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Simple TS implementations of some Rust structures

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Rust-TS: Clean Implementation of Various Rust Style Structs

98% test coverage

This package seeks to implement useful structs from Rust for use in TypeScript Projects.

At this time this includes

  • Option Enum (mostly complete)
  • Result Enum (mostly complete)
  • Iterator (mostly complete)
  • DequeueVec/CircularBuffer (partial)

There is also some progress made on an implementation for Future to allow Promises to be awaited as a property expression, and not only with the await keyword.


pnpm add @ekwoka/rust-ts

Result<T,E> and Option<T>

These two Enums in Rust are used to handle methods and functions that can error, by returning a Result variant, and those that can return nothing, by returning an Option variant.

Since these two do serve somewhat similar purposes, they also have fairly similar interfaces. This documentation will only lightly touch on the usage/benefits of these structs as a whole, but you can refer to materials on these structs in a Rust context for more info there.

Result Variants

The Result<T,E> type is made up of two variants:

  • Ok<T>: Indicates the action was successful, and contains the proper return value
  • Err<E>: Indicates the action was not successful, and contains the error encountered.

A function can return a Result<T,E> type and then return both of these different objects based on what happens. These contain the same interface, with different behaviors, allowing a nice type safe and expressive handling of errors.

return decrypt(data)
	.map(item => ({ statusCode: 200, body: item }))
	.unwrapOrElse(error => ({ statusCode: 500, body: error }))

Here, decrypt returns a Result that indicates if the data could be properly decrypted, or an error if it fails to decrypt, and then the success condition and the error condition can be handled expressively to conform them into the same shape object, without needing statements or other behaviors.

This can be chained near endlessly, allowing a focus on the happy path, and deferring error handling, similar to .then/.catch but with better flow.

return decrypt(data)
	.map(item => ({ statusCode: 200, body: item }))
	.unwrapOrElse(error => ({ statusCode: 500, body: error }))

Here, similar to before, decrypt returns a Result, but now we pass the Ok values into other functions that also return Result variants (JSON.tryParse and Item.tryFrom). If decrypt is Ok then we try to parse it as JSON, if that is Ok, we try to validate it as an Item, and if that is okay, we return a 200 status.

If any of those is Err we return a 500 and the error info.

Now, the code can focus on the happy path, with the errors handled later.

Naturally, this makes a ton of sense to those familiar with Rust.

Option Variants

The Option<T> type is made up of two variants:

  • Some<T>: Indicates the presence of a value
  • None: Indicates the lack of a value

This is similar to the Result variants, but instead of indicating success and error states, indicates the presence of a value or lack of a value. For example:


Is the id not matching to a user an error? Not really. Any kind of find action, something not being found is quite different from there being an error in the process of searching. So you can return an Option to indicate, in this case, the specified user may or may not exist.

return getUser(userID).orElse(createUser)

Here, we can get the user, or create it if it doesn't exist already, all in an expression that allows focusing on the happy path


To use the Result and Option enums, you can simply import their appropriate Variant for your case and instantiate it as a class (except for None which is already instantiated as a constant instance)

import { Ok, Err } from '@ekwoka/rust-ts'

export const tryParseJSON = (data: string) => {
  // to handle thrown exceptions from native methods
  try {
    // if successful will return an `Ok`
    return new Ok(JSON.parse(data))
  } catch (e) {
    // if an error was thrown, will return an `Err`
    return new Err(e) 

// attempts to parse and logs the hello key to the console if it exists. Does nothing on an error
tryParseJSON('{"hello":"world"}').map(data => data.hello).andThen(console.log)

While this pattern would allow your own methods to return Err and you can handle them and do transformations and pass back the modified Result, we would need to wrap other things that might throw exceptions to catch and convert them to Err.

As a utility, this package exposes Try and TryAsync to help wrap such methods to return a Result instead of throwing.

import { Some, None } from '@ekwoka/rust-ts'

export getUser = (userID: string) => {
  // checks the user list for a matching user object
  const maybeUser = users.find(user => === userID)

  // if the user is found, return Some<User>
  if (maybeUser) return new Some(maybeUser)

  // if no user, return None
  return None

getUser(1).map(user => user.items)

Note: None does not need to be instantiated with new. You just reuse the same instance for all None since they have no internal data of relevance. This also allows them to be directly compared None === None . You can import none to get the class constructor if there is reason to need unique None values, but this is not recommended.

