map/reduce/each functionality, with a example.

map_reduce.js

	var type, each, filter, map, reduce;
	type = function (arg) {
		//filter out the type of the object, by getting the [[Class]] property.
		return Object.prototype.toString.call(arg).slice(8, -1).toLowerCase();
	};
	each = function (arg, fn) {
		var typef = type(arg); // get the type of the argument, but the real one.
		var i, len = (arg && arg.length), value;
		if(typef === 'array') { //if we're seeing an array,
			for (i = 0, len = arg.length; i < len; i++) {
				value = fn.call(arg, arg[i], i); //just use the function
				if (value === false) break;
			}
		} else { //else, loop as an object.
			for (i in arg) {
				value = fn.call(arg, arg[i], i);
				if (value === false) break;
			}
		}
		return arg;
	};
	map = function (array, fn) {
		//the returned array.
		var newArray = [];
		each(array, function (val) {
			//push the value the function returns into the array.
			newArray.push(fn.call(array, val));
		});
		//return it.
		return newArray;
	};
	filter = function (array, fn) {
		//the filtered elements.
		var filtered = [];
		each(array, function (val) {
			var value = fn.call(array, val);
			if (value === true) { //if the element passes the criteria,
				filtered.push(val); //add it to the filtered elements.
			}
		});
		//return it.
		return filtered;
	};
	reduce = function (array, fn, base) {
		var ret = base; //start the return value with the base.
		each(array, function (val) {
			ret = fn.call(array, ret, val); //glue it with the elements that were passed from the function
		});
		//return the summed up result.
		return ret;
	};
	
	//compute the average age of these adults.
        var testArray = [
            {name: 'john', age: 21},
            {name: 'sammy', age: 15},
            {name: 'barbara', age: 45},
            {name: 'ferris', age: 67}
        ];
        
        //contabilize the adult ages.
        var adultAges = //map the ages that are less than 18, but first, 
        		map(
        			//filter out the ones below 18
        			filter(
        				testArray, function (person) {
        					return person.age >= 18;
        				}
        			), 
        			//and then map over the resulting set to make an array of just ages.
        			function (person) {
        				return person.age;
        			}
        		);
        
        //calculate the average, or, in other words, sum up all the ages, and then divide them by the total of ages.
        var averageAge = Math.round(
        			reduce(
        				adultAges, function (base, age) {
        					return base + age;
        				}, 0 //start with 0.
        			) / adultAges.length
        		); // round up everything, and we have 44.