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A toolbelt for writing functional javascript.


Cull is a toolbelt for writing functional javascript. It is based on these core values:

  • Pure JavaScript - no programming in strings
  • Pure functions - prefer side-effect free functions wherever possible
  • No chaining - chaining obscures side-effects and invites spaghetti code
  • No wrapping - work with native arrays and DOM elements
  • Fail early - complains loudly about API misuse in development

This is totally a work in progress. The API may change until we reach the big one-oh.

A consistent API

In an effort to create a consistent API, here are some basic function parameter guidelines:

Function first, collection second

The readability of

map(name, filter(underage, persons))


map(filter(persons, underage), name)

tells us which order to put the parameters. It is hard to spot what name belongs to, dangling at the end. This gets harder with more nested functions.

You might argue that when you inline the functions, the second form looks better. For maximum visual pleasure, don't inline your functions.

Function list

Documentation and examples

isList (list)

Is list an object with a numeric length, but not a DOM element?

assert(cull.isList({ length: 4 }));
refute(cull.isList({ length: 4, tagName: "DIV" }));

toList (value)

Returns a version of value that is an actual Array.

assert.equals(cull.toList(1), [1]);
assert.equals(cull.toList(null), []);
assert.equals(cull.toList(undefined), []);
var args = function () { return arguments; };
assert.isArray(cull.toList(args(1, 2, 3)));

doall (fn, list)

Calls fn on every item in list, presumably for side-effects. Returns the list.

var result = [];
cull.doall(function (item) {
}, [1, 2, 3, 4]);
assert.equals(result, [16, 9, 4, 1]);

isFunction (fn)

Is fn a function?

assert(cull.isFunction(function () {}));

reduce (fn, initial, items)

Returns the result of applying fn to initial and the first item in list, then applying fn to that result and the 2nd item, etc.

Can also be called without initial, in which case the first invocation of fn will be with the first two items in list.

var list = [1, 2, 3, 4];
var add = function (a, b) { return a + b; };
assert.equals(cull.reduce(add, list), 10);
assert.equals(cull.reduce(add, 5, list), 15);

all (pred, list)

Is pred truthy for all items in list?

refute(cull.all(isEven, [1, 2, 3, 4]));
assert(cull.all(isEven, [2, 4, 6, 8]));

some (pred, list)

Is pred truthy for any items in list?

assert(cull.some(isEven, [1, 2, 3, 4]));
refute(cull.some(isEven, [1, 3, 5, 7]));

onlySome (pred, list)

Is pred truthy for at least one item in list, and also falsy for at least one item in list?

assert(cull.onlySome(isEven, [1, 2, 3, 4]));
refute(cull.onlySome(isEven, [2, 4, 6, 8]));
refute(cull.onlySome(isEven, [1, 3, 5, 7]));

trim (string)

Returns string with white space at either end removed.

assert.equals(cull.trim("  abc  "), "abc");
assert.equals(cull.trim("  abc  def  "), "abc  def");

identity (arg)

Returns arg unchanged.

assert.equals(cull.identity(4), 4);

defined (o)

Is o neither undefined nor null?


unary (fn)

Returns a version of fn that only accepts one argument.

var add = function (a, b) { return a + b; };
assert.isNaN(cull.unary(add)(2, 3));

prop (name)

Returns a function that takes one argument and returns its name-property.

var persons = [
    { firstName: "John", age: 23 },
    { firstName: "Suzy", age: 27 },
    { firstName: "Peter", age: 35 }
assert.equals("firstName"), persons),
              ["John", "Suzy", "Peter"]);
assert.equals("age"), persons),
              [23, 27, 35]);

func (name, args)

Returns a function that takes one argument and calls its name-function with args (optional).

var f = cull.func("getId");
var obj = { getId: function () { return 42; } };
assert.equals(42, f(obj));

eq (x)

Returns a function that takes one argument and returns true if it is equal to x.

var isFive = cull.eq(5);

compose (fns, thisp)

Returns a function that calls the last function in fns, then calls the second to last function in fns with the result of the first, and so on, with an optional this-binding in thisp.

var identity = cull.compose();
assert.equals(identity(2, 3), 2);

callWith ()

Takes any number of arguments, and returns a function that takes one function and calls it with the arguments.

var add = function (a, b) { return a + b; };
var fn = cull.callWith(1, 2);
assert.equals(fn(add), 3);

partial (fn)

Takes a function fn and any number of additional arguments, fewer than the normal arguments to fn, and returns a function. When called, the returned function calls fn with the given arguments first and then additional args.

var fn = function (a, b) { return a + b; };
var curried = cull.partial(fn, 3);
assert.equals(curried(5), 8);

bind (obj, callee)

Returns a function that calls callee with obj as this. callee can be a function, or it can be a string - in which case it will be used to look up a method on obj.