None behaves a lot like undefined or null in JavaScript, but with actual methods similar to Some that allow it to be used expressively without separate statements or extra checks to handle the None case up front. This can allow None to propagate naturally through many operations.

In systems level languagesnull is different than what it is in JavaScript (it's a null pointer, and not a specific thing) so some value of None over null is lost in JavaScript, but the expressive handling of Option types is still a major boon.

Typing Function Returns

To handle typing function signatures that return Result and Option types, you can import the type and use it in the type signature.

import type { Result, Option } from '@ekwoka/rust-ts'

declare const tryParse = (data: string) => Result<unknown, SyntaxError>
declare const getUser = (userId: string) => Option<User>

Naturally, Ok<T> and Err<E> implement Result<T,E> and Some<T> and None implement Option<T>


These two structs are very similar in their APIs. They mostly feature the same methods, with almost matching semantics, but can vary slightly. This guide to the Methods will go over the methods once as if it is a single struct, and present the signatures for each, and any accompanying differences between the two where they exist.

Shared methods will be presented first, and struct specific will be at the end.

constructor<T>(value: T)

The constructors for these are all quite simple. They take in a value (for Ok and Some) or error (for Err) and store it internally, returning the object.

unwrap(): T

Returns the inner value for Ok<T> and Some<T>. Throws an error on Err and None.

This can be used safely after checks that handle Err and None cases. Otherwise, this throws an exception for attempting to unwrap and unsafe value. As such, using unwrap means that you as a developer need the inner value and have validated that it exists.

It is recommended to use expect instead of unwrap in many cases (or one of the following unwrapOr methods), which allows you to provide a message for why you as a developer EXPECT that the value is safe to unwrap, so others reading it can know.

unwrapOr(defaultValue: T): T

This returns the inner value for Ok<T> and Some<T> and otherwise returns the provided value in the case of None or Err.

unwrapOrElse(op: (error: E) => T): T (Result)
unwrapOrElse(op: () => T): T (Option)

Returns the inner value for Ok<T> and Some<T>, and otherwise calls the passed in callback and returns its value for Err and None types. When a Result is Err, the Err is passed to the callback.

unwrapErr(): E (Result only)

Returns the inner value of an Err and otherwise throws an exception.

expect(message: string): T

Returns the inner value for Ok<T> and Some<T>. Otherwise throws an exception.

This is, code wise, almost exactly the same as unwrap, and the semantic distinction is that the message value can inform other developers why the code is unsafely accessing the inner value.

const config = readFile('./config.toml').expect('Config file must be present for application to run')

The passed in message is a part of the exception thrown, but the message is not meant to be an error message. It is meant to explain why the unwrapping of the value should be safe. Think this value is safe because <MESSAGE>.

andThen<U>(op: (value: T) => U): U | Err<E> (Result)
andThen<U>(op: (value: T) => U): U | None (Option)

Calls the passed in callback with the inner value for Ok<T> and Some<T> types returning that callbacks return value, otherwise simply returns this for Err and None types.

This can be useful for performing a final action on valid types without unwrapping them, or to help with flattening values. For instance, passing an Ok inner value to another function that returns a Result, using andThen will prevent you from ending up Result<<Result<T,E>, E2> value.

orElse<U>(op: (E) => U): U | Ok<T> (Result)
orElse<U>(op: () => U): U | Some<T> (Option)

Similar to andThen but operates on Err and None values, calling the callback, and returning it's return type, or the Ok / Some variant.

map<U>(op: (value: T) => U): Result<U, E>
map<U>(op: (value: T) => U): Option<U>

Passes the inner value to the callback and uses the returned value as the inner value of a new struct for Ok<T> and Some<T> variants, and does nothing with Err and None variants.

This, among the other methods, indicates that Result and Option structs are Monads, that wrap an inner value and allow performing operations based on their internal values returning other structs of the same or similar types.

This is not unlike how Promise work in JavaScript, where .then and .catch operate on the resolved or rejected values of one Promise and return another Promise with a new inner value.

mapErr<U>(op: (error: E) => U): Result<T, U> (Result only)

Similar to the above map but operates only on the Err<E> internal value, and returning a new Err<U>, while returning the original Ok<T>.

mapOr<U>(op: (value: T) => U, defaultValue: U): U

This and the following mapOrElse run the callback (with provided inner value from Ok and Some variants) and then directly return the value, while providing an alternative default value in the case of Err or None variants.

