When exporting a module like this, what happens?

Question

I was looking up some database connection Google searches when I saw something that exported an instance of a module as such

const foo = () => {
// Do stuff
};
...

module.exports = foo();

I don't know what this is called but how does nodejs treat exporting a function invocation vs an object or the function itself (without calling it)?

Thank you

Answer 1

The foo function only gets called once no matter how many times you require the module.

This is very simplified explanation of what is happening behind the scenes in Node.js

// cache for modules
var modules = {};

// very simplified require function
function require(name) {

    // check cache
    if (modules[name])
        // so if it has already been required it returns the cached result
        return modules[name].module.exports;

    // it will resolve path to the required module
    // and loads the file content
    // not showing here

    var obj = { module: { exports: {}}};
    
    // node will wrap the code in a function similar to bellow
    function module(module, exports){
    
        const foo = () => {
        // Do stuff
        };
        ...
        
        module.exports = foo();
    
    };
    
    module(obj.module, obj.module.exports);
    
    // and now cache it
    modules[name] = obj;

   return obj.module.exports;
}