Web modules are exclusive to Velo and enable you to write functions that run server-side in the backend, and easily call them in your client-side code. With web modules you can import functions from backend into files or scripts in page code or public files, knowing they will run server-side in the backend. Velo handles all the client-server communication required to enable this access. See the advanced tip below if you want to know how this communication is handled. Advanced: How Web Modules Work - Behind the Scenes When you import a web module on the client-side you get a proxy function to the web module function. This proxy function uses an XMLHttpRequest to invoke the function in the backend. The runtime listens to those invocations and calls the appropriate function. The arguments and return value are serialized using JSON. When to Use Web Modules Web module functions contain code you would typically want or need to run in the backend. Your code may have security risks if it runs in the frontend, or you may want to access other web services. For example, let's say you want to enable your site visitor to send an email via a third-party provider. You would write your function that sends the email in the backend, like this: {fetch} ; API_KEY = ; { fetch( + API_KEY, { : , : .stringify({address, subject, body})
 }).then( { (response.status >= && response.status < ) response.text(); (response.statusText);
 });
}; // Filename: backend/sendEmail.jsw  (web modules need to have a *.jsw* extension) import from 'wix-fetch' // wix-fetch is the API we provide to make https calls in the backend const "the api key for the backend service that sends email" export ( ) function sendEmail address, subject, body return "https://a-backend-service-that-sends-email.com/send?APIKey=" method 'post' body JSON ( ) function response if 200 300 return else throw new Error You need this code to run in the backend for two reasons: It includes your API key to the third-party service, which you don't want to expose in the client. You are calling a third-party web service, which you can only do in the backend because of . cross-origin resource sharing (CORS) You would then import the function from your web module into your frontend file and use it: {sendEmail} ; {
 sendEmail(
         $w( ).value,
         $w( ).value,
         $w( ).value)
         .then( { .log( );
        }
     );
} import from 'backend/sendEmail.jsw' export ( ) function sendButton_onClick event "#addressInput" "#subjectInput" "#bodyInput" ( ) function console "email was sent" Using Web Module Functions in Backend You can import a web module function into another module in backend. Calling a Function in a Web Module Unlike regular modules that allow you to export functions, objects, and other items, you can only export functions from web modules. Web modules also always return a promise. This is true even if, in the implementation of the function, it returns a value. For example, if your web module function returns a value, like this: { factor1 * factor2;
} // Filename: aModule.jsw (web modules need to have a *.jsw* extension) export ( ) function multiply factor1, factor2 return When you call the function, it still returns a promise that resolves to the value. So you need to use it like this: {multiply} ;
multiply( , ).then( { .log(product); }); import from 'backend/aModule' 4 5 ( ) function product console // Logs: 20 Debugging Web Modules You can log messages to the console in web modules and they will be displayed in the client's console log, even though the code is running in the backend. Web Module Permissions Because Web Modules allow you to call your backend code from the frontend, it's important that you limit which visitors can access their functionality by setting their permissions. See to learn more. About Web Module Permissions Previously published at https://support.wix.com/en/article/velo-web-modules-calling-backend-code-from-the-frontend

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Velo Promises in Action: Key Tips to Call the Asynchronously-Run Functions

Dealing with Velo Web Modules: Our Advanced Tips for Improved Function Import

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