example of a context argument

context.js

function Tree(value, left, right) {
    if (!(this instanceof Tree))
        return new Tree(value, left, right);
    this.value = value;
    this.left = left || null;
    this.right = right || null;
}

Tree.prototype.toString = function toString() {
    return "(" + JSON.stringify(this.value) + " " + this.left + " " + this.right + ")";
};

// An implementation that computes the max depth of a tree purely
// by reference to the results of sub-computations.
function maxDepth1(tree) {
    return tree === null
         ? 0
         : Math.max(maxDepth1(tree.left), maxDepth1(tree.right)) + 1;
}

// An implementation that computes the max depth of a tree by
// carrying a context argument representing the current depth.
function maxDepth2(tree, depth) {
    depth = depth || 0;
    if (tree === null)
        return depth;
    return Math.max(maxDepth2(tree.left, depth + 1),
                    maxDepth2(tree.right, depth + 1));
}

var t1 = Tree("foo", Tree("bar", Tree("baz")), Tree("quux"));
var t2 = null;
var t3 = Tree("mumble", t1, t1);

function test(tree, expected) {
    console.log("test: " + tree);
    var a1 = maxDepth1(tree), a2 = maxDepth2(tree);
    console.log(a1 === expected && a2 === expected ? "PASSED" : "FAILED");
    console.log();
}

test(t1, 3);
test(t2, 0);
test(t3, 4);