To add methods to a component, use invokers (sparingly) or members. Don't add them directly to 'that' in an init function (i.e. don't use "that.save = function () {...};).

The members option is essentially a simpler version of invokers. The right-hand side of each property is evaluated as an IoC expression and assigned to the top-level component path held on the left-hand side. Unlike invokers, methods are evaluated only once during component construction.

fluid.defaults("my.component", {
    ...
    members: {
        width: "{that}.options.width", // promotes the 'size' option to a property of the component, i.e. 'that.size'
        computeArea: "my.component.computeArea" // component method will resolve to the free function
    }
});