Optionally takes additional arguments that are partially applied.

var func = this.spy();
var obj = {};
var bound = cull.bind(obj, func);
assert.equals(func.thisValues[0], obj);{});
assert.equals(func.thisValues[1], obj);
assert.equals(func.thisValues[2], obj);

flatten (list)

Flatten list recursively and return a list of non-list values

assert.equals(cull.flatten([1, 2, 3, 4]), [1, 2, 3, 4]);

indexOf (needle, list)

Return the first index of needle in list, otherwise < 0

assert.equals(1, cull.indexOf("b", ["a", "b", "c"]));

uniq (list)

Return a list with only the unique values in list

assert.equals(cull.uniq([1, 2, 3, 4]), [1, 2, 3, 4]);

first (fn, list)

Return the first item in list for which fn returns true

var items = [1, 2, 3, 4];
var even = function (i) { return i % 2 === 0; };
assert.equals(cull.first(even, items), 2);

select (fn, list)

Return a new list containing the items from list for which fn is true

var items = [0, 1, 2, null, 3, 4, undefined, 5, 6];
var result = (i) { return !!i; }, items);
assert.equals(result, [1, 2, 3, 4, 5, 6]);

difference (list, other)

Return a list with the items present in list but not in other

var result = cull.difference([1, 2, 3, 4], [2, 3]);
assert.equals(result, [1, 4]);

intersection (list1, list2)

Return a list with the items present in both list1 and list2

var result = cull.intersection([1, 2, 3, 4], [2, 3, 5]);
assert.equals(result, [2, 3]);

keys (object)

Return a list of enumerable own property keys in object

    id: 1,
    num: 42,
    name: "Mr"
}), ["id", "num", "name"]);

values (object)

Return a list of enumerable own property values in object

    id: 1,
    num: 42,
    name: "Mr"
}), [1, 42, "Mr"]);

map (fn, list)

Returns a new list consisting of the result of applying fn to the items in list.

var square = function (num) { return num * num; };
assert.equals(, [1, 2, 3]), [1, 4, 9]);

negate (pred)

Returns the complement of pred, ie a function that returns true when pred would be falsy, and false when pred would be truthy.

var isOdd = cull.negate(isEven);

reject (pred, list)

Returns a new list of the items in list for which pred returns nil.

var items = [1, 2, 3, 4, 5];
var odd = function (n) { return n % 2; };
assert.equals(cull.reject(odd, items), [2, 4]);

concat (list1, list2)

Returns a new list with the concatenation of the elements in list1 and list2.

var a = [1, 2, 3];
var b = [4, 5, 6];
assert.equals(cull.concat(a, b), [1, 2, 3, 4, 5, 6]);

partition (n, list)

Returns a new list with the items in list grouped into n-sized sublists.

The last group may contain less than n items.

var n = 2;
var result = cull.partition(n, [1, 2]);
assert.equals(result, [[1, 2]]);

mapdef (fn, list)

Returns a new list consisting of the result of applying fn to the items in list, but filtering out all null or undefined values from both list and the resulting list.

this.list = [
    { id: 1 },
    { id: 2 },
    { different: false },
    { id: 3 }

mapcat (fn, list)

Returns the result of applying concat to the result of applying map to fn and list. Thus function fn should return a collection.

var dbl = function (single) { return [single, single]; };
    cull.mapcat(dbl, [1, 2, 3]),
    [1, 1, 2, 2, 3, 3]

zipmap (keys, vals)

Returns an object with keys mapped to vals. Superflous keys or vals are discarded.

var keys = ["a", "b", "c"];
var vals = [1, 2, 3];
assert.equals(cull.zipmap(keys, vals),
              {"a": 1, "b": 2, "c": 3});

interpose (sep, list)

Returns a new list of all elements in list separated by sep.

var result = cull.interpose(":", [1, 2, 3]);
assert.equals(result, [1, ":", 2, ":", 3]);

after (obj, name, fn)

Advices the method name on obj, calling fn after the method is called. fn is called with the return value of the method as its first argument, then the methods original arguments. If fn returns anything, it will override the return value of the method.

cull.after(this.obj, "fn", function (ret, x) {
    this.list.push(ret * x);
this.obj.fn(3, 2);
assert.equals(this.obj.list, [3, 3]);

before (obj, name, fn)

Advices the method name on obj, calling fn before the method is called. fn is called with the same arguments as the method.;
cull.before(this.obj, "fn", function (x) {
    this.list.push(- 1);

around (obj, name, fn)

Advices the method name on obj, calling fn instead of the method. fn receives the original method as its first argument, and then the methods original arguments. It is up to the advicing function if and how the original method is called.

cull.around(this.obj, "fn", function () {});
assert.equals(this.obj.list, []);


Copyright © 2012-2013, Christian Johansen and Magnar Sveen. Cull.js uses semantic versioning. Code released under the BSD license. Documentation released under CC Attribution-Share Alike.