AngularJS custom validator, an excellent example of duck typing

A StackOverflow answer usually is not worth writing a blog post, but this time I am sharing something, which 1) is an invaluable idea for developers using AngularJS 2) may serve as a beginning of deeper theoretical considerations regarding types in programming languages.

The link to before-mentioned SO:
http://stackoverflow.com/questions/18900308/angularjs-dynamic-ng-pattern-validation

I have created a codepen of this case:

See the Pen AngularJS ng-pattern — an excellent example of duck typing by Przemyslaw S. (@przemsen) on CodePen.0

In the codepen you can see a JavaScript object set up in ng-pattern directive, which should typically be a regular expression. The point is that AngularJS check for being a regular expression is simply a call to .test() function. So we can just attach such test function to an object and implement whatever custom validation logic we need. In here we can see the beauty of duck typing allowing us to freely customize behavior of the framework.

I said this could be a beginning of a discussion on how to actually understand duck typing. It is not that obvious and programmers tend to understand it intuitively rather than precisely, though it is not a term with any kind of formal definition. I recommend Eric Lippert’s blog post on the subject: https://ericlippert.com/2014/01/02/what-is-duck-typing/.

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