This is not unlike chaining .map.unwrapOr.

mapOrElse<U>(op: (value: T) => U, opErr: (error: E) => U): U (Result)
mapOrElse<U>(op: (value: T) => U, opNone: () => U): U (Option)

Calls the first callback with the Ok or Some inner value if present, otherwise calls the second callback with the Err internal value or nothing for Err and None variants.

This is similar to chaining .map.unwrapOrElse. This signature is also similar to how .then works on Promise where the first callback handles resolved values, and the second handles rejected values.

inspect(inspector: (value: T) => void): Result<T, E> | Option<T>

Calls the inspector callback with the inner value of Ok and Some variants, and then returns the original Result or Option. As the name implies, this is a nice way to inspect the inner value in a method chain that might otherwise require multiple expressions to store an intermediary value and then inspect it and then continue processing. Most simply just used as .inspect(console.log).

inspectErr(inspector: (error: E) => void): Result<T, E> (Result only)

The same as inspect but only calls the callback on Err<E> with the error value. Returns the original Result struct.

flatten(): Result<I,IE> | Option<I>

There can be cases where you might end up with nested Result or Option values, like Result<Result<number, string>, Error> or Option<Option<string>>.

Naturally, these can be annoying to deal with safely, especially in chains where the inner value could be a Result<number, string>|number.

This flattens these values. So a Result<Result<number, string>|string, Error> would become a Result<number|string, string | Error>.

The types here can get quite complicated to get to a point of actually being Type Safe, so expect that there could be issues in particularly complex types. The difficulty mainly comes from allowing a known type of Ok<Ok<number>> flatten to Ok<number> and not resolve to a Result<number, never>, which I just don't think is quite tackled yet. Most of the time you won't be operating on known Ok or Err variants, so this method should work with no type issues on Result and Option types.

Result exclusive API

isOk(): this is Ok<T>

Returns true if the Result<T, E> is actually and Ok<T>, otherwise false

isErr(): this is Err<E>

Returns true if the Result<T, E> is actually and Err<E>, otherwise false

ok(): Option<T>

Returns a Some<T> in the event of the Result<T,E> being Ok<T>, otherwise returns None (for Err<E>)

Option exclusive API

isSome(): this is Some<T>

Returns true if the Option<T> is Some<T> otherwise false

isNone(): this is None

Returns true if the Option<T> is, in fact, None otherwise false

okOr<E>(error: E): Result<T, E>

Returns an Ok<T> for Some<T> otherwise returns an Err<E> for None created from the passed in error.


This is a simple and clean implementation of the Iter trait from Rust, named as RustIterator to avoid conflicts with the current abstract Iterator interface in TypeScript, or the upcoming Iterator interface in JavaScript.

Right now, iterators in JavaScript SUCK. They're really bad. You can call next() check if the iterator is done , do a for..of loop, or spread it into an Array. That's not a whole lot.

Doing more active work with iterable values is left to TransformStream implementations that are required to be async and involve a lot more work currently (though compose in Node and Bun makes it quite a bit easier). Or of course, just doing a bunch of loops, or resorting to the various Array methods, which can perform unnecessary work, consume more memory, and generally just be inefficient.

Note: the Array methods will often be more performant than this Iter implementation. This is due in part both to the fact the Array methods are highly optimized in the native side of the runtimes, and that the implementations of native Iterator and Generator which this relies on are needlessly wasteful. There are still plenty of cases, especially with potentially infinite data, where this iterator implementation can reduce the total amount of work done. The main case would be in large lists, that will have many map and filter style operations but where you only want the first n values. the Array methods would need to map and filter the entire list, even if only 5 final values are used.

The main benefit of this Iterator implementation, is not performance over existing native alternatives, but in the breadth of operations offered for iterating over the data contained.


RustIterator<T> implements both the Iterable<T> and Iterator<T> interfaces, which, by definition, means it implements IterableIterator<T>.

What this means is that you can use it in all cases where an Iterable or Iterator is needed, like in for..of loops, spreading into a list (...) or destructuring [first, second] = iter.

