What is `require` in the content of Backbone.js?

This is an extract of the Backbone.js source (near the very top):

// Require Underscore, if we're on the server, and it's not already present.   
var _ = root._;
if (!_ && (typeof require !== 'undefined')) _ = require(开发者_如何学运维'underscore')._;

At no point is require defined. What is it?

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In other words Backbone.js can be used with non-browser JavaScript backend languages that follow the CommonJS spec like Node.js.

Here's the bit about require in Node.js docs.

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It's checking for the presence of require from the CommonJS module specification. It's saying that if the global object doesn't contain _, try to require the underscore module (if require is defined) and get _ from there.