F(unctional) util(ities). Resistance is futile.
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A collection of F(unctional) Util(ities). Resistance is futile.

Mostly, these are generic utilities that could conceivably be part of a library like lodash/fp, but for some reason or other are not.



Version History/Changelog

See our changelog


npm i -S futil or npm i -S futil-js

This package requires lodash/fp, so make sure that's available in your app.


import * as F from futil or import F from futil or import {x,y,z} from futil




(fn, a, b) -> fn(a, b) If fn is a function, call the function with the passed-in arguments. Otherwise, return false.


(fn, a, b) -> fn(a, b) If fn is a function, call the function with the passed-in arguments. Otherwise, return fn.


(a, Monoid f) -> f[a] :: f a Binds a function of an object to it's object.


(f, [g1, g2, ...gn]) -> a -> f([g1(a), g2(a), ...]) http://ramdajs.com/docs/#converge. Note that f is called on the array of the return values of [g1, g2, ...gn] rather than applied to it.

comply (alias: composeApply)

(f, g) -> x -> f(g(x))(x) A combinator that combines compose and apply. f should be a 2 place curried function. Useful for applying comparisons to pairs defined by some one place function, e.g. var isShorterThanFather = F.comply(isTallerThan, fatherOf)


Implement defer, ported from bluebird docs and used by debounceAsync


A _.debounce for async functions that ensure the returned promise is resolved with the result of the execution of the actual call. Using _.debounce with await or .then would result in the earlier calls never returning because they're not executed - the unit tests demonstate it failing with _.debounce.


(f1, f2, ...fn) -> f1Arg1 -> f1Arg2 -> ...f1ArgN -> fn(f2(f1)) Flurry is combo of flow + curry, preserving the arity of the initial function. See https://github.com/lodash/lodash/issues/3612.



handleItem -> handleLastItem -> iterator Creates an iterator that handles the last item differently for use in any function that passes (value, index, list) (e.g. mapIndexed, eachIndexed, etc). Both the two handlers and the result are iterator functions that take (value, index, list).



([f1, f2, ...fn]) -> !f1(x) && !f2(x) && ...!fn(x) Creates a function that checks if none of the array of predicates passed in returns truthy for x


(condition, onTrue, onFalse) -> x -> (T(condition)(x) ? onTrue(x) : onFalse(x)) http://ramdajs.com/docs/#ifElse. The transform function T supports passing a boolean for condition as well as any valid argument of _.iteratee, e.g. myBool = applyTest(x); F.ifElse(myBool, doSomething, doSomethingElse);


(condition, onTrue) -> x -> (T(condition)(x) ? onTrue(x) : _.identity(x)) http://ramdajs.com/docs/#when. T extends _.iteratee as above.


(condition, onFalse) -> x -> (T(condition)(x) ? _.identity(x) : onFalse(x)) http://ramdajs.com/docs/#unless. T extends _.iteratee as above.


when curried with Boolean


when curried with exists



[f1, f2, ...fn] -> _.map(_.flow(fn)) Maps a flow of f1, f2, ...fn over a collection.


f -> x -> f(find(f, x)) A version of find that also applies the predicate function to the result. Useful when you have an existing function that you want to apply to a member of a collection that you can best find by applying the same function.


(index, val, array|string) -> array|string Inserts value into an array or string at index

Collection Algebras or composable/recursive data types


(a -> b) -> [a] -> [b] Maps a function over an iterable. Works by default for Arrays and Plain Objects.


(a -> b) -> [a] -> [b] Maps a function over a recursive iterable. Works by default for nested Arrays, nested Plain Objects and mixed nested Arrays and Plain Objects. Also works for any other iterable data type as long as two other values are sent: a mapping function, and a type checker (See the unit tests for deepMap).

Lodash Conversions

These are conversions of lodash fp methods.

