000D   x000D  function MyConstructor data  transport    x000D      this data   data  x000D      transport on  data   function      x000D          alert this data   x000D          x000D    x000D   x000D     Mock transport object x000D  var transport     x000D      on  function event  callback    x000D          setTimeout callback  1000   x000D        x000D     x000D   x000D     called as x000D  var obj   new MyConstructor  foo   transport   x000D   x000D   x000D    However  I m not able to access the data property of the created object inside the callback  It looks like this does not refer to the object that was created but to an other one   I also tried to use an object method instead of an anonymous function   function MyConstructor data  transport        this data   data      transport on  data   this alert      MyConstructor prototype alert   function         alert this name        but it exhibits the same problems   How can I access the correct object

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It s all in the  magic  syntax of calling a method   object property      When you get the property from the object and call it in one go  the object will be the context for the method  If you call the same method  but in separate steps  the context is the global scope  window  instead   var f   object property  f      When you get the reference of a method  it s no longer attached to the object  it s just a reference to a plain function  The same happens when you get the reference to use as a callback   this saveNextLevelData this setAll     That s where you would bind the context to the function   this saveNextLevelData this setAll bind this      If you are using jQuery you should use the   proxy method instead  as bind is not supported in all browsers   this saveNextLevelData   proxy this setAll  this

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this in JS  The value of this in JS is 100  determined by how a function is called  and not how it is defined  We can relatively easily find the value of this by the  left of the dot rule    When the function is created using the function keyword the value of this is the object left of the dot of the function which is called If there is no object left of the dot then the value of this inside a function is often the global object  global in node  window in browser   I wouldn t recommend using the this keyword here because it is less explicit than using something like window  There exist certain constructs like arrow functions and functions created using the Function prototype bind   a function that can fix the value of this  These are exceptions of the rule but are really helpful to fix the value of this   Example in nodeJS module exports data    module data      This outside a function in node refers to module exports object console log this    const obj1         data   quot obj1 data quot       met1  function              console log this data              met2       gt            console log this data              const obj2         data   quot obj2 data quot       test1  function              console log this data              test2  function              console log this data         bind obj1       test3  obj1 met1      test4  obj1 met2      obj2 test1    obj2 test2    obj2 test3    obj2 test4    obj1 met1 call obj2    Output   Let me walk you through the outputs 1 by 1  ignoring the first log starting from the second    this is obj2 because of the left of the dot rule  we can see how test1 is called obj2 test1     obj2 is left of the dot and thus the this value  Even though obj2 is left of the dot  test2 is bound to obj1 via the bind   method  So the this value is obj1  obj2 is left of the dot from the function which is called  obj2 test3    Therefore obj2 will be the value of this  In this case  obj2 test4   obj2 is left of the dot  However  arrow functions don t have their own this binding  Therefore it will bind to the this value of the outer scope which is the module exports an object which was logged in the beginning  We can also specify the value of this by using the call function  Here we can pass in the desired this value as an argument  which is obj2 in this case

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What you should know about this this  aka  quot the context quot   is a special keyword inside each function and its value only depends on how the function was called  not how when where it was defined  It is not affected by lexical scopes like other variables  except for arrow functions  see below   Here are some examples  function foo         console log this         normal function call foo        this  will refer to  window      as object method var obj    bar  foo   obj bar        this  will refer to  obj      as constructor function new foo        this  will refer to an object that inherits from  foo prototype   To learn more about this  have a look at the MDN documentation   How to refer to the correct this Use arrow functions ECMAScript 6 introduced arrow functions  which can be thought of as lambda functions  They don t have their own this binding  Instead  this is looked up in scope just like a normal variable  That means you don t have to call  bind  That s not the only special behavior they have  please refer to the MDN documentation for more information  function MyConstructor data  transport        this data   data      transport on  data        gt  alert this data       Don t use this You actually don t want to access this in particular  but the object it refers to  That s why an easy solution is to simply create a new variable that also refers to that object  The variable can have any name  but common ones are self and that  function MyConstructor data  transport        this data   data      var self   this      transport on  data   function             alert self data              Since self is a normal variable  it obeys lexical scope rules and is accessible inside the callback  This also has the advantage that you can access the this value of the callback itself  Explicitly set this of the callback - part 1 It might look like you have no control over the value of this because its value is set automatically  but that is actually not the case  Every function has the method  bind  docs   which returns a new function with this bound to a value  The function has exactly the same behavior as the one you called  bind on  only that this was set by you  No matter how or when that function is called  this will always refer to the passed value  function MyConstructor data  transport        this data   data      var boundFunction    function        parenthesis are not necessary         alert this data                  but might improve readability        bind this       lt - here we are calling   bind         transport on  data   boundFunction      In this case  we are binding the callback s this to the value of MyConstructor s this  Note  When a binding context for jQuery  use jQuery proxy  docs  instead  The reason to do this is so that you don t need to store the reference to the function when unbinding an event callback  jQuery handles that internally  Set this of the callback - part 2 Some functions methods which accept callbacks also accept a value to which the callback s this should refer to  This is basically the same as binding it yourself  but the function method does it for you  Array map  docs  is such a method  Its signature is  array map callback   thisArg    The first argument is the callback and the second argument is the value this should refer to  Here is a contrived example  var arr    1  2  3   var obj    multiplier  42    var new arr   arr map function v        return v   this multiplier     obj       lt - here we are passing  obj  as second argument  Note  Whether or not you can pass a value for this is usually mentioned in the documentation of that function method  For example  jQuery s   ajax method  docs  describes an option called context   This object will be made the context of all Ajax-related callbacks    Common problem  Using object methods as callbacks event handlers Another common manifestation of this problem is when an object method is used as callback event handler  Functions are first-class citizens in JavaScript and the term  quot method quot  is just a colloquial term for a function that is a value of an object property  But that function doesn t have a specific link to its  quot containing quot  object  Consider the following example  function Foo         this data   42      document body onclick   this method     Foo prototype method   function         console log this data       The function this method is assigned as click event handler  but if the document body is clicked  the value logged will be undefined  because inside the event handler  this refers to the document body  not the instance of Foo  As already mentioned at the beginning  what this refers to depends on how the function is called  not how it is defined  If the code was like the following  it might be more obvious that the function doesn t have an implicit reference to the object  function method         console log this data       function Foo         this data   42      document body onclick   this method     Foo prototype method   method   The solution is the same as mentioned above  If available  use  bind to explicitly bind this to a specific value document body onclick   this method bind this    or explicitly call the function as a  quot method quot  of the object  by using an anonymous function as callback   event handler and assign the object  this  to another variable  var self   this  document body onclick   function         self method        or use an arrow function  document body onclick        gt  this method

