guide updates on Duktape.line() and Duktape.act()

website/guide/duktapebuiltins.html

@@ -378,6 +378,63 @@ debugging and diagnosis, e.g. when estimating rough memory usage of objects.</p>
 </table>
 </div>
 
+<h3>line()</h3>
+
+<p>Get the line number of the call site.  This may be useful for debugging, e.g.:</p>
+<pre class="ecmascript-code">
+log.info('reached line:', Duktape.line());
+</pre>
+
+<div class="note">
+As <code>Duktape.act()</code> provides more information, this function
+will probably be removed.
+</div>
+
+<h3>act()</h3>
+
+<p>Get information about a call stack entry.  Takes a single number argument
+indicating depth in the call stack: -1 is the top entry, -2 is the one below
+that etc.  Returns an object describing the call stack entry, or <code>undefined</code>
+if the entry doesn't exist.  Example:</p>
+<pre class="ecmascript-code">
+function dump() {
+    var i, t;
+    for (i = -1; ; i--) {
+        t = Duktape.act(i);
+        if (!t) { break; }
+        print(i, t.lineNumber, t.function.name, Duktape.enc('jsonx', t));
+    }
+}
+
+dump();
+</pre>
+
+<p>The example, when executed with the command line tool, currently prints
+something like:</p>
+<pre>
+-1 0 act {lineNumber:0,pc:0,function:{_func:true}}
+-2 4 dump {lineNumber:4,pc:16,function:{_func:true}}
+-3 10 global {lineNumber:10,pc:5,function:{_func:true}}
+</pre>
+
+<p>The interesting entries are <code>lineNumber</code> and <code>function</code>
+which provides e.g. the function name.</p>
+
+<p>You can also implement a helper to get the current line number using
+<code>Duktape.act()</code>:</p>
+<pre class="ecmascript-code">
+function getCurrentLine() {
+    // indices: -1 = Duktape.act, -2 = getCurrentLine, -3 = caller
+    var a = Duktape.act(-3) || {};
+    return a.lineNumber;
+}
+print('running on line:', getCurrentLine());
+</pre>
+
+<div class="note">
+The properties provided for call stack entries may change between versions.
+</div>
+
 <h3>gc()</h3>
 
 <p>Trigger a forced mark-and-sweep collection.  If mark-and-sweep is disabled,