Ins (Rearg False)

getIn, hasIn, includesIn, pickIn lodash/fp is great, but sometimes the curry order isn't exactly what you want. These methods provide alternative orderings that are sometimes more convenient. The idea of In methods is to name them by convention, so when ever you need a method that actually takes the collection first (e.g. a get where the data is static but the field is dynamic), you can just add In to the end (such as getIn which takes the object first)

Ons (Immutable False)

extendOn, defaultsOn, mergeOn, setOn, unsetOn, pullOn lodash/fp likes to keep things pure, but sometimes JS can get pretty dirty. These methods are alternatives for working with data that--for whatever the use case is--needs to be mutable Any methods that interact with mutable data will use the On convention (as it is some action occuring On some data)

Indexed (Cap False)

mapIndexed, eachIndexed, reduceIndexed, mapValuesIndexed lodash/fp caps iteratees to one argument by default, but sometimes you need the index. These methods are uncapped versions of lodash's methods. Any method with uncapped iteratee arguments will use the Indexed convention.



joinString -> [string1, string2, ...stringN] -> string1 + joinString + string2 + joinString ... + stringN Joins an array after compacting. Note that due to the underlying behavior of _.curry no default join value is supported -- you must pass in some string with which to perform the join.


[string1, string2, ...stringN] -> string1 + '.' + string2 + '.' ... + stringN Compacts and joins an array with '.'


filterFunction -> [string1, string2, ...stringN] -> string1 + '.' + string2 + '.' ... + stringN Compacts an array by the provided function, then joins it with '.'


[a] -> [a] Returns an array of elements that are repeated in the array.


([[], [], []]) -> [[], []] Takes any number of ranges and return the result of merging them all.

Example: [[0,7], [3,9], [11,15]] -> [[0,9], [11,15]]


(val, array) -> array Return array with val pushed.


(from, to, array) -> array Moves a value from one index to another


[a, b...] -> a -> b Creates a function that takes an element of the original array as argument and returns the next element in the array (with wrapping). Note that (1) This will return the first element of the array for any argument not in the array and (2) due to the behavior of _.curry the created function will return a function equivalent to itself if called with no argument.


(k, v, [a]) -> { k(a): v(a) } Creates an object from an array by generating a key/value pair for each element in the array using the key and value mapper functions.


A version of _.zipObjectDeep that supports passing a function to determine values intead of an array, which will be invoked for each key.


[a, b] -> {a:true, b:true} Converts an array of strings into an object mapping to true. Useful for optimizing includes.


['a', 'b', 'c'] -> [['a'], ['a', 'b'], ['a', 'b', 'c']] Returns a list of all prefixes. Works on strings, too. Implementations must guarantee that the orginal argument has a length property.


string -> {encode: array -> string, decode: string -> array} Creates an object with encode and decode functions for encoding arrays as strings. The input string is used as input for join/split.


{ encode: ['a', 'b'] -> 'a.b', decode: 'a.b' -> ['a', 'b'] } An encoder using . as the separator


{ encode: ['a', 'b'] -> 'a/b', decode: 'a/b' -> ['a', 'b'] } An encoder using / as the separator


((a, a) -> Boolean) -> [a] -> [[a]] Returns an array of arrays, where each one of the arrays has one or more elements of the original array, grouped by the first function received. Similar to Haskell's groupBy.


(any, array) -> array Removes an element from an array if it's included in the array, or pushes it in if it doesn't. Immutable (so it's a clone of the array).


bool -> value -> list -> newList Just like toggleElement, but takes an iteratee to determine if it should remove or add. This is useful for example in situations where you might have a checkbox that you want to represent membership of a value in a set instead of an implicit toggle. Used by includeLens.


f -> array -> [array[0], f(), array[n], ....) Puts the result of calling f in between each element of the array. f is a standard lodash iterator taking the value, index, and list. If f isn't a function, it will treat f as the value to intersperse. See https://ramdajs.com/docs/#intersperse.

Note: Intersperse can be used with JSX components! Specially with the differentLast iterator:

Example with words (toSentence is basically this flowed into a _.join('')):

> F.intersperse(differentLast(() => 'or', () => 'or perhaps'), ['first', 'second', 'third'])
['first', 'or', 'second', 'or perhaps', 'third']

Example with React and JSX:

let results = [1, 2, 3]
return <div>
      _.map(x => <b>{x}</b>),
      F.intersperse(F.differentLast(() => ', ', () => ' and '))


1, 2 and 3.



(k, v) -> {k: v} Creates an object with a key and value.


(v, k) -> {k: v} Flipped version of singleObject.


({a, b}) -> [{a}, {b}] Breaks an object into an array of objects with one key each.


Remove properties with falsey values.

Example: ({ a: 1, b: null, c: false }) -> {a:1}


Check if the variable is an empty object ({}).