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Another approach  which is the standard way since DOM2 to bind this within the event listener  that let you always remove the listener  among other benefits   is the handleEvent evt method from the EventListener interface   var obj       handleEvent e           always true     console log this     obj           document body addEventListener  click   obj     Detailed information about using handleEvent can be found here  https   medium com  WebReflection dom-handleevent-a-cross-platform-standard-since-year-2000-5bf17287fd38

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Here are several ways to access parent context inside child context -   You can use bind   function    Store reference to context this inside another variable see below example   Use ES6 Arrow functions  Alter code function design architecture - for this you should have command over  design patterns in javascript    1  Use bind   function    function MyConstructor data  transport        this data   data      transport on  data     function              alert this data          bind this          Mock transport object var transport         on  function event  callback            setTimeout callback  1000               called as var obj   new MyConstructor  foo   transport       If you are using underscore js - http   underscorejs org  bind   transport on  data     bind function          alert this data      this      2 Store reference to context this inside another variable    function MyConstructor data  transport      var self   this    this data   data    transport on  data   function         alert self data               3 Arrow function    function MyConstructor data  transport      this data   data    transport on  data        gt        alert this data

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First  you need to have a clear understanding of scope and behaviour of this keyword in the context of scope   this  amp  scope      there are two types of scope in javascript  They are       1  Global Scope     2  Function Scope   in short  global scope refers to the window object Variables declared in a global scope are accessible from anywhere On the other hand function scope resides inside of a function variable  declared inside a function cannot be accessed from outside world normally this keyword in global scope refers to the window object this inside function also refers to the window object So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing   -------------------------------------------------------------------------------- -                                                                              - -   Global Scope                                                               - -     globally  this  refers to window object                                  -      -                                                                              - -         function outer function callback                                     - -                                                                              - -                  outer function scope                                        - -                  inside outer function this  keyword refers to window object -                                                                              - -              callback       this  inside callback also refers window object  -  -                                                                              - -                                                                              - -         function callback function                                           - -                                                                              - -                    function to be passed as callback                         - -                                                                              - -                   here  THIS  refers to window object also                   - -                                                                              - -                                                                              - -                                                                              - -         outer function callback function                                     - -            invoke with callback                                              - --------------------------------------------------------------------------------     Different ways to manipulate this inside callback functions   Here I have a constructor function called Person  It has a property called name and four method called sayNameVersion1 sayNameVersion2 sayNameVersion3 sayNameVersion4  All four of them has one specific task Accept a callback and invoke it The callback has a specific task which is to log the name property of an instance of Person constructor function   function Person name        this name   name      this sayNameVersion1   function callback           callback bind this              this sayNameVersion2   function callback           callback              this sayNameVersion3   function callback           callback call this             this sayNameVersion4   function callback           callback apply this            function niceCallback            function to be used as callback      var parentObject   this      console log parentObject       Now let s create an instance from person constructor and invoke different versions of sayNameVersionX   X refers to 1 2 3 4   method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance   var p1   new Person  zami      create an instance of Person constructor   bind     What bind do is to create a new function with the this keyword set to the provided value   sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function   this sayNameVersion1   function callback       callback bind this      this sayNameVersion2   function callback       callback       first one bind this with callback inside the method itself And for the second one callback is passed with the object bound to it   p1 sayNameVersion1 niceCallback     pass simply the callback and bind happens inside the sayNameVersion1 method  p1 sayNameVersion2 niceCallback bind p1      uses bind before passing callback     call     The first argument of the call method is used as this inside the function that is invoked with call attached to it   sayNameVersion3 uses  call to manipulate the this to refer to the person object that we created  instead of the window object   this sayNameVersion3   function callback       callback call this      and it is called like the following    p1 sayNameVersion3 niceCallback      apply     Similar to call  first argument of apply refers to the object that will be indicated by this keyword   sayNameVersion4 uses apply to manipulate this to refer to person object  this sayNameVersion4   function callback       callback apply this      and it is called like the following Simply the callback is passed   p1 sayNameVersion4 niceCallback

