When should I use require() and when to use define()?

From the require.js source code (line 1902):

/**
 * The function that handles definitions of modules. Differs from
 * require() in that a string for the module should be the first argument,
 * and the function to execute after dependencies are loaded should
 * return a value to define the module corresponding to the first argument's
 * name.
 */

The define() function accepts two optional parameters (a string that represent a module ID and an array of required modules) and one required parameter (a factory method).

The return of the factory method MUST return the implementation for your module (in the same way that the Module Pattern does).

The require() function doesn't have to return the implementation of a new module.

Using define() you are asking something like "run the function that I am passing as a parameter and assign whatever returns to the ID that I am passing but, before, check that these dependencies are loaded".

Using require() you are saying something like "the function that I pass has the following dependencies, check that these dependencies are loaded before running it".

The require() function is where you use your defined modules, in order to be sure that the modules are defined, but you are not defining new modules there.

With define you register a module in require.js that you can then depend on in other module definitions or require statements. With require you "just" load/use a module or javascript file that can be loaded by require.js. For examples have a look at the documentation

My rule of thumb:

• Define: If you want to declare a module other parts of your application will depend on.

• Require: If you just want to load and use stuff.

"define" method for facilitating module definition and "require" method for handling dependency loading

define is used to define named or unnamed modules based on the proposal using the following signature:

define(
module_id /*optional*/, 
[dependencies] /*optional*/, 
definition function /*function for instantiating the module or object*/
);

require on the other hand is typically used to load code in a top-level JavaScript file or within a module should you wish to dynamically fetch dependencies

Refer to https://addyosmani.com/writing-modular-js/ for more information.

General rules:

1. You use define when you want to define a module that will be reused

2. You use require to simply load a dependency

   //sample1.js file : module definition 
   define(function() {
         var sample1 = {};
         //do your stuff
        return sample1;
    });
   
   //sample2.js file : module definition and also has a dependency on jQuery and sample1.js
   define(['jquery', 'sample1'], function($,sample1) {
       var sample2 = {
           getSample1:sample1.getSomeData();
       };
       var selectSomeElement = $('#someElementId');
       //do your stuff....
       return sample2;
   });
   
   //calling in any file (mainly in entry file)
   require(['sample2'], function(sample2) {
       // sample1 will be loaded also
   });
   

Hope this helps you.