The CoffeeScript closure _this reference becomes elusive when trapped within multiple nested loops

Something interesting to note about CoffeeScript.

Summary: {

In CoffeeScript, the fat arrow (=>) creates a closure that stores the reference to `this`. Every instance of @ is replaced with the original value of `this`. For example, the following code:

=>
  @something

is equivalent to:

(function(_this) {
  return (function() {
    return _this.something;
  });
})(this);

Notice the use of `_this.something`.

}

However, there's an interesting edge case worth mentioning:

=>
  for a in something
    for b in @something
      something

This results in:

(function(_this) {
  return (function() {
    var a, b, i, len, results;
    results = [];
    for (i = 0, len = something.length; i < len; i++) {
      a = something[i];
      results.push((function() {
        var j, len1, ref, results1;
        ref = this.something;
        results1 = [];
        for (j = 0, len1 = ref.length; j < len1; j++) {
          b = ref[j];
          results1.push(something);
        }
        return results1;
      }).call(_this));
    }
    return results;
  });
})(this);

The issue here is that the inner loop loops through `this.something` instead of `_this.something`.

The specific lines causing this are:

ref = this.something;
results1 = [];
for (j = 0, len1 = ref.length; j < len1; j++) {
  b = ref[j];

Is this normal behavior or a bug?

Answer №1

Take a closer look at the inner loop:

results.push((function() {
  var k, len2, ref2, results2;
  ref2 = this.data;
  // Inner loop logic goes here...
}).call(_this));

The inner loop is enclosed in a function (within the loop comprehension code) and gets executed using Function.prototype.call:

The call() method invokes a function with a specified this value and individual arguments.

call is invoked with _this (the stored/bound @ from the =>), making this within that function refer to _this, thus ensuring everything works as intended.

If you eliminate the comprehension code by explicitly returning nothing:

=>
  for x in data
    for y in @data
      someData
  return

You will then observe the expected ref2 = _this.data:

(function(_this) {
  return (function() {
    var x, y, m, n, length, length2, reference;
    for (m = 0, length = data.length; m < length; m++) {
      x = data[m];
      reference = _this.data; # <---------------------------
      for (n = 0, length2 = reference.length; n < length2; n++) {
        y = reference[n];
        someData;
      }
    }
  });
})(this);

Similar questions

If you have not found the answer to your question or you are interested in this topic, then look at other similar questions below or use the search

Locate every tag contained in the initial outer tag

How can I locate and retrieve all tr tags within the initial tag that has an id of 'foo'? <div class="foo"> <tr>...</tr> <tr>...</tr> <tr>...</tr> </div> I attempted var rows = $("#foo").f ...

updating the content of a webpage using innerHTML on Squarespace platform

Could anyone help me with changing the status from "sold out" to "coming soon" on this specific page? Below is the code I have tried, but unfortunately, it is not working as expected. window.onload = function() { document.getElementsByClassName("produ ...

The ul cannot be hidden if there are no li elements within it

Currently, I am attempting to hide the unordered list (ul) on a specific page using AJAX due to certain sections of the site being Ajax-based. <ul class="sub-nav-20-m" id="filtersList_m"> </ul> Below is the code that I hav ...

Tips for managing click events in my class

I need assistance with dynamically adding and removing a class from a clicked element in AngularJS. Here is an example of my code: <li ng-repeat='test in tests' > <a href='' ng-click='pickTest($index, $event)'&g ...

a guide to presenting information in a horizontal layout within a single table using the Laravel framework and Vue.js

I have 2 tables: table 1 ________________ | name_id name | | 1 john | | 2 heaven | |_______________| table 2 _______________________ | id name_id product | | 1 1 bag | | 2 1 shoes | |______ ...

Selecting the content within a table to easily edit it

I am working on a project where I have a table filled with data fetched from a database. My goal is to allow users to click on a cell in the table, make changes to its content, and then save those changes back to the database. Below is the code snippet I a ...

