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  1. PO
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    <p>Ok, I'm trying to explain it piece by piece:</p> <pre><code>Function.prototype.partial = function() { var fn = this, args = Array.prototype.slice.call(arguments); return function() { var arg = 0; for (var i = 0; i &lt; args.length &amp;&amp; arg &lt; arguments.length; i++) { if (args[i] === undefined) { args[i] = arguments[arg++]; //This line is where the confusion is } } return fn.apply(this, args); }; }; </code></pre> <p>First line:</p> <pre><code>var fn = this, args = Array.prototype.slice.call(arguments); </code></pre> <p>This stores the value of <code>this</code> and the value of <code>arguments</code> in two variables, because both values would get overriden in the following function block:</p> <pre><code>return function() { //inside here arguments will be whatever is passed to this returned function later. }; </code></pre> <p>The for loop:</p> <pre><code>var arg = 0; for (var i = 0; i &lt; args.length &amp;&amp; arg &lt; arguments.length; i++) { } </code></pre> <p>It will basically loop through all arguments that were passed to the <code>partial</code> function and exits earlier if <code>arg &gt;= arguments.length</code>.</p> <pre><code>if (args[i] === undefined) { args[i] = arguments[arg++]; //This line is where the confusion is } </code></pre> <p>So, if an argument of the <code>args</code> array is undefined, we replace it with the next argument in the <code>arguments</code> array. When every arguments are replaced, the original function gets called with the merged arguments array:</p> <pre><code> return fn.apply(this, args); </code></pre> <p>Here is how it would work in practice:</p> <pre><code> function xy(arg1, arg2) { console.log(arg1 + " / " + arg2); } var p1 = xy.partial("foo", undefined); p1("bar") //"foo / bar" var p2 = xy.partial(undefined, "bar"); p2("foo") //"foo / bar" var p3 = xy.partial("foo"); p3("bar") //"foo / undefined" --&gt; because you have to activly pass "undefined" otherwise the arguments array is too short </code></pre> <p>And last but not least, how this code workes in detail with the example <code>p1 = xy.partial("foo", undefined);</code> :</p> <pre><code>//lets call xy.partial("foo", undefined); Function.prototype.partial = function() { //fn = xy //args = ["foo", undefined] var fn = this, args = Array.prototype.slice.call(arguments); //return function that is assigned to p1 //lets call p1("bar") return function() { //arguments = ["bar"] var arg = 0; //for (var i = 0; i &lt; 2 &amp;&amp; arg &lt; 1; i++) for (var i = 0; i &lt; args.length &amp;&amp; arg &lt; arguments.length; i++) { //first iteration: //args[0] === "foo" -&gt; nothing happend //second iteration: //args[1] === undefined -&gt; args[1] = arguments[0] (= "bar"); arg++; if (args[i] === undefined) { args[i] = arguments[arg++]; //This line is where the confusion is } } //at this point: args = ["foo", "bar"]; //now just call the function with the merged array return fn.apply(this, args); }; }; </code></pre>
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    1. POUnderstanding Jon Resig's implementation of partial application
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    1. COYou deserve more than 1 upvote for such a detailed answer. Thanks a lot. Also I made an edit. Please verify the same.
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    2. CO@Geek Thanks! The edit was correct of course :)
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