A library for safe functional programming in JavaScript, with first-class support for TypeScript
The true-myth npm package provides a set of utilities for working with types that represent the presence or absence of a value (Option) and the success or failure of an operation (Result). It is inspired by functional programming concepts and aims to make error handling and value management more robust and expressive.
Option
The Option type represents a value that may or may not be present. It can be either Some (containing a value) or None (containing no value). The match method allows for pattern matching on the Option type.
const { Option } = require('true-myth');
const someValue = Option.of(42);
const noneValue = Option.none();
someValue.match({
Some: (value) => console.log(`Got a value: ${value}`),
None: () => console.log('Got nothing')
});
noneValue.match({
Some: (value) => console.log(`Got a value: ${value}`),
None: () => console.log('Got nothing')
});Result
The Result type represents the outcome of an operation that can either succeed (Ok) or fail (Err). The match method allows for pattern matching on the Result type.
const { Result } = require('true-myth');
const successResult = Result.ok('Success!');
const failureResult = Result.err('Failure!');
successResult.match({
Ok: (value) => console.log(`Operation succeeded with value: ${value}`),
Err: (error) => console.log(`Operation failed with error: ${error}`)
});
failureResult.match({
Ok: (value) => console.log(`Operation succeeded with value: ${value}`),
Err: (error) => console.log(`Operation failed with error: ${error}`)
});Chaining and Transformations
Both Option and Result types support chaining and transformations using methods like map. This allows for functional-style transformations of the contained values.
const { Option, Result } = require('true-myth');
const someValue = Option.of(42);
const transformedValue = someValue.map(value => value * 2);
transformedValue.match({
Some: (value) => console.log(`Transformed value: ${value}`),
None: () => console.log('Got nothing')
});
const successResult = Result.ok(42);
const transformedResult = successResult.map(value => value * 2);
transformedResult.match({
Ok: (value) => console.log(`Transformed result: ${value}`),
Err: (error) => console.log(`Operation failed with error: ${error}`)
});Folktale is a suite of libraries for generic functional programming in JavaScript. It provides similar types like Maybe (equivalent to Option) and Result, along with a broader set of functional programming utilities. Folktale is more comprehensive and includes additional features like data validation and asynchronous control flow.
Monet is a library that brings functional programming concepts to JavaScript. It includes types like Maybe and Either, which are similar to Option and Result in true-myth. Monet also provides a rich set of combinators and utilities for functional programming, making it a more feature-rich alternative.
fp-ts is a library for typed functional programming in TypeScript. It provides a wide range of functional programming utilities, including Option and Either types. fp-ts is highly type-safe and integrates well with TypeScript's type system, making it a strong choice for TypeScript projects.
True Myth provides safe, idiomatic null, error, and async code handling in TypeScript, with Maybe, Result, and Task types that are really nice.
README • API docs • Source • Intro blog post
True Myth provides standard, type-safe wrappers and helper functions to help you with three extremely common cases in programming:
You could implement all of these yourself – it's not hard! – but it's much easier to just have one extremely well-tested library you can use everywhere to solve this problem once and for all.
See the docs for setup, guides, and API docs!
tsconfig.json:
moduleResolution: use "Node16" or laterstrict: truepackage.json
type: "module" (or else use import() to import True Myth into a commonJS build)For details on using a pure ES modules package in TypeScript, see the TypeScript handbook's guide.
This project follows the current draft of the Semantic Versioning for TypeScript Types specification.
-private module are publicSize of the ESM build without tree-shaking (yes, these are in bytes: this is a pretty small library!):
| file | size (B) | terser1 (B) | terser and brotli2 (B) |
|---|---|---|---|
| -private/utils.js | 888 | 321 | 166 |
| index.js | 646 | 273 | 108 |
| maybe.js | 18683 | 3553 | 889 |
| result.js | 14111 | 3258 | 812 |
| task/delay.js | 3901 | 649 | 259 |
| task.js | 51219 | 7254 | 1997 |
| test-support.js | 473 | 142 | 89 |
| toolbelt.js | 3739 | 890 | 277 |
| unit.js | 656 | 58 | 57 |
| total3 | 94316 | 16398 | 4654 |
Notes:
true-myth/test-support, you will not pay for it. However, some parts of the library do depend directly on other parts: for example, toolbelt uses exports from result and maybe, and Task makes extensive use of Result under the hood.The design of True Myth draws heavily on prior art; essentially nothing of this is original – perhaps excepting the choice to make Maybe.of handle null and undefined in constructing the types. In particular, however, True Myth draws particular inspiration from:
Option and Result types and their associated methodsMaybe and Result implementationsMaybe and Result types and their
associated functionsUsing terser 5.37.0 with --compress --mangle --mangle-props. ↩
Generated by running gzip -kq11 on the result of the terser invocation. ↩
This is just the sum of the previous lines. Real-world bundle size is a function of what you actually use, how your bundler handles tree-shaking, and how the results of bundling compresses. Notice that sufficiently small files can end up larger after compression; this stops being an issue once part of a bundle. ↩
9.0.0 (2025-04-16)
Now with nicer internals. cleaner public APIs, a brand new docs site! For more details, see the announcement blog post.
Maybe.of(unknown) -> Maybe<{}> (@chriskrycho)background section (@chriskrycho)nothing() (@chriskrycho)as const annotations (@chriskrycho)Delay re-export to delay (@chriskrycho)FAQs
A library for safe functional programming in JavaScript, with first-class support for TypeScript
The npm package true-myth receives a total of 224,834 weekly downloads. As such, true-myth popularity was classified as popular.
We found that true-myth demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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