Q:

How to require or include scripts in indesign?

How do I load another script file and run a method on it?
I'm using InDesign javascript and I don't know how to include multiple files in the same script.

A:

Three options: import, app.doScript, and $.evalFile. I prefer $.evalFile. See app.doScript vs $.evalFile
Working Example:
C:\script1.jsx
(function() {
    $.evalFile(new File("/c/script2.jsx"));
    var sFullName = g_script2.combineName("John", "Doe");
    $.writeln(sFullName);
    return "Success";
})();

C:\script2.jsx
g_script2 = {
    combineName: function(sFirstName, sLastName) {
        return sFirstName + " " + sLastName;
    }
};

If script2.jsx is not located in the root of the C drive, modify script 1 with its true location.
Explanation:

Script 1 creates and executes an anonymous function to avoid polluting the global namespace. If it didn't do this, sFullName would be global.
Script 1 executes Script 2.
Script 2 creates an object and stores it to the global variable g_script2.
Script 1 calls the combineName method of script 2. It is important to note here that all of the files of your script will share the same global namespace, which is how script 1 can access g_script2. However, this also means that no two files should ever have the same name for a function or variable, unless they're kept inside a global object like in this example.
The combineName function is run, and returns a string.
Script 1 prints the name, then returns "Success". Since that's the last object on the stack, it is returned as the script result.

A:

ExtendScript provides preprocessor directives for including external scripts. 
This directive inserts the contents of target file into the current script file at the location of the statement. So after the statement you will be able to call any method like it’s a method of the current script:
#target InDesign;    

// Include other script file that includes a function otherScriptFileMethod
#include "otherScriptFile.jsx"

// Now you can call the method as it was written in this script file
otherScriptFileMethod();

