{"id":3860,"date":"2022-08-20T20:42:34","date_gmt":"2022-08-20T15:12:34","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-how-to-access-the-correct-this-inside-a-callback\/"},"modified":"2022-08-20T20:42:34","modified_gmt":"2022-08-20T15:12:34","slug":"solved-how-to-access-the-correct-this-inside-a-callback","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-how-to-access-the-correct-this-inside-a-callback\/","title":{"rendered":"(Solved) How to access the correct `this` inside a callback"},"content":{"rendered":"

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What you should know about this<\/code><\/h2>\n

this<\/code> (aka “the context”) is a special keyword inside each function and its value only depends on how<\/em> 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:<\/p>\n

function foo() {\n    console.log(this);\n}\n\n\/\/ normal function call\nfoo(); \/\/ `this` will refer to `window`\n\n\/\/ as object method\nvar obj = {bar: foo};\nobj.bar(); \/\/ `this` will refer to `obj`\n\n\/\/ as constructor function\nnew foo(); \/\/ `this` will refer to an object that inherits from `foo.prototype`\n<\/code><\/pre>\n
To learn more about this<\/code>, have a look at the MDN documentation<\/a>.<\/p>\n
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How to refer to the correct this<\/code><\/h2>\n
Use arrow functions<\/a><\/h3>\n
ECMAScript 6 introduced arrow functions<\/em>, which can be thought of as lambda functions. They don’t have their own this<\/code> binding. Instead, this<\/code> is looked up in scope just like a normal variable. That means you don’t have to call .bind<\/code>. That’s not the only special behavior they have, please refer to the MDN documentation for more information.<\/p>\n
function MyConstructor(data, transport) {\n    this.data = data;\n    transport.on('data', () => alert(this.data));\n}\n<\/code><\/pre>\n
Don’t use this<\/code><\/h3>\n
You actually don’t want to access this<\/code> in particular, but the object it refers to<\/em>. 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<\/code> and that<\/code>.<\/p>\n
function MyConstructor(data, transport) {\n    this.data = data;\n    var self = this;\n    transport.on('data', function() {\n        alert(self.data);\n    });\n}\n<\/code><\/pre>\n
Since self<\/code> 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<\/code> value of the callback itself.<\/p>\n
Explicitly set this<\/code> of the callback – part 1<\/h3>\n
It might look like you have no control over the value of this<\/code> because its value is set automatically, but that is actually not the case.<\/p>\n
Every function has the method .bind<\/code> [docs]<\/sup><\/em><\/a>, which returns a new function with this<\/code> bound to a value. The function has exactly the same behavior as the one you called .bind<\/code> on, only that this<\/code> was set by you. No matter how or when that function is called, this<\/code> will always refer to the passed value.<\/p>\n
function MyConstructor(data, transport) {\n    this.data = data;\n    var boundFunction = (function() { \/\/ parenthesis are not necessary\n        alert(this.data);             \/\/ but might improve readability\n    }).bind(this); \/\/ <- here we are calling `.bind()` \n    transport.on('data', boundFunction);\n}\n<\/code><\/pre>\n
In this case, we are binding the callback’s this<\/code> to the value of MyConstructor<\/code>‘s this<\/code>.<\/p>\n
Set this<\/code> of the callback – part 2<\/h3>\n
Some functions\/methods which accept callbacks also accept a value to which the callback’s this<\/code> should refer to. This is basically the same as binding it yourself, but the function\/method does it for you. Array#map<\/code> [docs]<\/sup><\/em><\/a> is such a method. Its signature is:<\/p>\n
array.map(callback[, thisArg])\n<\/code><\/pre>\n
The first argument is the callback and the second argument is the value this<\/code> should refer to. Here is a contrived example:<\/p>\n
var arr = [1, 2, 3];\nvar obj = {multiplier: 42};\n\nvar new_arr = arr.map(function(v) {\n    return v * this.multiplier;\n}, obj); \/\/ <- here we are passing `obj` as second argument\n<\/code><\/pre>\n
Note:<\/strong> Whether or not you can pass a value for this<\/code> is usually mentioned in the documentation of that function\/method. For example, jQuery’s $.ajax<\/code> method [docs]<\/sup><\/em><\/a> describes an option called context<\/code>:<\/p>\n
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This object will be made the context of all Ajax-related callbacks.<\/p>\n<\/blockquote>\n
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Common problem: Using object methods as callbacks\/event handlers<\/h2>\n
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 “method” 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 “containing” object.<\/p>\n
Consider the following example:<\/p>\n
function Foo() {\n    this.data = 42,\n    document.body.onclick = this.method;\n}\n\nFoo.prototype.method = function() {\n    console.log(this.data);\n};\n<\/code><\/pre>\n
The function this.method<\/code> is assigned as click event handler, but if the document.body<\/code> is clicked, the value logged will be undefined<\/code>, because inside the event handler, this<\/code> refers to the document.body<\/code>, not the instance of Foo<\/code>.
\nAs already mentioned at the beginning, what this<\/code> refers to depends on how the function is called<\/strong>, not how it is defined<\/strong>.
\nIf the code was like the following, it might be more obvious that the function doesn’t have an implicit reference to the object:<\/p>\n
function method() {\n    console.log(this.data);\n}\n\n\nfunction Foo() {\n    this.data = 42,\n    document.body.onclick = this.method;\n}\n\nFoo.prototype.method = method;\n<\/code><\/pre>\n
The solution<\/strong> is the same as mentioned above: If available, use .bind<\/code> to explicitly bind this<\/code> to a specific value<\/p>\n
document.body.onclick = this.method.bind(this);\n<\/code><\/pre>\n
or explicitly call the function as a “method” of the object, by using an anonymous function as callback \/ event handler and assign the object (this<\/code>) to another variable:<\/p>\n
var self = this;\ndocument.body.onclick = function() {\n    self.method();\n};\n<\/code><\/pre>\n
or use an arrow function:<\/p>\n
document.body.onclick = () => this.method();\n<\/code><\/pre>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            4<\/span> <\/p><\/div>\n<\/div>\n
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solved How to access the correct `this` inside a callback <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] What you should know about this this (aka “the context”) 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 … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[396,333,397],"class_list":["post-3860","post","type-post","status-publish","format-standard","hentry","category-solved","tag-callback","tag-javascript","tag-this"],"yoast_head":"\n(Solved) How to access the correct `this` inside a callback - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-how-to-access-the-correct-this-inside-a-callback\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"(Solved) How to access the correct `this` inside a callback - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] What you should know about this this (aka “the context”) 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. 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Note:<\/strong> When a binding context for jQuery, use jQuery.proxy<\/code> [docs]<\/sup><\/em><\/a> 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.<\/p>\n