explaingit

jashkenas/underscore

★ 27,339JavaScriptAudience · developerComplexity · 2/5MaintainedLicenseSetup · easy

TLDR

A JavaScript utility library providing ready-made functions for working with lists, objects, and data, like looping, filtering, and transforming, without writing repetitive code.

Mindmap

mindmap
  root((repo))
    What it does
      Loop through items
      Filter and transform
      Combine data
      Handle objects
    Use cases
      Browser scripts
      Server-side code
      Data processing
      Avoid boilerplate
    Tech stack
      JavaScript
      Functional helpers
    Key features
      No built-in conflicts
      Reusable functions
      Well-established

Things people build with this

USE CASE 1

Loop through arrays and objects without writing manual for-loops in your JavaScript code.

USE CASE 2

Filter and transform lists of data with clean, readable functions instead of nested loops.

USE CASE 3

Combine multiple values into a single result using reduce and similar aggregation functions.

USE CASE 4

Handle common object and array tasks in both browser and Node.js environments without external dependencies.

Tech stack

JavaScript

Getting it running

Difficulty · easy Time to first run · 5min
Use freely for any purpose, including commercial use, as long as you keep the copyright notice.

In plain English

Underscore.js is a JavaScript library, a collection of ready-made helper functions that make working with data in JavaScript easier. Think of it as a toolbox full of shortcuts for common programming tasks. The problem it solves: JavaScript doesn't come with many built-in ways to work with lists of data or collections of objects. Tasks like "go through every item in a list," "filter out items that don't match a condition," or "transform every item into something else" require writing the same repetitive code over and over. Underscore provides clean, reusable functions for all of these tasks so you don't have to reinvent the wheel. It includes functions like each (loop through items), map (transform items), reduce (combine items into one result), and filter (keep only matching items), these are called "functional programming" helpers, meaning they treat functions as building blocks you can pass around. Crucially, Underscore does all this without modifying JavaScript's built-in objects, which avoids conflicts with other libraries. You'd reach for Underscore when you're writing JavaScript and want a dependable set of utility tools to handle data manipulation, array processing, or object handling without writing repetitive boilerplate code. It's used directly in browsers or in server-side JavaScript environments. The project is open source and has been around long enough to be a well-established reference in the JavaScript ecosystem.

Copy-paste prompts

Prompt 1
Show me how to use Underscore.js map() and filter() to transform and filter an array of user objects.
Prompt 2
How do I use Underscore.js reduce() to sum up all the prices in a shopping cart array?
Prompt 3
Give me an example of using Underscore.js each() to loop through an object and log its keys and values.
Prompt 4
How do I use Underscore.js to find the first item in an array that matches a condition?
Prompt 5
Show me how to chain multiple Underscore.js functions together to process a list of data.
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Generated 2026-05-18 · Model: sonnet-4-6 · Verify against the repo before relying on details.