Reference and array clone

In JavaScript, object and array are passed by reference.

Sometimes you may come into trouble if you don’t keep this in mind, especial in
recursion.

Let’s see an example:

First, prepare a JavaScript array to present a tree data:

    var treeData = [{ "name": "root1",
        "kids": [{ "name": "leaf1" }, { "name": "leaf2"}] },
        { "name": "root2"}];

The expected html structure is:

    <ul>
        <li>root1
            <ul>
                <li>leaf1</li>
                <li>leaf2</li>
            </ul>
        </li>
        <li>root2</li>
    </ul>

Our JavaScript code:

    function resolveTree(rootNode, kids) {
        var ulNode = $("<ul />").appendTo(rootNode);
        $.each(kids, function(i, item) {
            var liNode = $("<li />").html(item.name).appendTo(ulNode);
            if (item.kids) {
                resolveTree(liNode, item.kids);
            }
        });
    } 

    resolveTree($("#tree"), treeData);

Next, we want to store the tree node’s path.

For example, leaf1 - [root1, leaf1], leaf2 - [root1, leaf2], root2 - [root2]

We modifiy the resolveTree function:

    function resolveTree(rootNode, kids, path) {
        var ulNode = $("<ul />").appendTo(rootNode);
        $.each(kids, function(i, item) {
            var liNode = $("<li />").html(item.name).appendTo(ulNode);
            path.push(item.name);
            liNode.data("path", path).click(function(event) {
                event.stopPropagation();
                alert($(this).data("path"));
            });
            if (item.kids) {
                resolveTree(liNode, item.kids, path);
            }
            path.pop();
        });
    }
    resolveTree($("#tree"), treeData, []);

The logic must be right, but the result is suprising.

No matter which node i click, the alert box contains an empty string.

What’s the matter?

oh… the array is passed by reference….

The variable path in the code - liNode.data(”path”, path) - is a reference. Then
we want a clone of the array.

There are many method to create a array clone.

    // Method 1
    var newArray = [];
    for (var i = 0; i < oldArray.length; i++) {
        newArray.push(oldArray[i]);
    } 

    // Method 2
    var newArray = oldArray.slice(0); 

    // Method 3
    var newArray = [].concat(oldArray);

The final solution:

    $(function() { 

        var treeData = [{ "name": "root1", "kids": [{ "name": "leaf1" }, { "name": "leaf2"}] }, { "name": "root2"}]; 

        function resolveTree(rootNode, kids, path) {
            var ulNode = $("<ul />").appendTo(rootNode);
            $.each(kids, function(i, item) {
                var liNode = $("<li />").html(item.name).appendTo(ulNode);
                path.push(item.name);
                liNode.data("path", [].concat(path)).click(function(event) {
                    event.stopPropagation();
                    alert($(this).data("path"));
                });
                if (item.kids) {
                    resolveTree(liNode, item.kids, path);
                }
                path.pop();
            });
        } 

        resolveTree($("#tree"), treeData, []);
    });