Question

This is code from Crockford’s JavaScript: The Good Parts.

var results = [];

var walkDOM = function (node,func) {
        func(node);                     //What does this do?
        node = node.firstChild;
        while(node) {
            walkDOM(node,func);
            node = node.nextSibling;
        }

    };

I understand the code except for func(node). I guess the point is to pass node as a parameter in function func, but how will the browser understand it this way? node and func could be anything–so when the function is called it could read like this:

walkDOM(document.body,function(att) {
          node.getAttribute(att);
          results.push(node);
          });

When func is passed, walkDOM will process function(att) {…}(document.body)–which wouldn’t make any sense. So why has Crockford chosen to include func(node)?

Answer 1

Looks to me like the func is used for doing something to every node in the tree.

For example, if I wanted to alert the tag name for every node in the entire tree:

walkDOM(document.body, function(node) {
    alert(node.tagName);
});

In your example function:

walkDOM(document.body,function(att) {
      node.getAttribute(att);
      results.push(node);
      });

… you have named the node parameter to att, but that doesn’t magically make in a name of an attribute. I would expect a “variable ‘node’ is not defined” when node.getAttribute(att) is ran, because node is being set to att… there is no node in that function’s scope.