Handle Optional Parameters with Default Values in JavaScript

ArgumentAssigner.js

function assignArguments(values, expression) {				
	// determine how many arguments are needed for each parameter
	// grab all of the defaults, too
	var needed = 1;
	var argsNeeded = {};
	var defaults = {};
	var queue = [expression];				
	for (var queueIndex = 0; queueIndex < queue.length; ++queueIndex) {
		var node = queue[queueIndex];
		if (typeof node === 'string') {
			// the node is a required parameter
			argsNeeded[node] = needed;
			++needed;
		} else if (node instanceof Array) {
			// the node is a list of parameters
			for (var index = 0; index !== node.length; ++index) {
				queue.push(node[index]);
			}
		} else {
			// the node is a list of optional parameters with defaults
			// make sure there isn't more than one parameter
			var count = 0;
			for (var key in node) {
				if (node.hasOwnProperty(key)) {
					if (count !== 0) {
						throw new Error('A default parameter list had more than one value.');
					}
					defaults[key] = node[key];
					queue.push(key);
					++count;
				}
			}
		}
	}
	
	// determine the order that the parameters appear
	var parameters = [];
	var stack = [expression];
	while (stack.length > 0) {
		var node = stack.pop();
		if (typeof node === 'string') {
			// the node is a required parameter
			parameters.push(node);
		} else if (node instanceof Array) {
			// the node is a list of parameters
			var index = node.length;
			while (index !== 0) {
				--index;
				stack.push(node[index]);
			}
		} else {
			// the node is an optional parameter with defaults
			// we'll check for multiple parameters
			var count = 0;
			for (var key in node) {
				if (node.hasOwnProperty(key)) {
					if (count > 0) {
						throw new Error('A default parameter list had more than one value.');
					}
					stack.push(key);
					++count;
				}
			}
		}
	}
	
	var result = {};
	var valueIndex = 0;
	for (var index = 0; index !== parameters.length; ++index) {
		var parameter = parameters[index];
		var needed = argsNeeded[parameter];
		if (needed > values.length) {
			if (parameter in defaults) {
				result[parameter] = defaults[parameter];
			}
		} else {
			result[parameter] = values[valueIndex];
			++valueIndex;
		}
	}
	
	return result;
}