meteor/docs/source/packages/underscore.md

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underscore	Documentation of Meteor's `underscore` package.

Underscore is a utility-belt library for JavaScript that provides support for functional programming. It is invaluable for writing clear, concise JavaScript in a functional style.

The underscore package defines the _ namespace on both the client and the server.

{% pullquote warning %} We have slightly modified the way Underscore differentiates between objects and arrays in collection functions. The original Underscore logic is to treat any object with a numeric length property as an array (which helps it work properly on NodeLists). In Meteor's version of Underscore, objects with a numeric length property are treated as objects if they have no prototype (specifically, if x.constructor === Object. {% endpullquote %}