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We can not bind this to setTimeout    as it always execute with global object  Window   if you want to access this context in the callback function then by using bind   to the callback function we can achieve as   setTimeout function        this methodName      bind this   2000

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You Should know about  quot this quot  Keyword  As per my view you can implement  quot this quot  in three ways  Self Arrow function Bind Method  A function s this keyword behaves a little differently in JavaScript compared to other languages  It also has some differences between strict mode and non-strict mode  In most cases  the value of this is determined by how a function is called  It can t be set by assignment during execution  and it may be different each time the function is called  ES5 introduced the bind   method to set the value of a function s this regardless of how it s called  and ES2015 introduced arrow functions which don t provide their own this binding  it retains this value of the enclosing lexical context   Method1  Self - Self is being used to maintain a reference to the original this even as the context is changing  It s a technique often used in event handlers  especially in closures   Reference   https   developer mozilla org en-US docs Web JavaScript Reference Operators this function MyConstructor data  transport        this data   data      var self   this      transport on  data   function              alert self data              Method2  Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression  although without its own bindings to the this  arguments  super  or new target keywords  Arrow function expressions are ill-suited as methods  and they cannot be used as constructors  Reference  https   developer mozilla org en-US docs Web JavaScript Reference Functions Arrow functions   function MyConstructor data  transport        this data   data      transport on  data      gt            alert this data              Method3 Bind- The bind   method creates a new function that  when called  has its this keyword set to the provided value  with a given sequence of arguments preceding any provided when the new function is called  Reference  https   developer mozilla org en-US docs Web JavaScript Reference Global objects Function bind   function MyConstructor data  transport        this data   data      transport on  data   function             alert this data          bind this

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The question revolves around how this keyword behaves in javascript  this behaves differently as below   The value of this is usually determined by a function execution context  In the global scope  this refers to the global object  the window object   If strict mode is enabled for any function then the value of this will be undefined as in strict mode  global object refers to undefined in place of the window object  The object that is standing before the dot is what this keyword will be bound to  We can set the value of this explicitly with call    bind    and apply   When the new keyword is used  a constructor   this is bound to the new object being created  Arrow Functions don   t bind this     instead  this is bound lexically  i e  based on the original context   As most of the answers suggest  we can use Arrow function or bind   Method or Self var  I would quote a point about lambdas  Arrow function  from Google JavaScript Style Guide  Prefer using arrow functions over f bind this   and especially over goog bind f  this   Avoid writing const self   this  Arrow functions are particularly useful for callbacks  which sometimes pass unexpectedly additional arguments   Google clearly recommends using lambdas rather than bind or const self   this So the best solution would be to use lambdas as below  function MyConstructor data  transport      this data   data    transport on  data        gt        alert this data            References   https   medium com tech-tajawal javascript-this-4-rules-7354abdb274c arrow-functions-vs-bind

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Currently there is another approach possible if classes are used in code    With support of class fields  it s possible to make it next way   class someView       onSomeInputKeyUp    event    gt            console log this      this refers to correct value                 someInitMethod                           someInput addEventListener  input   this onSomeInputKeyUp    For sure under the hood it s all old good arrow function that bind context but in this form it looks much more clear that explicit binding   Since it s Stage 3 Proposal you will need babel and appropriate babel plugin to process it as for now 08 2018

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The trouble with  quot context quot  The term  quot context quot  is sometimes used to refer to the object referenced by this  Its use is inappropriate because it doesn t fit either semantically or technically with ECMAScript s this   quot Context quot  means the circumstances surrounding something that adds meaning  or some preceding and following information that gives extra meaning  The term  quot context quot  is used in ECMAScript to refer to execution context  which is all the parameters  scope  and this within the scope of some executing code  This is shown in ECMA-262 section 10 4 2   Set the ThisBinding to the same value as the ThisBinding of the calling execution context  which clearly indicates that this is part of an execution context  An execution context provides the surrounding information that adds meaning to the code that is being executed  It includes much more information than just the thisBinding  So the value of this isn t  quot context quot   it s just one part of an execution context  It s essentially a local variable that can be set by the call to any object and in strict mode  to any value at all

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I was facing problem with Ngx line chart xAxisTickFormatting function which was called from HTML like this   xAxisTickFormatting   quot xFormat quot   I was unable to access my component s variable from the function declared  This solution helped me to resolve the issue to find the correct this  Hope this helps the Ngx line chart  users  instead of using the function like this  xFormat  value   string     return value toString     this oneComponentVariable    gives wrong result     Use this   xFormat    value    gt          console log this         now you have access to your component variables    return value   this oneComponentVariable

[javascript] How to access the correct `this` inside a callback?

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