The following methods/properties are how these interfaces are implemented.

next(): IteratorResult<T>

Returns the next yielded value from the iterator as an IteratorResult<T>. This consumes one value of the upstream Iterator

interface IteratorResult<T> {
  done: boolean;
  value: T | undefined
done: Boolean

Allows introspecting into whether the Iterator is capable of yielding new values.

This does not consume any part of the Iterator, and as such does not actually KNOW if the Iterator is done, just indicates if the Iterator has previously completed. This means an Iterator with a done value of false may or may not actually have an additional value, but a done of true means the Iterator should not yield any new values. Iterator instances that were previously done can still yield new values (and mark themselves as not done) in the future, on a technical level. RustIterator instances will generally never actually do this, as nearly every method will return a fused iterator that will never again yield a new value.

[Symbol.iterator](): RustIterator<T>

Returns itself, as an Iterator.

This method, called with the well-known Symbol @@iterator is used internally when doing Array destructuring, spreading, or for..of looping over Iterable objects. This is all that is needed to implement the Iterable abstract interface.

In this case, the method simply returns the same RustIterator instance it is called on, not a distinct object.

Checking maybeIterator[Symbol.iterator]() === maybeIterator is the most common way to check if an object is an Iterator, as all native Iterator implement Iterable and return themselves for this method. Checking for the existence of a next method is much less of a guarantee.

`new RustIterator(upstream: Iterable)

Constructs a new RustIterator from an Iterable.

This will call the @@iterator method on the Iterable and store the returned Iterator internally. This will not otherwise consume any Iterator passed in. If the Iterable supplied, is an Iterator then consuming the RustIterator would consume that Iterator.

Creating a RustIterator from an upstream RustIterator will return a new RustIterator that wraps the previous, not simply the same RustIterator. It will not clone the values, or any other magic.

nextChunk(n: number): IteratorResult<T[]>

Returns an IteratorResult of an Array containing the next n values from the Iterator. If the Iterator yields less than n values, the IteratorResult will be marked done and only include all those values yielded up to n. There is no guarantee in this method that n values will be returned, just that no more than n will be returned.

new PeekableRustIterator<T>(upstream: Iteratable<T>)

PeekableRustIterator is a special kind of RustIterator that enables special behavior to allow you to inspect the next value to be yielded by next, without consuming the Iterator.

peek(): IteratorResult<T>

Returns the IteratorResult that will next be yielded when calling next. This can allow the developer to check the next value, and change course, without consuming the value in the Iterator.

Due to how Iterator work, this WILL consume the next value from the upstream Iterator as that value will need to be consumed to inspect it. If you split iterators out and consume them in different places strategically, this will block that value from being able to be yielded by other consumers of that Iterator. That value is stored internally and will be returned the next time next is called.

peekable(): PeekableRustIterator<T>

on both PeekableRustIterator and RustIterator, the peekable method returns a PeekableRustIterator. Naturally returning itself, and a new instance respectively.

peeked: IteratorResult<T>

If the Iterator is currently in the state of having been peeked (peek has been called since the last time next was called), this will return that IteratorResult, otherwise undefined. This is primarily internal for handling the peeked value.

Extends RustIterator<T>

All of RustIterator methods and properties are available on PeekableRustIterator

Consuming Methods

There are many different styles of methods on RustIterator beyond the basic interfaces presented above. I've loosely grouped these into Consuming, Iterating, and Special categories.

  • Consuming: Immediately consumes all or a portion of the Iterator returning a value made from them (ex. collect/reduce)
  • Iterating: Returns a new RustIterator that will transform the values when consumed, but does not consume the original Iterator (ex. map/filter)
  • Special: Anything else, mainly methods that will return a new RustIterator while also consuming the previous Iterator (ex reverse/sort)

These are semantically grouped, as the mental model for how you might use them in more complex applications is distinct.

All of the methods in this group consume the Iterator, partially or in full, performing all upstream work to yield those values. If an Iterator infinitely yields values, these can potentially lock the thread entirely.

collect(): T[]

Probably the most important Consuming method, collect consumes the Iterator and places all of the values into an Array. Very useful for passing the values out to things that need Array or storing an intermediary collection of the values, to allow multiple iterations.

forEach(f: (val: T) => void): void

See Array.forEach

Consumes the Iterator, calling f with each value.

fold<A = T>(fn: (acc: A, item: T) => A, initial?: A): A

Similar to Array.reduce.

Consumes the Iterator, calling fn with each value and the initial as an accumulator. The returned value from each fn call, is passed as the acc.