Check if the variable is not an empty object ({}).


Omit properties whose values are empty objects.

Example: { a:1, b:{}, c:2 } -> {a:1, c:2} (TODO remame to omitEmptyObjects)


Checks if an object's property is equal to a value.


Returns true if object keys are only elements from signature list. (but does not require all signature keys to be present)


Similar to _.matches, except it returns true if 1 or more object properties match instead of all of them. See https://github.com/lodash/lodash/issues/3713.




sourcePropertyName -> targetPropertyName -> sourceObject -> sourceObject Rename a property on an object.

Example: renameProperty('a', 'b', { a: 1 }) -> { b: 1 }


'b' -> { a: true, b: [1, 2] } -> { a: true, b: 1 }, { a: true, b: 2} Just like mongo's $unwind: produces an array of objects from an object and one of its array-valued properties. Each object is constructed from the original object with the array value replaced by its elements. Unwinding on a nonexistent property returns an empty array.


Flatten an object with the paths for keys.

Example: { a: { b: { c: 1 } } } => { 'a.b.c' : 1 }.


Unlatten an object with the paths for keys.

Example: { 'a.b.c' : 1 } => { a: { b: { c: 1 } } }.


Deprecated in favor of lodash update Applies a map function at a specific path

Example: mapProp(double, 'a', {a: 2, b: 1}) -> {a: 4, b: 1}.


_.get that returns the target object if lookup fails


_.get that returns the prop if lookup fails


Flipped alias


A _.get that takes an array of paths (or functions to return values) and returns the value at the first path that matches. Similar to _.overSome, but returns the first result that matches instead of just truthy (and supports a default value)


Flipped cascade


A _.get that takes an array of paths and returns the first path that matched


A _.get that takes an array of paths and returns the first value that has an existing path


A _.get that takes an array of paths and returns the first path that exists


newKey -> {a:x, b:y} -> [{...x, newKey: a}, {...y, newKey: b}] Opposite of _.keyBy. Creates an array from an object where the key is merged into the values keyed by newKey. Example: F.unkeyBy('_key')({ a: { status: true}, b: { status: false }) -> [{ status: true, _key: 'a' }, { status: false, _key: 'b' }]. Passing a falsy value other than undefined for newKay will result in each object key being pushed into its corresponding return array member with itself as value, e.g. F.unkeyBy('')({ a: { status: true}, b: { status: false }) -> [{ status: true, a: 'a' }, { status: false, b: 'b' }]. Passing undefined will return another instance of F.unkeyBy.


(from, to) -> simpleDiff Produces a simple flattened (see flattenObject) diff between two objects. For each (flattened) key, it produced a from and a to value. Note that this will omit any values that aren't present in the deltas object.


(from, to) -> [simpleDiffChanges] Same as simpleDiff, but produces an array of { field, from, to } objects instead of { field: { from, to } }


(from, to) -> diff Same as simpleDiff, but also takes in count deleted properties. Note: We're considering not maintaining this in the long term, so you might probably have more success with any existing library for this purpose.


(from, to) -> [diffChanges] Same as simpleDiffArray, but also takes in count deleted properties. Note: We're considering not maintaining this in the long term, so you might probably have more success with any existing library for this purpose.


A _.pick that mutates the object


Like _.mergeAll, but concats arrays instead of replacing. This is basically the example from the lodash mergeAllWith docs.


{ a: [x, y, z], b: [x] } -> { x: [a, b], y: [a], z: [a] } Similar to _.invert, but expands arrays instead of converting them to strings before making them keys.


key -> { a: { x: 1 }, b: { y: 2 } } -> { a: { x: 1, key: 'a' }, b: { y: 2, key: 'b' } } Iterates over object properties and stamps their keys on the values in the field name provided.



'asdf' -> '(asdf)' Wraps a string in parenthesis.


Maps _.trim through all the strings of a given object or array.


string -> string Converts strings like variable names to labels (generally) suitable for GUIs, including support for acronyms and numbers. It's basically _.startCase with acronym and number support.


string -> {value:string, label:string} Creates a {value, label} which applies autoLabel the string parameter on puts it on the label property, with the original on the value property. You can also pass in an object with value or with both value and label.


