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POIssue with Coffeescript comprehensions and closures
 

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  1. POIssue with Coffeescript comprehensions and closures
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    <p>I ran into a problem when converting some JavaScript to CoffeeScript. The basic issue is I went from using $.each to CoffeeScript comprehensions and I now have a problem where the last value of the comprehension is being captured in closures. Lets start with the original JavaScript:</p> <pre><code>function bindKeyboardControls(websocket) { var moveKeyMap = { Down: ['down', 's'], Up: ['up', 'w'], Left: ['left', 'a'], Right: ['right', 'd'] }; $.each(moveKeyMap, function (direction, keys) { $.each(keys, function (_, key) { $(document).bind('keydown', key, function () { move(websocket, direction); }); }); }); }; </code></pre> <p>Here is my first attempt with CoffeeScript:</p> <pre><code> bindKeyboardControls = (websocket) -&gt; moveKeyMap = Down: ['down', 's'] Up: ['up', 'w'] Left: ['left', 'a'] Right: ['right', 'd'] for direction, keys of moveKeyMap for key in keys $(document).bind('keydown', key, -&gt; move(websocket, direction)) null </code></pre> <p>Why doesn't this work? Well here's the generated JavaScript:</p> <pre><code>bindKeyboardControls = function(websocket) { var direction, key, keys, moveKeyMap, _i, _len; moveKeyMap = { Down: ['down', 's'], Up: ['up', 'w'], Left: ['left', 'a'], Right: ['right', 'd'] }; for (direction in moveKeyMap) { keys = moveKeyMap[direction]; for (_i = 0, _len = keys.length; _i &lt; _len; _i++) { key = keys[_i]; $(document).bind('keydown', key, function() { return move(websocket, direction); }); } } return null; }; </code></pre> <p>Do you see how the 'direction' variable is declared at the top of the function? The function that is being passed in to document.bind is creating a closure on that variable, so by the time the function runs, direction will always be equal to the last value ('Right').</p> <p>Here is a fixed version that is somewhat ugly:</p> <pre><code> bindKeyboardControls = (websocket) -&gt; moveKeyMap = Down: ['down', 's'] Up: ['up', 'w'] Left: ['left', 'a'] Right: ['right', 'd'] for direction, keys of moveKeyMap for key in keys ((d) -&gt; $(document).bind('keydown', key, -&gt; move(websocket, d)))(direction) null </code></pre> <p>I could also just go back to using $.each. So I do have some solutions, but is there a better one?</p>
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