{"id":60,"date":"2015-01-18T09:20:32","date_gmt":"2015-01-18T09:20:32","guid":{"rendered":"http:\/\/www.mbarsinai.com\/blog\/?p=60"},"modified":"2015-02-21T21:11:06","modified_gmt":"2015-02-21T21:11:06","slug":"uninheritance-using-undefined-in-javascript","status":"publish","type":"post","link":"https:\/\/www.mbarsinai.com\/blog\/2015\/01\/18\/uninheritance-using-undefined-in-javascript\/","title":{"rendered":"Uninheritance Using Undefined in Javascript"},"content":{"rendered":"

Javascript has a value called undefined<\/code>. It’s an interesting little fella, which is the value of everything that is not defined. One common use of it is trolling your javascript console:<\/p>\n


\n> defined
\nReferenceError: defined is not defined
\n> var o = {}
\nundefined
\n> o.defined == undefined
\ntrue
\n<\/code><\/p>\n

Ha ha, silly javascript, not can’t even tell defined from it’s opposite. Except that it can. And it’s also very elegant.<\/p>\n

See, unlike other dynamic languages that requires programs to test the symbol table for symbol presence (e.g. ruby’s defined?()<\/code> method), javascript does not have to test the table, since every valid expression (such as a variable reference) has a value. It just may be that this is undefined<\/code>.<\/p>\n

Common inheritance models allows programmers to refine behavior (when overriding methods) or to extend them (when adding new methods). But javascript, with it’s undefined<\/code> and its prototype-based inheritance, has a new type of inheritance – inheritance with method deletion<\/strong>. <\/p>\n

Suppose we want to model vehicles. Vehicles can go, er, I mean go()<\/code>, and they also have a number of doors. Easy enough, here’s the vehicle object, and a car and a truck.<\/p>\n


\nvar vehicle = Object.create({});
\nvehicle.go = function() { return this.name + \" is going\"; };
\nvehicle.doors = function() { return this.doorCount; };
\n\/\/ make a truck
\nvar truck = Object.create( vehicle );
\ntruck.doorCount = 2;
\ntruck.name=\"Truck\";
\n\/\/ make a car
\nvar car = Object.create( vehicle );
\ncar.doorCount = 5;
\ncar.name=\"Car\";
\n<\/code><\/p>\n

This works fine (all examples here were executed using Firefox Developer Edition<\/a>):
\n
\n> var aCar = Object.create(car);
\nundefined
\n> aCar.name=\"MyLittlePony\";
\n\"MyLittlePony\"
\n> aCar.go()
\n\"MyLittlePony is going\"
\n<\/code><\/p>\n

Nothing new so far. But how do you go about modeling a broken car? When using languages that use class-based inheritance, such as Java, one might result to throwing an IllegalStateException<\/code>, since brokenCar<\/code> is a vehicle<\/code> and thus must have a go()<\/code> method. In javascript, however, there’s another option:<\/p>\n


\n> var brokenCar = Object.create( car )
\nundefined
\n> brokenCar.go = undefined;
\n<\/code><\/p>\n

And now:
\n
\n> brokenCar.doors()
\n5
\n<\/code>
\nBut
\n
\n> brokenCar.go()
\nTypeError: brokenCar.go is not a function
\n<\/code><\/p>\n

In other words, brokenCar<\/code> is a car<\/code> that can’t go, just like in real life, where a broken car is a car that can’t go.<\/p>\n

Achievement “modeling bliss” unlocked.<\/p>\n","protected":false},"excerpt":{"rendered":"

Javascript has a value called undefined. It’s an interesting little fella, which is the value of everything that is not defined. One common use of it is trolling your javascript console: > defined ReferenceError: defined is not defined > var o = {} undefined > o.defined == undefined true Ha ha, silly javascript, not can’t […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":4,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[3],"tags":[10,9],"class_list":["post-60","post","type-post","status-publish","format-standard","hentry","category-programming","tag-javascript","tag-modeling"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p3NnQg-Y","_links":{"self":[{"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/posts\/60","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/comments?post=60"}],"version-history":[{"count":6,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/posts\/60\/revisions"}],"predecessor-version":[{"id":67,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/posts\/60\/revisions\/67"}],"wp:attachment":[{"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/media?parent=60"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/categories?post=60"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.mbarsinai.com\/blog\/wp-json\/wp\/v2\/tags?post=60"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}