Check the tag of post #public or #backend

If it's public

Create a public .js file in your Wix account

Name it as a "template.js"

copy the main code

function fullName(firstName, lastName){
    return firstName + ' ' + lastName
}

and past it on template.js

Final Public file

//in template.js
export function fullName(firstName, lastName){
    return firstName + ' ' + lastName
}

Now you can reuse this function in any page

Just by importing the module name

import { fullName } from 'public/template';

// general import syntax
import { moduleName } from 'public/filename.js';

Let say we have 2 input element with first name and last name

After a button click

We will show the full name in a text

Final Page code

//Importing module from template js file
import { fullName } from 'public/template'

$w.onReady(()=>{
    $w('#btn').onClick(()=>{
        let firstName = $w('#inFirstName').value;
        let lastName = $w('#inLastName').value;
        //Calling the module
        $w('#textDisplayFullName').text = fullName(firstName, lastName);   
    })
});

- inFirstName - id of an input element

- inLastName - id of an input element

- textDisplayFullName - id of text element

- btn - id of a button element