If no initial is passed, the very first item yielded by the Iterator is used as the accumulator, with the first call of fn being passed both the first and second yielded values.

This is the generalized form of reduce that allows the acc type to be different from T.

reduce(fn: (acc: T, item: T) => T, initial?: T): T

Similar to Array.reduce but requires acc be the same type as T

Internally uses fold

Consumes the Iterator, calling fn with each value and the provided initial as an accumulator. If no initial is provided, the first value will be used as the accumulator.

The value returned from each calling of fn is passed as the first argument (acc) to the following call of fn, with the final iterations return value being returned.

sum(): T where T = number | string | bigint

Internally uses reduce

Consumes the Iterator, adding the values together (with the + operator), returning the final value (a summed number or bigint or a concatenated string)

This will only have predictable results on string, number and bigint type Iterator. Other primitives and objects can have unpredictable and not type safe results.

max(): T | undefined where T = number | bigint | string

Internally uses reduce

Returns the maximum value (with the > operator) yielded by the Iterator.

This will only have predictable results on string, number and bigint type Iterator. Other primitives and objects can have unpredictable and not type safe results.

string values will be sorted by the first codepoint that differs between two string values, with later codepoint being returned. Characters that are made of multiple codepoint are treated as two separate codepoint. This mainly applies to non-English texts and Emojis.

min(): T | undefined where T = number | bigint | string

Internally uses reduce

Returning the minimum value (with the < operator) yielded by the Iterator.

This will only have predictable results on string, number and bigint type Iterator. Other primitives and objects can have unpredictable and not type safe results.

string values will be sorted by the first codepoint that differs between two string values, with earlier codepoint being returned. Characters that are made of multiple codepoint are treated as two separate codepoint. This mainly applies to non-English texts and Emojis.

find(checker: (item: T) => unknown): T | null

Calls the checker with each yielded value, returning the first value that results in a truthy value. Returns null if no such value is found.

The Iterator is only consumed up until the first match. Any remaining values could still be yielded. Calling find multiple times could be used like filter in cases where the filtering condition may change as the Iterator is iterated.

any(checker: (item: T) => unknown): boolean

Internally uses find

See to Array.some

Returns true if any yielded value returns true when passed to the checker, otherwise false.

Will return false if the Iterator yields no values

all(checker: (item: T) => unknown): boolean

Internally uses any

See Array.every

Returns true is all the yielded values return true when passed to the checker, otherwise false.

Will return true if the Iterator yields no values

position(checker: (item: T) => boolean): number | null

Similar Array.findIndex, except that it will not return -1 on no match

Returns the 0-index of the first yielded value that returns true when passed to the checker.

Returns null when no yielded values match

findIndex(checker: (item: T) => boolean): number | null

Alias for position

count(): number

Returns the count of items returned by the Iterator.

last(): T | undefined

Returns the final value yielded by the Iterator. If the Iterator is already done or never yields a value before becoming done, returns undefined.

advanceBy(n: number): void

Advances the Iterator n steps consuming those values.

nth(n: number): T | undefined

Returns the nth value yielded by the Iterator. If the Iterator becomes done before n values, returns undefined

Iterating Methods

All of the following methods return a new RustIterator without consuming any values of the previous. This new Iterator will yield transformed, filtered, or other modified values of the previous Iterator.

map<S>(f: (val: T) => S): RustIterator<S>


Will yield the result of passing each value to f.

filter(f: (val: T) => boolean): RustIterator<T>

See Array.filter

Will yield only values that, when passed to f, return a truthy value.

take(n: number): RustIterator<T>

Will yield only the first n values

takeWhile(f: (val: T) => boolean): RustIterator<T>

Will continue to yield values until a value, when passed to f returns a falsy value.

Will not yield the first value that returns falsy, but it will consume that value from the upstream Iterator.

stepBy(n: number): RustIterator<T>

Will yield only ever nth value from the upstream Iterator.

enumerate(): RustIterator<[number, T]>

Will yield tuples of the 0-index and the value.

This is useful for having access to the index like is available in the Array methods

arrayChunks<N extends size = 1>(size: N)

Will yield tuples of size length of values from the Iterator.

If the end of the Iterator is reached and the internal chunk is not yet size length, the chunk is yielded as is. The missing values will be undefined.

inspect(fn: (val: T) => void): RustIterator<T>

Yields every value as is, but first passes the value to fn.

