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wtf is core-js?

skevy/core-js — explained in plain English

Analysis updated 2026-07-17 · repo last pushed 2016-01-17

JavaScriptAudience · developerComplexity · 2/5DormantSetup · easy

TL;DR

core-js is a polyfill library that adds missing modern JavaScript features to older browsers, so developers can write clean modern code that still works everywhere.

Mindmap

mindmap
  root((core-js))
    What it does
      Polyfills modern JS features
      Supports older browsers
      Modular includes
    Tech stack
      JavaScript
      npm
    Use cases
      Use Promise in old browsers
      Use Array methods everywhere
      Deduplicate arrays with Set
    Audience
      Frontend developers
      Build tool maintainers
    Features
      ES5 basics
      ES6 collections
      String improvements

Code map

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filefunction / class

Why would anyone build with this?

REASON 1

Use modern JavaScript methods like Promise or Array.from() while still supporting old browsers like IE8.

REASON 2

Deduplicate an array using Array.from(new Set(...)) even where Set isn't natively supported.

REASON 3

Include only the specific polyfills your app needs to keep bundle size small.

What's in the stack?

JavaScriptnpm

How it stacks up

skevy/core-jsa15n/a15na15n/checkout-validation
LanguageJavaScriptJavaScriptJavaScript
Last pushed2016-01-172019-04-072014-09-04
MaintenanceDormantDormantDormant
Setup difficultyeasyeasyeasy
Complexity2/52/52/5
Audiencedevelopergeneraldeveloper

Figures from each repo's GitHub metadata at analysis time.

How do you spin it up?

Difficulty · easy Time to first run · 5min

Yoink these prompts

Prompt 1
Show me how to install core-js and import only the polyfills my project needs.
Prompt 2
Help me configure core-js with Babel so my app supports older browsers automatically.
Prompt 3
Give me an example of using core-js to polyfill Array.find() and Array.findIndex().
Prompt 4
Explain how to use core-js as a standalone library without modifying built-in objects.

Frequently asked questions

wtf is core-js?

core-js is a polyfill library that adds missing modern JavaScript features to older browsers, so developers can write clean modern code that still works everywhere.

What language is core-js written in?

Mainly JavaScript. The stack also includes JavaScript, npm.

Is core-js actively maintained?

Dormant — no commits in 2+ years (last push 2016-01-17).

How hard is core-js to set up?

Setup difficulty is rated easy, with roughly 5min to a first successful run.

Who is core-js for?

Mainly developer.

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