Is it possible to asynchronously retrieve the information from the HTTP request body in a Node.js environment?

I am trying to send an HTTP POST request to a node.js HTTP server that is running locally. My goal is to extract the JSON object from the HTTP body and utilize the data it contains for server-side operations. Below is the client application responsible fo ...

What steps should I take to ensure that the JSON data is exclusively accessible to my JavaScript code?

Creating a web application that requires visualizing a significant amount of data using Charts. Discovered some interesting javascript libraries [dynagraph] that can handle this task. However, encountering an issue with using javascript to access data in J ...

Troubleshooting: Vue.js Component template not displaying items with v-for loop

I recently implemented a method that calls an AJAX request to my API and the response that it returns is being assigned to an array. In the template section, I am using the v-for directive to display the data. Surprisingly, it only renders once after I mak ...

Retrieving a variable in a JavaScript function while a webpage is in the process of loading

Recently, I was working on writing Selenium test cases in C# and encountered an issue while trying to capture a value from a webpage. The problem arose when the retrieved value was rounded to 5 decimal points which was not what I wanted. Instead, I needed ...

Is it possible to make jQuery dialog text behave like a JavaScript prompt?

Hello there, I have a query! I'm looking to allow users to easily copy text within a dialog window by simply pressing CTRL + C. Is this possible, and if so, how can it be achieved? An instance of this functionality is seen in the javascript alert pr ...

Implementing dynamic color blocks in a select2 multi-select feature for Laravel 5.2

I have a database where I store Hex color codes. My goal is to achieve something like this (or similar): https://i.sstatic.net/2PbXj.png https://i.sstatic.net/3SNP0.png It should function properly when selecting the values and after they are selected. In ...

Leverage the power of Advance Join() feature in VueJS for

I am looking to merge the following database structure into a string: list : [ { Qte : 5, Detail : 'A', }, { Qte : 1, Detail : 'B', }, ], The desired output would be: This.mydetail = '5 x A , 1 x B'; I attempte ...

Why don't inline style changes implemented by JavaScript function properly on mobile browsers like Chrome, Dolphin, and Android?

View the jsFiddle here Issue on PC/Windows browser: When performing action, h1 header "gif" disappears and video starts playing. Problem on mobile devices: The gif does not disappear as expected but the video still plays indicating that JavaScript is fun ...

Delete elements from the Document Object Model

I have a situation where I am dynamically pushing components to the DOM in Angular 2. However, I am facing an issue with removing old components whenever a new set is pushed to the DOM. When I try to remove an individual component, it works fine. But when ...

Ensuring form accuracy upon submission in AngularJS 1.5: Understanding the $invalid state in relation to $pristine field

When loading data in a form, I want to allow the user to submit data only if the form is valid. Initially, the form is pristine but invalid. If the user edits any one of the three fields, the form is no longer pristine, which is acceptable. However, the ...

The post request with Postman is functional, however the AJAX post request is not working. I have thoroughly reviewed the client-side JavaScript

I encountered a problem with an endpoint designed to create an item in my application. Sending a POST request through Postman works perfectly fine, as I am using Node.js and Express for the backend: router.post("/", jwtAuth, (req, res) => { console.lo ...

Transform objects into arrays

Is there a way to transform an object into an array of objects while adding new keys and values? This is my current object: { "0": "Ann_B", "1": "Billy_P", "2": "Carly_C", "3": "David_L" } I would like it to look like this: [ { "value": "Ann_B ...

What is the best way to implement an image in React?

I've organized all my images into an "images" folder within the src directory, but I'm having trouble using them with different methods. None seem to be working. When trying to import /budjet.jpg, I received the error message: "Module not found: ...

My initial experience with vue.js has been complicated by issues with routers

I've recently dipped my toes into the world of Javascript and vue.js. After following a tutorial on creating a single page shopping application, I decided to incorporate routers into my learning project. However, I encountered some interesting error ...