Toy map / reduce functions in JavaScript

map_reduce.js

// Define reduce, fn and list are required, init is an optional initial value
// which will be passed to fn with the first item in list
function reduce(fn, list, init) {
    var val = init,
        start = 0;
    if (val === undefined) {
        if (list.length > 1) {
            val = list[0];
            start = 1;
        } else {
            return list[0];
        }
    }
    for (var i = start, item; i < list.length; i++) {
        item = list[i];
        val = fn(val, item);
    }
    return val;
}

// Sum all numbers
reduce(function(a, b) {return a + b}, [1, 2, 3]); // => 6
// Sum all numbers to 10
reduce(function(a, b) {return a + b}, [1, 2, 3], 10); // => 16
// Multiply all numbers
reduce(function(a, b) {return a * b}, [1, 2, 3], 10); // => 60
// Passing a single value simply returns it
reduce(function(a, b) {return a + b}, [1]); // => 1

// Define map based on reduce
function map(fn, list) {
    return reduce(function(a, b) {
        return a.concat(fn(b));
    }, list, []);
}

// Double each number
map(function(a) {return a * 2}, [1, 2, 3]); // => [2, 4, 6]
// Each number to the power of 2
map(function(a) {return Math.pow(a, 2)}, [1, 2, 3]); // => [1, 4, 9]

// Define filter based on reduce
function filter(fn, list) {
    return reduce(function(a, b) {
        var result = fn(b);
        if (result) {
            return a.concat(b);
        } else {
            return a;
        }
    }, list, []);
}

// Numbers less than two
filter(function (a) {return a < 2}, [0, 1, 2, 3, 4]); // => [0, 1]
// Numbers divisible by 2
filter(function (a) {return (a % 2) == 0}, [0, 1, 2, 3, 4]); // => [0, 2, 4]

To run either copy and paste into the devtools console of a browser or use .load map_reduce.js in a Node.js REPL.