Creating prototypes for closures within loops

Question

Creating prototypes for closures within loops

Currently, I am developing a JavaScript library where it is crucial to avoid polluting the global namespace by containing all variables within one or two global variables. However, I have encountered a unique situation that requires the use of closures, but my usual approach has not been effective. Despite searching extensively, I have only found the traditional closure method which I typically rely on.

[...]
addFilters: function(filters) {
  for(filter in filters) {
    this.filters[filter] = filters[filter];
    this.Image.prototype[filter] = function() { //closures, how do they work?
      return (function(image, filter, arguments) {
        image.addQueue(filter, arguments);
      })(this, filter, arguments);
    };
  }
},
[...]

In the given code snippet, the Image.prototype functions and image.addQueue are failing to properly capture the value of 'filter', resulting in it being set to the last filter in the iteration each time.

To view the complete code with relevant sections highlighted, visit: http://pastebin.com/UVFTVPkh

For a live demonstration, you can access it here:

javascript closures

Answer 1

Answer №1

The factory function for generating functions is a bit off in your code. It's important to name the arguments of the function and ensure that the self-invocation is done correctly. To improve clarity, consider using a separate function to create the function assigned to this.Image.prototype[filter].

function createPrototypeFunction(image, filter, args) {
    return function(filter, args) {
        image.addQueue(filter, args);
    };
}

// snip...

for (filter in filters) {
    this.filters[filter] = filters[filter];
    this.Image.prototype[filter] = createPrototypeFunction(this, filter, args);
}

To improve the implementation, here's how you can utilize immediate function invocation:

for (filter in filters) {
    this.filters[filter] = filters[filter];
    this.Image.prototype[filter] = (function(image, filter, args) {
        return function(filter, args) {
            image.addQueue(filter, args);
        };
    })(this, filter, args);
}

Answer 2

The factory function for generating functions is a bit off in your code. It's important to name the arguments of the function and ensure that the self-invocation is done correctly. To improve clarity, consider using a separate function to create the function assigned to this.Image.prototype[filter].

function createPrototypeFunction(image, filter, args) {
    return function(filter, args) {
        image.addQueue(filter, args);
    };
}

// snip...

for (filter in filters) {
    this.filters[filter] = filters[filter];
    this.Image.prototype[filter] = createPrototypeFunction(this, filter, args);
}

To improve the implementation, here's how you can utilize immediate function invocation:

for (filter in filters) {
    this.filters[filter] = filters[filter];
    this.Image.prototype[filter] = (function(image, filter, args) {
        return function(filter, args) {
            image.addQueue(filter, args);
        };
    })(this, filter, args);
}

Answer 3

Answer №2

A closure is tied to the scope in which it is declared, rather than to the specific values of the variables within that scope at the time of declaration.
I will elaborate on why your code did not function as expected. Understanding this concept will reveal a simpler approach to solve the problem.
The functions created within each FOR loop are all linked to the same scope (the addFilters function scope). When these closures are executed, they access and read the variables from that common scope, resulting in them obtaining the same values.
Therefore, the crucial aspect here is to bind each closure to a distinct scope, which is achieved by enclosing the closures within an anonymous function: thereby creating a unique scope for each closure.

However, is it necessary to utilize an anonymous function invocation to establish a scope?
The answer is NO.
You can create a scope more clearly by utilizing the with statement.

Below is your code using this alternative method:

[...]
addFilters: function(filters) {
    for(filter in filters) {
        this.filters[filter] = filters[filter];
        with({ _filter: filter }) // This creates a new scope with the local variable `_filter` storing the value of `filter`
            this.Image.prototype[filter] = function(image, arguments){ image.addQueue(_filter, arguments); }; // The closure is now bound to the new scope
    }
},
[...]

The variables image and arguments serve as parameters for the closure, so there is no need to introduce them into the new scope. These values are provided when executing the closures.

In addition, there is no requirement to name the _filter variable differently. You can simply use filter, as it will overshadow the outer scope's filter.

Answer 4

A closure is tied to the scope in which it is declared, rather than to the specific values of the variables within that scope at the time of declaration.
I will elaborate on why your code did not function as expected. Understanding this concept will reveal a simpler approach to solve the problem.
The functions created within each FOR loop are all linked to the same scope (the addFilters function scope). When these closures are executed, they access and read the variables from that common scope, resulting in them obtaining the same values.
Therefore, the crucial aspect here is to bind each closure to a distinct scope, which is achieved by enclosing the closures within an anonymous function: thereby creating a unique scope for each closure.

However, is it necessary to utilize an anonymous function invocation to establish a scope?
The answer is NO.
You can create a scope more clearly by utilizing the with statement.

Below is your code using this alternative method:

[...]
addFilters: function(filters) {
    for(filter in filters) {
        this.filters[filter] = filters[filter];
        with({ _filter: filter }) // This creates a new scope with the local variable `_filter` storing the value of `filter`
            this.Image.prototype[filter] = function(image, arguments){ image.addQueue(_filter, arguments); }; // The closure is now bound to the new scope
    }
},
[...]

The variables image and arguments serve as parameters for the closure, so there is no need to introduce them into the new scope. These values are provided when executing the closures.

In addition, there is no requirement to name the _filter variable differently. You can simply use filter, as it will overshadow the outer scope's filter.

Creating prototypes for closures within loops

Answer №1

Answer №2

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