# getify/Functional-Light-JS

Pragmatic, balanced FP in JavaScript. @FLJSBook on twitter.

Repository: https://github.com/getify/Functional-Light-JS
Canonical: https://ross.abutalabs.com/products/functional-light-js
Homepage: http://FLJSBook.com
Language: JavaScript
License: NOASSERTION
License Family: other
Topics: book, javascript, functional-programming, training-materials, training-providers, education
Last push: 2023-12-26T15:46:02+00:00

## Health v2 (maintenance only)
Score: 32/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 0, release rhythm 35, longevity 100
- inputs: {"age_days": 3669, "days_push": 981, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: no_releases, no_license
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 16738, forks 1918 (observed 2026-08-28T04:11:17.090214+00:00)

## What it is
Functional-Light JavaScript is a complete book by Kyle Simpson offering a pragmatic, balanced introduction to functional programming concepts applied to JavaScript. It is freely readable online and also sold in ebook formats, focusing on practical FP fundamentals without heavy mathematical terminology.

## Use cases
- learn functional programming in javascript
- understand pure functions and immutability in js
- pragmatic fp without category theory jargon
- study closures currying and composition in javascript
- improve javascript code quality with fp principles
- free online book on functional javascript

## When to choose
- you already know JavaScript and want to apply FP ideas pragmatically
- you prefer plain-language explanations over heavy FP terminology
- you want a free, complete, in-depth text on FP in JS

## When to avoid
- you need a beginner-level intro to JavaScript itself
- you want a formal, mathematically rigorous treatment of FP
- you need a runnable library rather than learning material

## Facets
- artifact type: learning-resource
- maturity: stable
- function: developer-tools
- domain: programming-languages, tutorials, education
- platform: browser
- tags: book, functional-programming, javascript, free-online-book, kyle-simpson, nodejs

## Member repositories
- getify/Functional-Light-JS (main) score 32

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:11:17.090214+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-29T17:04:46.358377+00:00, confidence not recorded.
  - readme: https://github.com/getify/Functional-Light-JS (fetched 2026-08-28T04:11:17.090214+00:00, sha 8f86b4d07649)
  - homepage: http://FLJSBook.com (fetched 2026-08-29T08:02:25.420326+00:00, sha 7c4715f8987f)
  - site_page: https://leanpub.com/about (fetched 2026-08-29T08:02:25.439819+00:00, sha a3ffdd2e2a46)
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  - site_page: https://leanpub.com/authors (fetched 2026-08-29T08:02:25.429985+00:00, sha 8047e75e1d4b)
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  - site_page: https://leanpub.com/help/api (fetched 2026-08-29T08:02:25.436554+00:00, sha a3f97ef691e5)
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- Data as of 2026-08-30T08:39:29.467469+00:00.