One reason why Javascript rocks:
As a use case, imagine you want to restrict a certain set of functions to only run if you are logged in. Doing stuff like this is ridiculously easy with first-class functions.
Here’s a generic decoration example:
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(function() { var original = function(paramOne, paramTwo) { console.log('original: ' + paramOne + ' ' + paramTwo); }; var decorator = function(originalFn) { return function(paramOne, paramTwo) { console.log('decorator: ' + paramOne + ' ' + paramTwo); originalFn(paramOne, paramTwo); }; }; var fnTable = {}; var registerCallback = function(url, callback) { fnTable[url] = callback; }; registerCallback('/abc', original); registerCallback('/def', decorator(original)); fnTable['/abc']('one', 'two'); fnTable['/def']('three', 'four'); })(); |
Update: Here’s the specific way you’d do this with node.js/Express:
Since all of the URL handlers you register have the same signature, it’s easy to add precondition checks to the handlers via decorators.
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var decorator = function(originalFn) { return function(req, res) { if (req.session.userLoggedIn) { originalFn(req, res); } else { redirectSomewhere(res); } } } app.get('/url', decorator(original)); app.post('/other', decorator(original)); |
You get preconditions essentially for free, which is a damn sight better than adding the if() block to each and every handler function. Also, if you need more preconditions in the future, you can just stack them.