Override ‘Magento_Checkout/js/view/billing-address.js’ with Custom JS in Magento 2

There is no more skin folder but you can still use themes.

As a proof of concept I used you example with op-checkout-method.js and this this.

Preconditions:

• Magento2-beta11 installed
• Default theme active (blank).
• No files generated in the pub/static folder (remove the pub/static/frontend folder)

Actions:

• Copied the op-checkout-method.js file from it's module location app/code/Magento/Checkout/view/frontend/web/js/opc-checkout-method.js to the blank theme to app/design/frontend/Magento/blank/Magento_Checkout/web/js/opc-checkout-method.js
• edited the clone file and added a console.log('something') or alert('something') in the _create function of the mage.opcCheckoutMethod widget.
• cleared browser cache.

Result:

• When the checkout page loads I see my alert displayed or the text logged in the console.

Related Info:

If I run from cli php dev/tools/Magento/Tools/View/deploy.php (the script that publishes the static resources) my new js file gets placed in pub/static/frontend/Magento/blank/en_US/Magento_Checkout/js/opc-checkout-method.js

[EDIT]

I found a way to do it via a module.

In [Namespace]/[Module]/view/frontend/requirejs-config.js add this:

var config = {
    map: {
        '*': {
            'Magento_Checkout/js/opc-checkout-method':'[Namespace]_[Module]/js/opc-checkout-method'
        }
    }
};

Then create the file [Namespace]/[Module]/view/frontend/web/js/opc-checkout-method.js with your content.

For testing purposes I cloned the original file and just added again a console.log in the _create function.

Also remember to regenerate the public resources for frontend.

You need create requirejs-config.js in you module with

var config = {
    config: {
        mixins: {
            'Magento_Checkout/js/model/step-navigator': {
                'You_Module/js/step-navigator/pluggin': true
            }
        }
    }
};

And js/step-navigator/pluggin.js file with content like

define(function () {
    'use strict';

    return function (target) { // target == Result that 'Magento_Checkout/js/model/step-navigator' returns.
        // modify target
        var navigateTo = target.navigateTo;
        target.navigateTo = function(code, scrollToElementId) {
            if (code == ...) { // before  method
              ....
            }
            var result = navigateTo.apply(this, arguments);
            //after method call
            return  result;
        };
        return target.
    };
});