js zip function

README.md

A zip function in javascript using .reduce.

See Tom MacWright's post making juice with reduce / foldl.

See also _.zip and d3.zip for alternative implementations and sample usage.

zip.js

'use strict';
var assert = require('assert');


// zip
//
function zip(arrays) {

    return arrays.reduce(function (acc, arr, i) {

        while (acc.length < arr.length) {
            acc.push([]);
        }
        for (var j = 0; j < arr.length; ++j) {
            acc[j][i] = arr[j];
        }
        return acc;
    }, []);
}


var result = zip([ ['foo', 'bar'], ['apples', 'grapes'] ]);
var expected = [ ['foo', 'apples'], ['bar', 'grapes'] ];
assert.deepEqual(result, expected);


var result = zip([ ['foo', 'bar'], ['apples', 'grapes'], ['a', 'b', 'c'] ]);
var expected = [
    ['foo', 'apples', 'a'],
    ['bar', 'grapes', 'b'],
    []                          // [ , , 'c']
];
expected[2][2] = 'c';
assert.deepEqual(result, expected);



// following D3's implementation
// https://github.com/mbostock/d3/blob/master/src/arrays/zip.js
//
var zip = function (arrays) {

    var n = arrays.length;
    if (!n) { return []; }
    var max = maxArray(arrays);

    for (var i = -1, zips = new Array(max); ++i < max;) {
        for (var j = -1, n, zip = zips[i] = new Array(n); ++j < n;) {
            zip[j] = arrays[j][i];
        }
    }
    return zips;
};


function maxArray(arrays) {
    return arrays.reduce(function (max, arr) {
        var len = arr.length;
        return len > max ? len : max;
    }, 0);
}


var result = zip([ ['foo', 'bar'], ['apples', 'grapes'] ]);
var expected = [ ['foo', 'apples'], ['bar', 'grapes'] ];
assert.deepEqual(result, expected);


var result = zip([ ['foo', 'bar'], ['apples', 'grapes'], ['a', 'b', 'c'] ]);
var expect = [
    ['foo', 'apples', 'a'],
    ['bar', 'grapes', 'b'],
    [undefined, undefined, 'c']
];
assert.deepEqual(result, expect);