This allows accessing the value, primarily for debugging purposes, expressively in the method chain.

The most simple use is with console.log

scan<A = T, R = T>(fn: (state: [A], val: T) => R, initial: A): RustIterator<R>

Yields the result of passing the value to fn.

fn is also passed a tuple of initial as the starting state, and continues to pass that same state as an argument to each calling of fn. When you mutate state, this allows you to iterate while maintaining some kind of internal "memory" to the iteration, so it can have some sense of the past.

flat<D extends depth = 1>(depth?: D)

See Array.flat

Yields the individual values yielded by flattening the value (where T = Iterable) depth number of times.

This can allow multiple Iterable to be combined, to iteratively iterate over each successive Iterable.

For this purpose, only Iterable objects will be flattened. string, while Iterable, will not be flattened into individual characters or codepoint.

flatMap<S>(mapper: (val: T) => S)

See Array.flatMap

Yields the individual items yielded by flattening the result of calling mapper with the value. This will only flatten a single level.

This is similar to separately calling .map.flat with a depth of 1.

For this purpose, only Iterable objects will be flattened. string, while Iterable, will not be flattened into individual characters or codepoint

window<S extends size = 1>(n: S)

Yields tuples of n size containing a rolling window of values. Each subsequent yielded value will be the same as the previous, except with the head value removed, and a new value added at the tail.

No window will be yielded until n values are consumed, even if the Iterator becomes done before then. Similarly, once the Iterator is done, no more values will be yielded. This means each window yielded will be n size, each time, every time.

Special Methods

These methods don't cleanly fit into the above groups of methods. At this time, this mainly means they return a new RustIterator while consuming the previous, or otherwise having a behavior that is not simple to consider them as simply Iterating.

chain(other: Iterable<T>): RustIterator<T>

Individually yields all the values of other AFTER consuming all of the upstream Iterator.

`zip<S = T>(other: Iterable): RustIterator<[T, S]>

Successively yields a tuple of the next values of both the upstream Iterator and other.

This is useful for merging values together as pairs automatically.

cycle(): RustIterator<T>

Repeatedly yields each individual value of the upstream Iterator forever, resulting in an Iterator that can never be done.

As this requires storing all yielded values in memory, for large datasets, this means a lot of memory.

As the returned Iterator can NEVER end, Consuming methods could result in a blocked thread, if there have not been additional methods that limit the length of the Iterator (like take, takeWhile, find) that will eventually fuse the Iterator.

sort(compare?: (a: T, b: T) => number): RustIterator<T>

See Array.sort

Yields each value of the upstream Iterator, after sorting the values through compare.

By default this uses a lexigraphicCompare sort when no compare is passed. This emulates the native behavior of Array.sort, although mixed arrays of number | string can have strange results, as this will not turn all number to string when sorting. It is recommended to provide your own compare any time you have values that are not strictly number | bigint.

To accomplish this, the upstream Iterator is completely consumed and stored in memory, immediately upon calling this method, even if the returned RustIterator has not yielded any values.

To reduce the total work performed, and iteratively sort the values, a Bubble Sort algorithm is used.

When the resulting Iterator yields a value, the first value bubbles through being compared to each remaining value, being swapped as needed. When this process is done, the value is yielded.

As values are yielded, the internal storage of values is reduced in memory.

While Bubble Sort can increase total comparisons when needing to sort the entire list, it works nicely for this use case, as it doesn't prematurely sort any sub arrays while producing the next value. This is ideal for the purpose of an Iterator where you do as little work as possible until it is finally needed, and where not all values will actually need to be sorted, as in the following example.

const lowestThreePrices = prices.sort().take(3).collect()

While this will collect all of the prices, it will only sort out the lowest three values, discarding the remaining unsorted values.

Due to the naive nature of the sorting implementation, the order of like values (those that when compared with compare return 0) is not preserved from the original order. In fact, they will almost always be reversed. Maybe that's something to the future...

reverse(): RustIterator<T>

See Array.reverse

Individually yields all the values of the upstream Iterator in reverse order.

To accomplish this, the upstream Iterator is completely consumed and stored in memory, immediately upon calling this method, even if the returned RustIterator has no yet yielded any values.

The values in memory are not stored in reverse order, instead the values are yielded from the tail to the front.


