A simple tool for yielding items incrementally from a list.

Generator.js

/**
 *  A tool for yielding items incrementally from a list.
 *  Usage: 
 *      var gen = Generator([1, 2, 3], 0);
 *      gen.next();     // Yields 1
 *      gen.next();     // Yields 2
 *      gen.previous(); // Yields 1
 *
 *   @author JT Paasch
 *   @license MIT <http://opensource.org/licenses/MIT>
 */
var Generator = function(list, initial_index) {
    
    /**
     *  The current index. 
     *  @var Integer
     */
    var index; 
    
    /**
     *  The value to yield.
     *  @var Mixed
     */
    var yield_value;
    
    // Initialize the index, or throw an error if the index is invalid.
    if (list[initial_index] === undefined) {
	throw "No index '" + initial_index + "' in list: " + list;
    } else {
        index = initial_index;
    }
    
    /**
     *  Yield the next value in the list (if it exists).
     *  @return Mixed The value, or undefined.
     */ 
    var next = function() {
        yield_value = undefined;
        if (list[index + 1] !== undefined) {
            yield_value = list[index + 1];
            index += 1;
        }
        return yield_value;
    };
    
    /**
     *  Yield the previous value in the list (if it exists).
     *  @return Mixed The value, or undefined.
     */
    var previous = function() {
        yield_value = undefined;
        if (list[index - 1] !== undefined) {
            yield_value = list[index - 1];
            index -= 1;
        }
        return yield_value;
    };
    
    /**
     *  Return public methods.
     *  @return Object
     */ 
    return {
        next: next,
        previous: previous
    };
    
};