Question

In the first few lines of backbone.js, I don’t see why they are testing for undefined on exports or require

It seems obvious that it would be undefined as they did not set it. If it was a global(window) object then they would have said it explicitly.

root.exports  // they don't do this
root.require

Why do they check this?

typeof exports !== 'undefined'

and this

if (!_ && (typeof require !== 'undefined'))

and this from above

!_

Full Snippet

(function(){
    var root = this, 
        previousBackbone = root.Backbone,
        slice = Array.prototype.slice,      
        splice = Array.prototype.splice,
        _ = root._,
        Backbone;
    if (typeof exports !== 'undefined') {
        Backbone = exports;
    } else {
        Backbone = root.Backbone = {};
    }
    Backbone.VERSION = '0.9.2';
    if (!_ && (typeof require !== 'undefined')) {
        _ = require('underscore');
    }

Answer 1

That’s there to allow Backbone.js to be used as a Common.js module I believe. More details here: http://wiki.commonjs.org/wiki/Modules/1.1

Specifically this bit:

In a module, there is a free variable called “exports”, that is an object that the module may add its API to as it executes.

Also, this bit covers your question about require:

In a module, there is a free variable “require”, that is a Function. The “require” function accepts a module identifier. “require” returns the exported API of the foreign module.