CODEWITHSUNDEEP
javascriptthisfunction-pointers
user1485496
user1485496

Reputation: 23

Calling function by value of variable and keeping scope of this pointer

I have been fiddling with code to call a function with the name of the value of a variable and then keep the this scope when called, but the this pointer seems to be in context of the element I have used jQuery's bind method on, rather than the object the function I might be calling is within. To clarify here´s some code to illustrate the problem:

classname.prototype = {
    bindElementToFunction: function(element, functionToCall){
        $(element).bind("click", 
                        {realThis: this, functionToCall: functionToCall}, 
                        this.callFunction);
    },

    // I had hoped I could change the this pointer back to the object by running 
    // it through this function, I have tried using .apply and .call but I can't 
    // seem to get them to work with function pointers
    callFunction: function(event){
        var realThis = event.data.realThis;
        var functionToCall = event.data.functionToCall;
        functionToCall = realThis[functionToCall];
        // I have tried .apply and .call in several different ways but can't seem 
        // to get them to work in this context
        functionToCall(); 
    },
    abitraryFunction: function(){
        this.test();
    },
};

The problem here is then that everything works fine up until abitraryFunction where this is still referring to the element from the bind function. I have tried doing .apply() with the appropriate this pointers, but they do not seem to work.

So here's the question how do I change the context of the "this" pointer in combination with function pointers? Feel free to scrap all the code I have written, as long as I am able to do a bind function to an element that then runs a method within a object where "this" is refferring to the object the method is within.

Thanks

Upvotes: 2

Views: 535

Answers (1)

Stecman
Stecman

Reputation: 3090

I think the jQuery bind is making your code way more complicated than it needs to be. The JavaScript bind() function works perfectly:

http://jsfiddle.net/bQGWS/

By simply assigning a function to the onclick (or any other event hook) of an element, this is evaluated from the element's point of view and so points to the element itself.

When you use bind, you end up with a copy of the function where this is effectively replaced with the var you passed into bind().

classname = function(){}

classname.prototype = {
    method: function(){
        try {
            alert( this.othermethod() );
        } catch(e) {
            // Method doesn't exist in scope
            alert( 'Wrong scope :(');
        }
    },

    othermethod: function(){
        return 'hello desired scope!';
    },

    referenceToElement: function(elementId, functionname){
        var el = document.getElementById(elementId);

        // Just assigning the function as is
        el.onclick = this[functionname];
    },

    bindToElement: function(elementId, functionname){
        var el = document.getElementById(elementId);

        // Using the bind function to create a copy in the
        // scope of this (within the prototype)
        el.onclick = this[functionname].bind(this);
    }
}

var instance = new classname();
instance.referenceToElement('reference', 'method');
instance.bindToElement('bound', 'method');

Upvotes: 1

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