[string] -> [{value:string, label:string}] Applies autoLabelOption to a collection. Useful for working with option lists like generating select tag options from an array of strings.


insertAtIndex -> (index, val, string) -> string Insert a string at a specific index.

Example: (1, '123', 'hi') -> 'h123i'


array => string joins an array into a human readable string. See https://github.com/epeli/underscore.string#tosentencearray-delimiter-lastdelimiter--string

Example: ['a', 'b', 'c'] -> 'a, b and c'


(separator, lastSeparator, array) => string Just like toSentence, but with the ability to override the separator and lastSeparator

Example: (' - ', ' or ', ['a', 'b', 'c']) -> 'a - b or c'



regex -> string -> bool Just like ramda test, creates a function to test a regex on a string.


options:string -> string -> regex A curried implementation of RegExp construction.


options:string -> string -> (string -> bool) Makes and tests a RegExp with makeRegex and testRegex.


string -> string -> bool Returns true if the second string matches any of the words in the first string.


string -> string -> bool Returns true if the second string matches all of the words in the first string.


regex -> string -> [[number, number]] Returns an array of postings (position ranges) for a regex and string to test, e.g. F.postings(/a/g, 'vuhfaof') -> [[4, 5]]


start -> end -> regex -> input -> highlightedInput Wraps the matches for regex found in input with the strings start and end.

Example: ('<b>', '</b>', /h/, 'hi') -> '<b>h</b>i'


regex -> string -> [{text: string, start: number, end: number}] Returns an array of matches with start/end data, e.g. F.allMatches(/a/g, 'vuhfaof') -> [ { text: 'a', start: 4, end: 5 } ]



number -> bool Returns true if number is greater than one.


Language level utilities


Just throws whatever it is passed.


Tap error will run the provided function and then throw the first argument. It's like _.tap for rethrowing errors.

exists (alias: isNotNil)

Negated _.isNil


Returns true if the input has a length property > 1, such as arrays, strings, or custom objects with a lenth property


A curried, flipped _.add. The flipping matters for strings, e.g. F.append('a')('b') -> 'ba'


x -> bool Designed to determine if something has a meaningful value, like a ux version of truthiness. It's true for everything except null, undefined, '', [], and {}. Another way of describing it is that it's the same as falsiness except 0 and false are truthy and {} is falsey. Useful for implementing "required" validation rules.


x -> bool Opposite of isBlank


f -> x -> bool Recurses through an object's leaf properties and passes an array of booleans to the combinator, such as _.some, _.every, and F.none


A lens is a getter and setter pair, which can be used to interface to some part of an object graph. Methods that operate on lenses can encapsulate common operations independent of knowledge of their surrounding context. Unlike some traditional functional lenses (like Ramda's), the set methods here are generally mutable.

An object lens is simply an object that has a get and set function. An example of this is a mobx boxed observable.

A function lens is a lens expressed as a single function that takes the value to set or returns the current value if nothing is passed. Examples of this in the wild are knockout observables and jquery plugin api style methods.

The utilities in this library expect can accept either kind of lens, and utilities are provided to seamlessly convert between the two.


Lens stubs are primarily a reference implementation, but are useful for testing and mocking purposes


Takes a value and returns a function lens for that value


Takes a value and returns a object lens for that value

Lens Conversions

Methods to convert between lens types


Converts a function lens an object lens


Converts an object lens to a function lens

Lens Construction

This the main way you'll generally interact with the lens API


propertyName -> object -> { get: () -> object.propertyName, set: propertyValue -> object.propertyName } Creates an object lens for a given property on an object. .get returns the value at that path and set places a new value at that path. Supports deep paths like lodash get/set.


Takes an object and returns an object with lenses at the values of each path. Basically mapValues(lensProp).


value -> arrayLens -> includeLens An include lens represents membership of a value in a set. It's view and set functions allow you to read and set a boolean value for whether or not a value is in an array. If you change to true or false, it will set the underlying array lens with a new array either without the value or with it pushed at the end.

