|Written by Ian Elliot
|Thursday, 15 May 2014
Page 3 of 3
The Onclick example
Notice that assigning the Function object to a property of Button1 makes it a method of Button1 as well as of myObject - this is an example of one Function object serving two masters in that it is a method of myObject and Button1.
When you click the Button the Function is evaluated with this referencing the HTMLElement that is the Button. That is in this case the Function object is bound to the Button object.
Of course the correct way to do the job is to early bind the method.
With the addition of just bind(myObject) the event handler now works as the novice expects and displays the value of myObject.mySize.
It sometimes matters if methods are late or early bound - it all depends how you plan to use them.
Adding & Removing Object Methods
If you have an object then it is easy enough to add a new method.
the myNewthod function simply has to make use of this within its body as if it was being define as an object literal.
[Rest In Book]
Overriding Object Methods
Another important use of the bind method is that it can be used to override methods in object instances. If an object already has a method called myMethod then simply redefining it overrides it. That is:
overrides the existing myMethod and substitutes the new function.
This works but often you want to make use of the old definition of myMethod to implement the new version.
How can you do this?
A simple minded approach would try to use something like:
However this doesn't work because when you try to call the old method this is set incorrectly. That is
calls the old version of MyMethod with this set to the global scope, usually window.
The trick is to use bind to retain the correct call context:
Now a call to oldMyMethod() really does call the method with this set to myObject.
You can use this technique to override methods with code that calls the old method to get the job done.
[Rest In Book]
The need to use a call context, i.e. this, is a consequence of treating functions as first class objects and this is very well worthwhile.
There is a newer version of the draft of the book here.
More functions - parameters, pass by value, expressions and constructors.
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|Last Updated ( Wednesday, 13 September 2017 )