Outside of the JavaScript world, Array are commonly of a fixed size (known at compile time to arrange for a fixed memory space). Naturally, this means that Array cannot change in size, like having items added or removed, which is how JavaScript Array work.

In these languages, commonly Vector (or Vec) is used to indicate a sequential list of unknown size (like JavaScript Array).

So that's why it's a Vec, but what is the meaning of the Dequeue?

The Problem with Array

Both Rust Vec and JavaScript Array work in a specific way. They have one item at the start, followed by all the other items. Adding to the tail (push) and removing from the tail (pop) is simple. You just (provided there is enough memory) stick the item in the next spot, or take it out. Very simple. This is a structure commonly called a Stack. Last thing in is the first thing out. Very efficient.

When implementing some pathfinding algorithms, you may handle the next step to check with a Stack. A simple Depth First Search (DFS) will almost always use a Stack

However, adding to the head (unshift) and removing from the head (shift) are much more complicated. To add a new item to the head, you need to move every other item in the list over one. So the item at index 0 is moved to index 1 and the item at index 1 is moved to index 2 and so on. Similar for removing (just moving everything to a lower index).

It's common in many algorithms, to use an Array as a Queue, where new items are added to the tail, and items are removed from the head. When used like this, Array perform a lot of extra work to move every item forward in line. This can be responsible for significant slowdowns when the Queue gets to be thousands or tens of thousands of items long.

In many pathfinding algorithms, a Queue is used to schedule the search. A simple Breadth First Search (BFS) would use a Queue. Commonly, Djikstra and A* would use a Queue though they can technically be made to do either, for different reasons.

The Solution

So, if we can add many items to the tail of a Queue, but each time we remove an item from the head (or DE-QUEUE it!) there is a lot of extra work moving items around, how could we make the removing of items be as efficient as if we were using a Stack?

The answer is in how Array use the memory they are given.

Typically, at the System level, lists that can be of a dynamic length. They will start with some initial size (commonly 0) and as items are added, more memory is allocated to them, and when these allocations happen, the Array is moved around in memory to where it can have a single contiguous block of memory.

When items are removed, the allocated space is not reduced. Instead the Array still occupies that space, even with no values. In JavaScript, during garbage collection, the Array may be moved, and the memory resized.

The basic thing to understand is that the space in memory is fixed (when not growing).

So, if the memory is fixed, and we pop an item off the tail, the memory is not released from the Array, but that space now holds no value. So, what if we could, when we shift a value off the head, do the same thing? Just push the start of the actual item list of the Array at a space in memory that is not actually the beginning of the memory space?

Well, that's how a VecDequeue works!!! When an item is removed from the front, we just internally track the head of the Vec as being at a later index. This way, items do not actually need to be moved.

The Circular Buffer

Now, just doing that, and marching the memory along would be impractical. If we just keep pushing memory to the tail and adjusting the head back, we could end up with a memory space that is many gigabytes in size, but containing little real data! That's a horrible memory leak!

VecDequeue in Rust, and this VecDequeue in this package both handle this issue by having the memory instead be a Circular Buffer (which is why VecDequeue is exported under the alias CircularBuffer as well). In a Circular Buffer all of the list items are stored in order (as opposed to a Linked List that would store them scattered in memory), but the memory space is treated as being circular - that is, the next item after the one stored at the end of memory is at the beginning of the memory.

As items are added and removed, the offsets for the head and tail are moved around, modifying data, but leaving everything in place. When the tail wraps all the way around to the head, we increase the total memory, and do a quick shifting of the wrapped portion of the tail to be contiguous with the head portion in the new memory space.

This means we do still sometimes move items around in memory, but only as the buffer grows. To help reduce this, the size of the buffer is doubled whenever new memory is needed, not expanded one by one. This is a common handling of list memory.

Maybe we need a Demonstration...

In a typical Array, these operations would look like this:

empty is used here to refer to a spot in the list in memory that does not have a value. It may or may not be viewable when logging an Array as normally that only shows empty known items, not in memory space.