Lens Manipulation

Note: As of version 1.37, any manipulation function that takes a lens can also drop in a key and target object for an implicit lensProp conversion (e.g. you can do view(key, obj) instead of just view(lens))


Lens -> object.propertyName Gets the value of the lens, regardless of if it's a function or object lens


Lens -> (() -> object.propertyName) Returns a function that gets the value of the lens, regardless of if it's a function or object lens


propertyValue -> Lens -> object.propertyName Sets the value of the lens, regardless of if it's a function or object lens


Creates a function that will set a lens with the provided value


Takes an iteratee and lens and creates a function that will set a lens with the result of calling the iteratee with the provided value


Takes a lens and negates its value


Returns a function that will set a lens to true


Returns a function that will set a lens to false

Lens Consumption - DomLens

To help illustrate the potential use cases of the power of lenses, these are some functions that consume lenses in useful ways that are relevant in a DOM context including raw js, react, etc. They are pure functions, have no external dependencies, and are generally trivial - but hopefully they illustrate some interesting use cases.


lens -> {value, onChange} Takes a lens and returns a value/onChange pair that views/sets the lens appropriately. onChange sets with e.target.value


(value, lens) -> {checked, onChange} Creates an includeLens and maps view to checked and set to onChange (set with e.target.checked)


lens -> { onMouseOver, onMouseOut } Takes a lens and returns on onMouseOver which calls on on the lens and onMouseOut which calls off. Models a mapping of "hovering" to a boolean.


lens -> { onFocus, onBlur } Takes a lens and returns on onFocus which calls on on the lens and onBlur which calls off. Models a mapping of "focusing" to a boolean.


field -> lens -> {[field], onChange} Utility for building lens consumers like value and checkboxValues


(field, getValue) -> lens -> {[field], onChange} Even more generic utility than targetBinding which uses getEventValue to as the function for a setsWith which is mapped to onChange.


Aspects provide a functional oriented implementation of Aspect Oriented Programming. An aspect wraps a function and allows you run code at various points like before and after execution. Notably, aspects in this library allow you to have a shared state object between aspects and are very useful for automating things like status indicators, etc on functions.

There is a lot of prior art in the javascript world, but most of it assumes a vaguely object oriented context. The implementation in futil-js is done in just 20 lines of code and seems to capture all of the use cases of AOP.

Note: To do OO style AOP with this these aspects, just use lodash's _.update method and optionally boundMethod from futil if this matters

Caveat: While you can and should compose (or _.flow) aspects together, don't put non aspects in the middle of the composition. Aspects rely on a .state property on the wrapped function that they propagate through, but the chain will break if a non-aspect is mixed in between. Additionally, if you need external access to the state, make sure the aspects are the outer most part of the composition so the .state property will be available on the result of the composition.


{options} -> f -> aspectWrapped(f) The aspect api takes an options object and returns a function which takes a function to wrap. The wrapped function will be decorated with a state object and is equivalent to the original function for all arguments.

Options supports the following parameters:

Name Description
init: (state) -> () A function for setting any inital state requirements. Should mutate the shared state object.
after: (result, state, params) -> () Runs after the wrapped function executes and recieves the shared state and the result of the function. Can be async.
before: (params, state) -> () Runs before the wrapped function executes and receves the shared state and the params passed to the wrapped function. Can be async.
onError: (error, state, params) -> () Runs if the wrapped function throws an error. If you don't throw inside this, it will swallow any errors that happen.
always: (state, params) -> () Runs after the wrapped function whether it throws an error or not, similar to a Promise.catch

Example Usage:

let exampleAspect = aspect({
  before: () => console.log('pre run'),
  after: () => console.log('post run')
let f = () => console.log('run')
let wrapped = exampleAspect(f)
// Logs to the console:
// pre run
// run
// post run


This is a synchronous version of aspect, for situations when it's not desirable to await a method you're adding aspects to. The API is the same, but things like onError won't work if you pass an async function to the aspect.


There are a few basic aspects included on F.aspects (E.g. var loggedFunc = F.aspect(F.aspects.logs)(func)) because they seem to be universally useful. All of the provided aspects take an extend function to allow customizing the state mutation method (e.g. in mobx, you'd use extendObservable). If null, they default to defaultsOn from futil-js - check the unit tests for example usage.


Logs adds a logs array to the function state and just pushes in results on each run


Captures any exceptions thrown and set it on an error error it puts on state


Captures any exceptions thrown and pushes them sequentially into an errors array it puts on state


Adds a status property that is set to processing before the wrapped function runs and succeeded when it's done or failed if it threw an exception. Also adds shortcuts on state for processing, succeeded, and failed, which are booleans which are based on the value of status. Also adds a setStatus method which is used internally to update these properties.


