QUESTION: Can a function be set up as a constant in JS?
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From: Asen Bozhilov on 18 Mar 2010 17:10 Thomas 'PointedEars' Lahn wrote: > /** > * Returns <code>true</code> if the argument is a {@link Map} > * > * @param o : Object > * @return boolean > */ > Map.isInstance = function (o) { > return !!o && o.constructor === this; > > }; But this method is useless in ECMA-262-3 environments. For backward compatible can be good, but in inheritance pattern is not very well, because doesn't analyse prototype chain. function Map2(){} Map2.prototype = new Map(); Map2.prototype.constructor = Map2; print(Map.isInstance(new Map2())); //false What is a benefit from that method at all? It is try to make generalization, which isn't so precisely. I don't see reasons to test any objects in this way in implementations, which aren't support `instanceof` operator and `Object.prototype.isPrototypeOf (V)` method. If i need something like this, I'll do test over interface of object. For example if i need to call `getKey' method: if (typeof o.getKey == 'function') { o.getKey(key); } If i expect somewhere in my code `Map' instance, I'll write documentation about this case. And of course, I'll don't do it test above. Regards ;~)
From: kangax on 18 Mar 2010 18:38 On 3/18/10 8:49 AM, Thomas 'PointedEars' Lahn wrote: > JJ wrote: > >> // Action Constants >> function Action() {} >> // Action types, Action names >> Action.Type = { ADD: 0, UPDATE: 1, CLOSE: 2 }; >> Action.Name = [ "add.web", "save.web", "close" ]; > > Note that those are not real constants, but currently there is no better > interoperable way. > >> [...] >> if( !sStatus ) >> iActionType = Action.Type.ADD; >> else >> iActionType = Action.Type.UPDATE; > > Can (and IMHO should) be written as follows: > > var iActionType = (sStatus) ? Action.Type.UPDATE : Action.Type.ADD; Or even: var iActionType = Action.Type[sStatus ? 'UPDATE' : 'ADD']; [...] -- kangax
From: Andrea Giammarchi on 19 Mar 2010 05:15 Agreed, also instanceof simply checks the prototype and not the constructor If we want rely on constructors and their prototype, why create such method rather than do this check? if (o instanceof Map) { } it's even shorter than if (Map.isInstance(o)) { } then, as already said var MProto = Map.prototype; var m = new Map; // bye bye Map.prototype = {}; m instanceof Map; // false m.constructor === Map; // false MProto.isPrototypeOf(m); // true accordingly, believing that isInstance won't be changed, I would rather go for something like: Map.isInstance = (function (__proto__) { return function (o) { return __proto__.isPrototypeOf(o); }; }(Map.prototype)); Regards On Mar 18, 10:10 pm, Asen Bozhilov <asen.bozhi...(a)gmail.com> wrote: > Thomas 'PointedEars' Lahn wrote: > > /** > > * Returns <code>true</code> if the argument is a {@link Map} > > * > > * @param o : Object > > * @return boolean > > */ > > Map.isInstance = function (o) { > > return !!o && o.constructor === this; > > > }; > > But this method is useless in ECMA-262-3 environments. For backward > compatible can be good, but in inheritance pattern is not very well, > because doesn't analyse prototype chain. > > function Map2(){} > Map2.prototype = new Map(); > Map2.prototype.constructor = Map2; > > print(Map.isInstance(new Map2())); //false > > What is a benefit from that method at all? It is try to make > generalization, which isn't so precisely. I don't see reasons to test > any objects in this way in implementations, which aren't support > `instanceof` operator and `Object.prototype.isPrototypeOf (V)` method. > If i need something like this, I'll do test over interface of object. > For example if i need to call `getKey' method: > > if (typeof o.getKey == 'function') { > o.getKey(key); > > } > > If i expect somewhere in my code `Map' instance, I'll write > documentation about this case. And of course, I'll don't do it test > above. > > Regards ;~)
From: Thomas 'PointedEars' Lahn on 19 Mar 2010 08:26 Asen Bozhilov wrote: > Thomas 'PointedEars' Lahn wrote: >> /** >> * Returns <code>true</code> if the argument is a {@link Map} >> * >> * @param o : Object >> * @return boolean >> */ >> Map.isInstance = function (o) { >> return !!o && o.constructor === this; >> >> }; > > But this method is useless in ECMA-262-3 environments. For backward > compatible can be good, but in inheritance pattern is not very well, > because doesn't analyse prototype chain. It does not have to. > function Map2(){} > Map2.prototype = new Map(); This inheritance pattern is a problem to begin with. Often discussed, often disapproved of. > Map2.prototype.constructor = Map2; > > print(Map.isInstance(new Map2())); //false There is no guard against stupidity. > What is a benefit from that method at all? That a Map instance can be recognized for what it is. This is important because there are some methods that are only available to Map instances. RTSL. > if (typeof o.getKey == 'function') { > o.getKey(key); > } Doesn't apply here. PointedEars -- realism: HTML 4.01 Strict evangelism: XHTML 1.0 Strict madness: XHTML 1.1 as application/xhtml+xml -- Bjoern Hoehrmann
From: Thomas 'PointedEars' Lahn on 19 Mar 2010 08:28
Andrea Giammarchi wrote: > Agreed, also instanceof simply checks the prototype and not the > constructor The intention here was to avoid just that. > If we want rely on constructors and their prototype, why create such > method rather than do this check? > > if (o instanceof Map) { > > } > > it's even shorter than > > if (Map.isInstance(o)) { > > } And it is less compatible. > [non sequitur] > > [top post] Learn to quote. PointedEars -- Anyone who slaps a 'this page is best viewed with Browser X' label on a Web page appears to be yearning for the bad old days, before the Web, when you had very little chance of reading a document written on another computer, another word processor, or another network. -- Tim Berners-Lee |