Multiple lines of comments are used to indicate the internal steps the list may use to ahndle the code action

const array = new Array(3) // [empty, empty, empty]

array.push(1) // [1, empty, empty]

array.push(2) // [1, 2, empty]

array.push(3) // [1, 2, 3]

  // | [empty, 2, 3]
  // | [2, empty, 3]
  // | [2, 3, empty]

array.push(4) // [2, 3, 4]

  // | [empty, 3, 4]
  // | [3, empty, 4]
  // | [3, 4, empty]

  // | [3, empty, 4]
  // | [empty, 3, 4]
  // | [5, 3, 4]

Naturally, this gets worse and worse! While push and pop are O(1), shift and unshift are both O(n)!!

Okay, so how does a Circular Buffer work?

const array = new CicularBuffer(3) // [empty, empty, empty]

array.push(1) // [1, empty, empty]

array.push(2) // [1, 2, empty]

array.push(3) // [1, 2, 3]

array.shift() // [empty, 2, 3]

array.push(4) // [4, 2, 3]

array.shift() // [4, empty, 3]

array.unshift(5)  // [4, 5, 3]

Back to just O(1) updates!!

How does this shake out in practice?

In the benchmarks (in this repo run pnpm exec vitest bench), we see that the performance difference in different situations is as follows:

  • As a Queue (push to tail, shift from head): VecDequeue performs 4.7x faster than Array
  • As a Stack (push to tail, pop from tail): Array performs 1.1x faster than VecDequeue
  • With only head (unshift to head, shift from head): VecDequeue performs 17x faster than Array

These will of course depend, in reality, on how many operations, the distribution of those operations, and the size of the list. There were some situations where VecDequeue even out performed Array as a Stack.

So, Stack use cases may still benefit from just being a native Array, other cases can massively benefit from using a VecDequeue


Using a VecDequeue is very similar to using an Array, we just don't get the nice literal syntax.

import { VecDequeue } from '@ekwoka/rust-ts'

const array = new VecDequeue()

for (let i = 0; i < 100; i++)

Instance Methods

The goal for this API, though not quite there now, is to implement the whole Array interface, as well as a few of the useful methods of the VecDequeue struct from Rust. When the two have differently named methods that do the same thing, the primary name is the Array method, though many will be aliased with a camelcase version of the Rust name.

new VecDequeue<T>(initializer: Array<T> | number = 0)
new CircularBuffer<T>(initializer: Array<T> | number = 0)

The class is exported under both VecDequeue and CircularBuffer for convenience. It's the exact same thing.

When the initializer is an Array, the Vec buffer is initialized to the size of that array, the values are copied into the Vec buffer.

When the initializer is a number (or nothing, defaulting to 0), The Vec buffer is initialized to a size of the max of 1 or the number.

at(i: number): T

Returns the value at i index.

This is not the index within the internal buffer, but the 0-index from the head as it wraps around the circular buffer

get(i: number): T

Alias of at

set(i: number, v: T): void

Sets a value to the i index.

push(v: T): void

See Array.push

Pushes a value into the Vec at the tail.

This operation, performs a size check (by calling grow) before adding the value, in the event the buffer is full.

pop(): T | undefined

See Array.pop

Returns the value at the tail of the Vec removing it from the Vec, or undefined if none exists.

`unshift(v: T): void

See Array.unshift

Inserts a value to the Vec at the head.

This operation, performs a size check (by calling grow) before adding the value, in the event the buffer is full.

shift(): T | undefined

See Array.shift

Returns the value at the head of the Vec removing it from the Vec, or undefined if none exists.

first(): T | undefined

Returns the value at the head of the Vec, or undefined if none exists.

last(): T | undefined

Returns the value at the tail of the Vec, or undefined if none exists.

grow(): void

This is an internal method.

Checks if the buffer is full. If so, increases the buffer size by 100%, and moves any wrapped tail elements into the extended memory space.

[@@iterator](): Iterator<T>

Returns an Iterator over the items in the Vec, appropriately wrapping around the circular buffer.

Despite how the circular buffer, works, this has similar semantics to iterating over an array, in regards to if items are added and removed in the process. If you remove an item early in the list, the Iterator will appear to skip an item in its iteration.

toIter(): RustIterator<T>

Returns a RustIterator with the Vec as the upstream Iterator.

Static Methods

from<T>(arr: Array<T>): VecDequeue<T>

See Array.from

Creates a VecDequeue from an Array

`from(opt: { length: number }, mapper?: (v: number, i: number) => T): VecDequeue

See Array.from

Creates a VecDequeue of opt.length size by passing the 0-index into mapper for each item in the size.


Package last updated on 01 Jul 2024

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