Sets status to null after provided timeout (default is 500ms) elapses. If a null timeout is passed, it will never set status to null.


Prevents a function from running if it's state has processing set to true at the time of invocation


Flows together status, clearStatus, concurrency, and error, taking extend and timeout as optional parameters to construct the aspect


Utility for marking functions as deprecated - it's just a before with a console.warn. Takes the name of thing being deprecated, optionally deprecation version, and optionally an alternative and returns a higher order function which you can wrap deprecated methods in. This is what's used internally to mark deprecations. Includes a partial stack trace as part of the deprecation warning.


All tree functions take a traversal function so that you can customize how to traverse arbitrary nested structures.

Note: Be careful about cyclic structures that can result in infinite loops, such as objects with references to itself. There are cases where you'd intentionally want to visit the same node multiple times, such as traversing a directed acyclic graph (which would work just fine and eventually terminate, but would visit a node once for each parent it has connected to it) - but it's up to the user to be sure you don't create infinite loops.


A default check if something can be traversed - currently it is arrays and plain objects.


The default traversal function used in other tree methods if you don't supply one. It returns false if it's not traversable or empty, and returns the object if it is.


traverse -> (pre, post=_.noop) -> tree -> x A depth first search which visits every node returned by traverse recursively. Both pre-order and post-order traversals are supported (and can be mixed freely). walk also supports exiting iteration early by returning a truthy value from either the pre or post functions. The returned value is also the return value of walk. The pre, post, and traversal functions are passed the current node as well as the parent stack (where parents[0] is the direct parent).


traverse -> _iteratee -> tree -> newTree Structure preserving pre-order depth first traversal which clones, mutates, and then returns a tree. Basically walk with a _.cloneDeep first (similar to a tree map because it preserves structure). _iteratee can be any suitable argument to _.iteratee https://lodash.com/docs/4.17.5#iteratee


traverse -> (accumulator, initialValue, tree) -> x Just like _.reduce, but traverses over the tree with the traversal function in pre-order.


traverse -> tree -> [treeNode, treeNode, ...] Flattens the tree nodes into an array, simply recording the node values in pre-order traversal.


traverse -> f -> tree -> [f(treeNode), f(treeNode), ...] Like treeToArray, but accepts a customizer to process the tree nodes before putting them in an array. It's _.map for trees - but it's not called treeMap because it does not preserve the structure as you might expect map to do.


traverse -> tree -> [treeNodes] Returns an array of the tree nodes that can't be traversed into in pre-order.


(traverse, buildIteratee) -> ([_iteratee], tree) -> treeNode Looks up a node matching a path, which defaults to lodash iteratee but can be customized with buildIteratee. The _iteratee members of the array can be any suitable arguments for _.iteratee https://lodash.com/docs/4.17.5#iteratee


traverse -> transformer -> _iteratee -> tree -> result Similar to a keyBy (aka groupBy) for trees, but also transforms the grouped values (instead of filtering out tree nodes). The transformer takes three args, the current node, a boolean of if the node matches the current group, and what group is being evaluated for this iteratee. The transformer is called on each node for each grouping. _iteratee is any suitable argument to _.iteratee, as above.


traverse -> buildPath -> tree -> result Creates a flat object with a property for each node, using buildPath to determine the keys. buildPath takes the same arguments as a tree walking iteratee. It will default to a dot tree path.


(build, encoder) -> treePathBuilderFunction Creates a path builder for use in flattenTree. By default, the builder will look use child indexes and a dotEncoder. Encoder can be an encoding function or a futil encoder (an object with encode and decode functions)


prop -> treePathBuilderFunction Creates a path builder for use in flattenTree, using a slashEncoder and using the specified prop function as an iteratee on each node to determine the keys.


A utility tree iteratee that returns the stack of node indexes


A utility tree iteratee that returns the stack of node values


(traverse, buildIteratee) -> {walk, reduce, transform, toArray, toArrayBy, leaves, lookup, keyByWith, traverse, flatten, flatLeaves } Takes a traversal function and returns an object with all of the tree methods pre-applied with the traversal. This is useful if you want to use a few of the tree methods with a custom traversal and can provides a slightly nicer api. Exposes provided traverse function as traverse