HTML5 required attribute one of two fields

Question

I have a form with two required input fields:

Answer 1

Update 2020-06-21 (ES6):

Given that jQuery has become somewhat unfashionable in the JavaScript world and that ES6 provides some nice syntactic sugar, I have written a pure JS equivalent to the original answer:

document.addEventListener('DOMContentLoaded', function () {
    const inputs = Array.from(document.querySelectorAll('input[name=telephone], input[name=mobile]'));
    const inputListener = e => inputs.filter(i => i !== e.target).forEach(i => i.required = !e.target.value.length);

    inputs.forEach(i => i.addEventListener('input', inputListener));
});

Note that the above uses arrow functions and Array.from so does not work in IE11 without the use of a transpiler such as Babel.

Original Answer (jQuery):

I played around with some ideas and now have a working solution for this problem using jQuery:

jQuery(function ($) {
    var $inputs = $('input[name=telephone],input[name=mobile]');
    $inputs.on('input', function () {
        // Set the required property of the other input to false if this input is not empty.
        $inputs.not(this).prop('required', !$(this).val().length);
    });
});

  Telephone:
  
  
Mobile:
  
  

  

This uses the input event on both inputs, and when one is not empty it sets the required property of the other input to false.

I've written a jQuery plugin wrapping the above JavaScript code so that it can be used on multiple groups of elements.