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copied!<p>You have to follow <code>charge</code> more carefully. It is variable defined in <a href="https://github.com/mbostock/d3/blob/v2.9.2/src/layout/force.js#L11" rel="nofollow">line 11</a>:</p> <pre><code>charge = -30, </code></pre> <p>The function <code>force.charge</code> which you quoted is for <strong>setting</strong> the charge, it is <strong>not</strong> the function referred to in <code>+charge.call(this, nodes[i], i);</code>. Have a look at the second line of <code>force.charge</code>:</p> <pre><code>charge = typeof x === "function" ? x : +x; </code></pre> <p><code>x</code> can be a function (callback) <em>you</em> pass, to dynamically calculate the charge. The current node (<code>nodes[i]</code>) and the index of the node (<code>i</code>) will be passed to this callback, so that you can calculate the charge dynamically based on these values:</p> <pre><code>force.charge(function(node, index) { return index * 2; }); </code></pre> <p><code>x</code> (and therefore <code>charge</code>) can also be a number or numerical string. That's why it is tested beforehand whether <code>charge</code> is a function or not:</p> <pre><code>if (typeof charge === "function") { // function so we call it and pass the current node and index } else { // static value, the same for each node } </code></pre> <hr> <p>Apert from that, you can always pass any number of arguments to a function, no matter how many parameters it has defined. For example:</p> <pre><code>function foo() { alert([].join.call(null, arguments)); } foo('a', 'b'); </code></pre> <p>will alert <code>a,b</code>.</p> <hr> <p>To answer your questions: The arguments passed to <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/call" rel="nofollow"><code>.call()</code> <em><sup>[MDN]</sup></em></a> or <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/apply" rel="nofollow"><code>.apply()</code> <em><sup>[MDN]</sup></em></a> are passed in the same order to the function. So if I have a function <code>function foo(a, b, c)</code> then <code>foo.call(null, x, y)</code> would pass <code>x</code> as <code>a</code> and <code>y</code> as <code>b</code> (<code>c</code> would be <code>undefined</code>).</p> <p>The <code>+</code> operator is the <a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/Arithmetic_Operators#.2B_%28Unary_Plus%29" rel="nofollow"><em>unary plus</em> operator <em><sup>[MDN]</sup></em></a>, which simply converts the operand into a number.</p>

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