What are the benefits of using "var self = this" for synchronizing between a class and events?

Question

What are the benefits of using "var self = this" for synchronizing between a class and events?

Consider this straightforward code example (it's in AngularJS for simplicity, but the scenario is common in JavaScript):

angular.module('app',[]).
directive('myDir', function(){
    this.state = {a:1, b:2};

    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                // "this" refers to the element, not the directive
                this.state.a++;       
                this.state.b++;
                console.log(this.state);
            });
        }
    }
});

When the onclick callback is triggered, "this" points to the element rather than the directive function.

To remedy this, we often create a closure and use var self = this.

angular.module('app',[]).
directive('myDir', function(){
    // creating a closure for assistance
    var self = this;
    this.state = {a:1, b:2};
    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                self.state.a++;       
                self.state.b++;
                console.log(self.state);
            });
        }
    }
});

Although this method works and is commonly used, it may seem like a workaround in design. Is there a more optimal way to synchronize between a class and user events?

javascript angularjs

Answer 1

Answer №1

Upon examining your specific example, I find it to be lacking, although not necessarily for the reasons you may have initially thought. If you take a look at the revised version provided at http://plnkr.co/edit/K66o8tmRtnnfk8NZ8YPf?p=preview

app.directive('myDir', function(){
    // establishing a closed environment for protection
    var self = this;
    this.state = {a:1, b:2};
    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                self.state.a++;       
                self.state.b++;
                // Utilizing global scope (viewed as unfavorable)
                console.log(self === window);
            });
        }
    }
});

In this scenario, the `self` variable being defined is essentially equivalent to `window`, resulting in the storage of state information within the global scope. This is due to the fact that the function defining the directive does not involve a `new` statement targeting it, causing `this` to retain its default value of `window`.

If the function were targeted by a `new` statement, such as with a `controller` or a `service`, then setting `self = this` would be appropriate. However, in cases involving a `directive` or a `factory`, `this` remains unchanged from its default value of `window`. In such instances, it would be more advisable to simply define a local variable

var state = {a:1, b:2};

and access it through closures.

Another option is to utilize something like `bind` on each event handler to alter the reference of `this` within them. Nonetheless, there could arise situations where retaining the default binding inside the event handler is desired, potentially leading to inconsistencies between using `bind` and not utilizing it. This inconsistency might result in additional time spent troubleshooting, given the absence of a uniform approach for writing these functions.

Answer 2

Upon examining your specific example, I find it to be lacking, although not necessarily for the reasons you may have initially thought. If you take a look at the revised version provided at http://plnkr.co/edit/K66o8tmRtnnfk8NZ8YPf?p=preview

app.directive('myDir', function(){
    // establishing a closed environment for protection
    var self = this;
    this.state = {a:1, b:2};
    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                self.state.a++;       
                self.state.b++;
                // Utilizing global scope (viewed as unfavorable)
                console.log(self === window);
            });
        }
    }
});

In this scenario, the `self` variable being defined is essentially equivalent to `window`, resulting in the storage of state information within the global scope. This is due to the fact that the function defining the directive does not involve a `new` statement targeting it, causing `this` to retain its default value of `window`.

If the function were targeted by a `new` statement, such as with a `controller` or a `service`, then setting `self = this` would be appropriate. However, in cases involving a `directive` or a `factory`, `this` remains unchanged from its default value of `window`. In such instances, it would be more advisable to simply define a local variable

var state = {a:1, b:2};

and access it through closures.

Another option is to utilize something like `bind` on each event handler to alter the reference of `this` within them. Nonetheless, there could arise situations where retaining the default binding inside the event handler is desired, potentially leading to inconsistencies between using `bind` and not utilizing it. This inconsistency might result in additional time spent troubleshooting, given the absence of a uniform approach for writing these functions.

Answer 3

Answer №2

When dealing with services and controllers, using this is acceptable. However, it can get messy and incorrect within a directive's nested function scopes. In this case, this is only defined in the compile (referring to DDO), not in the directive factory function or pre/postlink functions.

A cleaner approach would be to pass the context to the callback function so that it operates in the correct context.

elem.on('click', angular.bind(state, function(){
    this.a++;       
    this.b++;
    console.log(this);
}));

An alternative way, more native to ES5, is:

elem.on('click', function(){
    this.a++;       
    this.b++;
    console.log(this);
}.bind(state));

This alternative method could be considered the recommended one, as long as lack of support from IE8 is not an issue.

Answer 4

When dealing with services and controllers, using this is acceptable. However, it can get messy and incorrect within a directive's nested function scopes. In this case, this is only defined in the compile (referring to DDO), not in the directive factory function or pre/postlink functions.

A cleaner approach would be to pass the context to the callback function so that it operates in the correct context.

elem.on('click', angular.bind(state, function(){
    this.a++;       
    this.b++;
    console.log(this);
}));

An alternative way, more native to ES5, is:

elem.on('click', function(){
    this.a++;       
    this.b++;
    console.log(this);
}.bind(state));

This alternative method could be considered the recommended one, as long as lack of support from IE8 is not an issue.

Answer 5

Answer №3

While your current approach is effective, there is an even better way to enhance any javascript code using the bind() method.

angular.module('app',[]).
directive('myDir', function(){
    // create a closure for the rescue
    var self = this;
    this.state = {a:1, b:2};
    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                self.state.a++;
                console.log(this.state.a == self.state.a);
            }.bind(self));
        }
    }
});

bind allows you to change the context of the code. You have the flexibility to pass anything as the context. Learn more at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

Answer 6

While your current approach is effective, there is an even better way to enhance any javascript code using the bind() method.

angular.module('app',[]).
directive('myDir', function(){
    // create a closure for the rescue
    var self = this;
    this.state = {a:1, b:2};
    return {
        link: function(scope, elem, attrs){
            elem.on('click', function(){
                self.state.a++;
                console.log(this.state.a == self.state.a);
            }.bind(self));
        }
    }
});

bind allows you to change the context of the code. You have the flexibility to pass anything as the context. Learn more at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

What are the benefits of using "var self = this" for synchronizing between a class and events?

Answer №1

Answer №2

Answer №3

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