Set or remove attribute depending on value of another element

[Daniel Lidström]

Hello,

I'd like to be able to set or remove an attribute from an input element depending on the value of another input element. The use case is the following: if the user enters a value for the number of strikes in a bowling game, then he must *also* enter a value for the number of misses. I am using ASP.NET MVC with unobtrusive jQuery validation, so the attribute I want to set is data-val-required. Here's my attempt so far:

Strikes input element:
<input type="number" value="" name="HomeTeam.Serie1.Game1.Strikes" id="HomeTeam_Serie1_Game1_Strikes" data-val-range-min="0" data-val-range-max="12" data-val-range="The field X must be between 0 and 12." data-val-number="The field X must be a number." data-val="true" data-bind="value: team_1_player1_strikes0, valueUpdate: 'afterkeydown'" class="valid">

Misses input element:
<input type="number" value="" name="HomeTeam.Serie1.Game1.Misses" id="HomeTeam_Serie1_Game1_Misses" data-val-range-min="0" data-val-range-max="12" data-val-range="The field Miss must be between 0 and 12." data-val-number="The field Miss must be a number." data-val="true" data-bind="enable: team_1_player1_strikes0, css: { 'input-validation-error': team_1_player1_strikes0 }, attr: { 'data-val-required': team_1_player1_strikes0 ? 'Number of misses required' : '' }" disabled="" data-val-required="Ange">

The important part is of course the data-bind attributes. The strikes input element will set the team_1_player1_strikes0 observable value when anything is entered. I would want the misses element to add data-val-required attribute when this happens, and remove the attribute when the strikes input element is cleared. My attempt doesn't work, obviously. How can I achieve this?
Thanks in advance!

[Casey Corcoran]

Looks like this will do the trick: attr:{'data-val-required':isRequired}

Where isRequired is an observable Boolean value. When set to false the
attribute is removed from the element.

[Daniel Lidström]

Thank you Casey! That would work for me.